The Tyranny of the Rocket Equation (2012) | Hacker News Reader

The Tyranny of the Rocket Equation (2012)

nasa.gov

214 pointsbane157 comments

Right. This has been known for a long time. It's why rockets aren't much better than they were 40 years ago. Chemical fuels are as good as they can get. Space travel with chemical fuels is just barely feasible.

In the 1960s, it was assumed that nuclear power would be necessary for space flight. Everybody involved knew the rocket equation. The original plan for Apollo included a nuclear upper stage. The engine (NERVA) was built and tested. A Nuclear Assembly Building at Canaveral was planned. But, because the goal was so narrow ("man, moon, decade"), the solution chosen was a disposable rocket the size of a 50-story building to send an RV-sized payload to the moon.

The crash of a nuclear rocket would produce a radioactive mess. Not Chernobyl or Fukishima sized, but at least small-town sized. Launching from an isolated island would help.

Various schemes have been tried or proposed to beat the rocket equation. Launching from a balloon was tried early. Launching from an aircraft is still used by Virgin Aerospace. It helps a little.

A space vehicle that's an air-breather while it's in the atmosphere and transitions to rocket mode once out has been proposed many times, but making something that's both a rocket and an airplane is hard and adds a lot of weight. As an airplane, it has to go hypersonic to get up enough speed that it's worth doing this. Building a hypersonic aircraft is very hard; so far, only a few small demo craft have done it. The National Space Plane (hypersonic single-stage-to-orbit) was proposed in the 1980s. Ben Rich, head of the Lockheed Skunk Works and the designer of the SR-71's propulsion system, declined to let Lockheed bid on it. (His comment: "We used titanium (on the SR-71). You know anything stronger?") Remember, it has to be strong at a few thousand degrees.

The same problems apply to launch track systems. Going hypersonic near the ground is possible; the Holloman AFB test track, 50,000 feet of very straight railroad track, has been used to reach Mach 8.6. The required acceleration is about 14g. Far too much for humans.

The "space elevator" requires not only unreasonable strong materials but the ability to put so much mass in space that you wouldn't need a space elevator if you had that kind of launch capacity.

I wouldn't dismiss the space elevator out of hand. It requires carbon nanotubes of a few meters length to achieve the required strength, and you wouldn't need to lift it pre-built - you could build it with a guideline and cable laying cars traveling up and down, adding to the cable, much like they do with suspension bridges. Long term, it seems like far and away the best approach if we can solve the materials science problem.

EDIT: I found this book to be a pretty great primer on the subject: http://www.amazon.com/Space-Elevator-Earth-Space-Transportat...

What would the guideline be made of?

The same stuff as the main cable, just thinner and less redundant. The main issue with the space elevator is the cable weight vs. cable strength.

I would certainly dismiss elevators because there is a much more practical alternative that doesn't require materials that don't exist in sufficient quantities to reach a quarter of the way to the moon.

The Lofstrom Loop (http://en.wikipedia.org/wiki/Launch_loop) could be built with materials we have today, although it requires sufficient amounts of money and land that only large countries or multibillionaires could attempt it.

I never have understood why the space elevator gets so much attention, and the launch loop is almost unknown. The elevator obviously had some high-profile proponents (like Arthur C. Clarke), but given how realistic the launch loop seems (only 8 billion dollars??), I would think it would be the thing everyone talks about.

The fact that it is not makes me think that it is less realistic or more constrained than it is made out to be. I certainly don't have the skills necessary to evaluate that, but I bet someone here does...

The launch loop must be perpetually active once established. That's a lot of continuous power needed for something we do pretty rarely--launch stuff into space.

And if the power turns off, it falls back to Earth. A heavy, high-speed belt falling 50 miles down along a length of 1,200 miles...not an easy problem to solve. Even if it drops into uninhabited territory, it's not going to be in great shape.

A space elevator is a passive system. Once established, it stays up. If you lose power you can't go up it, but it doesn't fall down. In that respect it is more like the passive infrastructure we're comfortable with, like highways, bridges, buildings, etc.

If the power is turned off the Launch Loop will not immediately fall out of the sky. As the loop slows it will gradually fall. This is also why most pictures show it operating over the ocean. We have a lot of ocean at the equator.

A space elevator is not passive. It needs active dampening systems to avoid resonances that would tear it apart. The failure modes for a space elevator are also pretty terrifying. The lower the break the less harmful it is, but higher breaks will cause huge amounts of damage. Worst case scenario, the whole cable comes down. That is 75000 km. (Assuming the traditional double-spool deployment.) The Earth's circumference is only 40000 km. It will wrap around the earth twice! 'Blue Mars' has more about what such a disaster might look like.

The loop is not more well known because it was only invented in 1981 and requires non-intuitive physics to explain. Space elevators are "simpler" (if you ignore the active control problems) and have been around since 1895, though the modern design dates to 1959.

> If the power is turned off the Launch Loop will not immediately fall out of the sky. As the loop slows it will gradually fall.

That's for a loop constructed only of superconductors and magnets. If you use active electromagnets instead of the superconductors for saving money (or if your superconductors get too hot), the failure mode is way more harmful.

Anyway, I really don't know why the launch loop is so overlooked. Specifically, I don't know why nobody's trying to build an intercontinental bridge. It may be because of our anti-nuclear culture, and that those things only make any sense when coupled with nuclear power, but that's just speculation.

The linked Wikipedia page addresses turning the loop on and off. In fact, it can't be run continuously because it would overheat.

Edit: actually, I may be mistaken. Here is one of the relevant passages:

"When at rest, the loop is at ground level. The rotor is then accelerated up to speed. As the rotor speed increases, it curves to form an arc. The sheath forces it to follow a curve steeper than the rotor's natural ballistic curve, which [clarification needed] , in turn, exerts a centrifugal force on the sheath, holding it aloft. The loop would be anchored to the ground to remain at a fixed height.

"Once raised, the structure requires continuous power to overcome the energy dissipated. Additional energy would be needed to power any vehicles that are launched."

So is this talking about raising it only the first time? And then it has to have continuous power for the remainder of it's operational life? It seems like having power off capability shouldn't be an insurmountable task.

The failure mode of a launch loop is really bad though. The constant power requirement is a pretty big problem and the solutions to loss of power aren't that promising. Compared to the elevator where there's a larger safety margin.

I liked Elon Musk's note on the space elevator; he basically said "we should look at it once we have a bridge from Los Angeles to Tokyo, because that's far easier to build".

I disagree with that argument. Great solutions require great problems, and moving things between LA and Tokyo just isn't anymore. Right now, shipping a person costs some $1k (return included) and takes less than 12 hours; a 40 ft container of cargo around $800 and 22 days.

Launching a satellite still costs millions. The closest thing to inexpensive space flight is probably Virgin Galactic's SpaceShipTwo, which is still in testing. Space remains interesting and unsolved. And as such, I'll bet you a beer we'll have a space elevator sooner than either a bridge or a tunnel directly connecting LA and Tokyo.

A better example of air launch is Pegasus [0]. An example of an air breather that has passed the physics plausibility test is Skylon [1].

0 - https://en.wikipedia.org/wiki/Pegasus_(rocket)

1 - https://en.wikipedia.org/wiki/Skylon_(spacecraft)

About your last point: if you have a space elevator you can have a mass going down to balance for the mass going up, which reduces the energy needed merely to working against the friction on the cable and the connection point at the top. That's cheap.

> Space travel with chemical fuels is just barely feasible.

To certain extent, that only applies while assuming rocket launches are hard to iterate. Otherwise it would be a matter of launching enough of them.

By iterate, I assume you mean a multi-stage rocket, which is, in theory, a simple solution to the problem; as well as the on only way we have actually gotten anything to space.

Another potential solution is asparagus staging, where you have multiple fuel tanks that are jettisoned when empty. Assuming you can construct a fuel cross-feed system so that you empty your outermost tanks while you inner tanks are still full, this gets you (mostly) the same benefits as an iterated rocket design. The outertanks also may have engines on them that are jettisoned when they are.

I do not believe that any rocket to date has used this system, However the SpaceX Falcon Heavy (planned to launch in 2015) includes this method. [0] As I understand it, they added a single asparagus stage (with two tanks/engines) to their first stage. Their staging sequence goes like: Fire all 3 lower engines. Jettison outer 2 lower rockets, leaving a single engine with a full tank. Jettison lower engine and fire the upper stage engine.

As a side note, Googling "asparagus staging" returns, with the exception of a single reference to the Falcon Heavy (as the 9th link), exclusively references to the game Kerbal Space Program for the first 3 pages. Tested through a Tor browser session that has not been tainted with my Kerbal related search history. However, the earliest use of the term I found (courtesy of [1]) was from 1997 [2] (Copyright page viewable from [3]). The Kerbal wiki [4] also claims that this is the first usage.

[0] http://en.wikipedia.org/wiki/Falcon_Heavy#cite_ref-nss201111... [1] http://www.reddit.com/r/KerbalSpaceProgram/comments/1b7hml/o... [2] http://books.google.com/books?id=C70gQI5ayEAC&pg=PA143&lpg=P... [3] http://www.amazon.com/gp/product/0486600610/ref=pd_lpo_sbs_d... [4]http://wiki.kerbalspaceprogram.com/wiki/Asparagus_staging

What about momentum exchange tethers, AKA skyhooks? The smaller ones require significantly less mass than a full space elevator, but mean that rockets need carry less fuel, because they're taking from the momentum of the tether. The lost momentum of the tether can then be replenished over time, using either efficient engines like ion drives, or if the tether is electrodynamic, using the Earth's magnetic field.

"A space vehicle that's an air-breather while it's in the atmosphere and transitions to rocket mode once out has been proposed many times, but making something that's both a rocket and an airplane is hard and adds a lot of weight."

For those interested, these people are working on it. http://www.reactionengines.co.uk/ (They're the Skylon makers mentioned in another comment here)

Now imagine if this planet was a bit more dense or bigger. Let's say 2g instead of 1. I wonder if we had any space vehicles at all right now, in that case.

EDIT: Nevermind, FTA:

> If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.

Anthropic principle my a$$. This Universe can barely support an expanding civilization.

We barely became self-aware, and the useful lifespan of the Sun is almost over - it will only make things more difficult for the Earth from now on.

If the planet is too small, it's not stable enough to support life. If it's too big, you're trapped there forever. And the range in between is narrow.

The useful lifespan of the Sun thing caught my attention, could you elaborate on that?

It's not going to happen tomorrow, but "soon" compared to all its previous history. It is estimated that in 1 billion years the Earth will be too hot for any meaningful ecosystems to exist. Things would start to degrade before that limit, of course.

Overall, it looks like life has basically one single shot at producing intelligence on an Earth-like planet. We popped out kinda towards the end.

That paragraph doesn't make much sense. It's assuming a single-stage rocket. Stages let you do better, and that's why every orbital rocket is staged. There will be a point where that becomes impractical (e.g. your rocket has to be the size of a mountain) but there isn't a point where it's physically impossible.

Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.

I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam over the course of a few minutes. And be able to hold that laser on the target.

Edit: The Saturn V held 3.2 million liters of fuel. And I think it takes about 2600 joules of energy to boil off 1 liter of water. So that is 8.3 billion joules of energy. If the flight time to orbit is 5 minutes, then a 27 megawatt laser should should do the trick.

Second edit -- I didn't see the "k" in front of joules on my random web searches for number of joules to boil off 1 liter of water. So that would be a 27 gigawatt laser (10 times the energy need for time travel). And, according to Retric below, I'm off even further. Point I was originally trying to get at is using a laser is more than a shade past impractical.

The Lightcraft design didn't carry it's own fuel:

http://en.wikipedia.org/wiki/Lightcraft

But how much benefit is that? Remember, rocket fuel doubles as reaction mass. This design STILL requires reaction mass, which will be essentially equal to the weight rocket fuel would have been. So all that's saved is the engines' own weight, which is a tiny fraction of the whole.

The big advantage would be that we're not limited to the exhaust velocity of combustion (which is inherent to using your reaction mass as fuel). If we can lase it hot enough to double the exhaust velocity, the rocket equation says we only need about a square root of the original amount of propellant and power, because almost all that fuel was lifting the rest of the fuel.

To move from the 85% propellant of rockets to 15% (somewhere between a fighter jet and a train, according to the article) we have to increase exhaust velocity of whatever we're pushing down to push us up by 12x.

One method of laser propulsion is using the laser to ablate a metal reaction mass. Since metal is much denser than water, and is converted into plasma, a much smaller reaction mass can be used. This method has a specific impulse of about 5000s, an order of magnitude higher than chemical rockets.

That would be useful for station-keeping for the ISS. I don't think NASA would let you point a laser strong enough to ablate metal at their station, though.

Rocket exhaust is normally plasma. Also, density is not really much of an issue unless it's really low. Rockets use liquid hydrogen which is normally ~1/14th the density of water.

The important bit is mass not density.

Your off by ~61,545 times. As, you don't want the energy to boil water, you want the energy you get from burning hydrogen.

1 liter of water is 11.19% hydrogen or ~0.1119kg. Hydrogen is 143 MJ/kg so 1 liter of water ~= 143,000,000J * 0.1119 = 16,001,700J.

So, you want a 1,661,715 megawatt laser. Good luck with that.

PS: You might be able to do laser assisted rocketry where you hit the combustion chamber with energy to increase exhaust velocity. But even that would take insane accuracy and a 100+ terawatt laser to be useful.

Ah, I didn't do the calculations myself, just did a couple random Google searches for how many joules it takes to boil off one liter of water (not how much it takes to bring a gallon to boiling point, which is of course much much less). Here's one site I used: https://www.physicsforums.com/threads/energy-required-to-boi..., but I didn't see the "k" in front of joules. Thanks.

Your still confusing a state change (boil) with a chemical change (burn). The reason this is important is how fast stuff comes out the bottom of your rocket is really important and pushing it faster takes more energy. To beat chemical rockets you need to use more energy to push harder.

It takes a kilowatt laser to lift a gram. A megawatt laser to lift a kilogram. A gigawatt laser to lift a metric ton.

Biggest chemical laser so far is about a megawatt. The laser diode people are making real progress, into the kilowatt range (http://teradiode.com/technology/) but gigawatt laser array are still a ways off.

A Saturn V at launch was 190 gigawatts.

NASA did a study on space elevators about a decade ago, and found that it would require carbon nanotubes several centimeters long, bound together by a realistically strong epoxy. Launching their design would require seven space-shuttle flights to deploy a minimal elevator, which you use to pull up additional construction material.

They addressed a lot of other practical issues too. Here's their final report (pdf), it's an interesting read.

http://www.niac.usra.edu/files/studies/final_report/521Edwar...

Ah neat, that looks like an extension of the book he wrote.

I get a bit worried when he dismisses chemical rockets for lifting the initial spool into space, and starts going on about electric drive... it seems like he should limit the number of technical revolutions required to make this work, if he wants this to succeed on a reasonable time scale...

I wonder about a fusion rocket drive, like Larry Niven wrote about (not the Bussard, the reaction drives).

There are two big problem in fusion energy research: plasma leaks, and high-energy neutrons. It seems to me that a fusion rocket answers both questions: just throw it all out the back. And hydrogen is abundant and cheap.

Because they're neutral, neutrons can't easily be deflected to throw them in some particular direction.

NERVA was fine if you didn't start it up until you were in LEO and took care to avoid any trajectories that might involve Earth reentry. But even for manned flight outside the atmosphere, shielding was an issue and servicing it was pretty much a suicide mission. Not insurmountable problems, but more than we needed for our modest goals at the time which didn't involve going beyond the Moon.

http://www.wired.com/2012/09/nuclear-flight-system-definitio...

Everyone Knows™ that putting stuff into space is expensive. Then Everyone Assumes™ that it's because of all the fuel. But no, fuel is cheap, hardware is what's expensive. If you look at the costs involved in putting something into orbit, the cost of fuel is a trivial detail, on the order of 1% of the total costs. Compare that to an airliner, where fuel is around 1/3rd of the total costs, or a car, where fuel can easily be over 50% of the total costs.

I happened to be reading up on rocket efficiency and I was surprised to learn that rockets are fairly efficient for launching stuff into orbit. Wikipedia uses the example of the Space Shuttle, where 16% of the energy in the propellants ends up in the kinetic and potential energy of the orbiter. That's pretty good!

The problem is that you use a rocket once and then throw it away. Imagine if your car was one-time-use. How often would you drive somewhere? How often would anyone drive anywhere? It wouldn't matter how efficient they are and it wouldn't matter if they didn't even require fuel at all.

Now, the rocket equation still comes into play here, because it means you need a lot of rocket for a little bit of payload. But the main problem is the one-time-use thing. A nuclear disposable rocket wouldn't improve things much. A reusable nuclear rocket would be great, but then so would a reusable chemical rocket.

This is the genius of SpaceX. For decades, rocket designers have looked at the rocket equation and tried their hardest to save fuel. SpaceX looked at the economics of rocketry and realized that fuel costs more or less don't matter, and instead concentrated on building their machines cheaply, and on making them reusable. We'll see how it works out, but if they succeed in making reusable rockets then they'll cut the cost of launches by an order of magnitude or more.

People have been trying to make reusable rockets since the beginning. The problem is that a reusable rocket is more complex, what means it's heavier, and that it needs much more fuel to launch, thus a bigger rocket, and the rocket equation makes everything astronomical.

Like everybody else, I cheering for SpaceX to solve this problem, but the challenge is not making a reusable rocket - it's making a light enough reusable rocket.

I'm not so sure. The Space Shuttle had a pretty big payload capacity. It had a lot of problems as well, but those were mostly due to being underfunded and hit with weird requirements beyond simple reusability. And that's really the only serious attempt at reusability that got beyond the early stages. It's still a pretty unknown area at this point, but I don't think building stuff sufficiently lightweight is necessarily the challenge. SpaceX doesn't seem to think it is, anyway. Their engines are not particularly high performance (meaning they need more fuel for the same job) and their rockets are engineered more for cost effectiveness than light weight. They seem to be making great strides precisely because they're not concentrating on weight. For example, their first stage reuse system involves carrying a bunch of extra fuel, where virtually every other attempt at reusability involved some sort of unpowered or nearly-unpowered landing after expending all fuel on the launch phase.

Fuel is indeed cheap (The Russkies are using kerosene and liquid oxygen---you can source those locally yourself.), but hardware isn't expensive, either. The expensive part is designing hardware that works correctly, given that it is 90% fuel.

If a car was a one-time-use vehicle, it would be much, much cheaper than it is now, when it's designed to last at least through the end of the warranty period. Likewise, the Space Shuttle was reusable, but that reusability never actually turned into a win.[2] (I have a rocket-scientist friend who might convincingly argue that putting the Space Shuttle's budget into building a Saturn V assembly line would have been a net win. Building the same rocket over and over seems to have worked for the Russians.)

There are two downsides to reusability, particularly for man-rated rockets: How do you land, and what do you have to do to turn it around.

As was pointed out in the article, IIRC, the Shuttle could put 120 tons into orbit, but 100 tons of that was coming back down with the re-entry vehicle. Kind of reduces the effective payload. I don't know the details of the SpaceX reusability design, but I'm wondering where the landing fuel comes from; if it rides the rocket the whole time, it's coming out of the 10%.

The turn-around part is bad, too. Take a look at [1] for the Shuttle. The interesting parts are:

* "Transfer engines to the Main Engine Processing Facility and service for future flights," and "When required, the orbital maneuvering system (OMS)/reaction control system (RCS) pods and forward RCS may be removed and taken to the Hypergol Maintenance Facility in KSC’s industrial area for maintenance." Yes, SOP involves major disassembly every time.

* "Visual inspections are made of the orbiter’s thermal protection system, selected structural elements, landing gear, and other systems to determine if they sustained any damage during the mission. Any damage to the thermal protection system must be repaired before the next mission." If!? I don't know the numbers, but after every flight, every tile was checked and a goodly number needed replacement. (They're not cheap, either.)

Is SpaceX flying man-rated yet?

Anyway, turn-around costs and not caring about re-entry make for a bit of cheapitude, too.

[1] http://www.nasa-klass.com/Curriculum/Get_Oriented%202/Space%...

[2] http://en.wikipedia.org/wiki/Criticism_of_the_Space_Shuttle_...

The Russians put stuff into orbit relatively cheaply, but not by a huge margin. Building the same rocket over and over again has worked out OK for them, but it hasn't dramatically reduced the cost of access to space.

The landing fuel for SpaceX's design is the same fuel as used to launch. A reusable Falcon 9 launch will have 30% less payload capacity than an expendable one, because of the need to save fuel for the landing. But the cost savings will be vastly more than 30%. So overall it's a big net win.

SpaceX isn't man-rated yet. They don't yet have a spacecraft that can carry people, so there's no point. That's being worked on, of course. The Dragon 2 spacecraft that will carry people includes a launch escape system, so it will be much more tolerant of launch mishaps than the Shuttle was, where the options for surviving a serious failure on launch were pretty much just "pray."

Turnaround costs and reentry shielding actually illustrate just how different SpaceX's approach is from the Shuttle's. They're only reusing the first stage for now. That means that there's no real need for a thermal protection system, so no worries with tiles. The engines are stressed a lot less, so they break less. The engines are also much less efficient (which is to say much less fragile) than the Shuttle's, and should need no refurbishment for subsequent launches. They're looking at reusing the capsules as well, but the heat shielding on those is tiny compared to what a Shuttle needed. They're not currently looking to reuse the second stage at all, but of course the rocket equation tells us that reusing the first stage is a much bigger deal. Just reusing the first stage gets you 90% of the cost savings of reusing everything.

Buran, the USSR's shuttle, was a reasonably good idea. Although it looked like the US shuttle, it was really a vehicle carried on a big booster; it had no main engines. It flew once, successfully, unmanned. The Boeing X-37 unmanned mini-shuttle is similar, and seems to work well. The USAF keeps sending one up, keeping it up for a year, and then bringing it down to land on a runway.

The Buran is really interesting. It's a pity it never got enough love. I'm not hugely familiar with it, but it does seem superficially like a better approach than the Shuttle.

It blows my mind that SpaceX’s would think that “fuel costs don’t matter” given that the company is run by Elon Musk, who when he’s not doing SpaceX is running Tesla and being Chairman of SolarCity where fuel costs, in particular the environmental fuel costs, are basically the only thing that matter. It takes the equivalent of 25,000 gallons of gas to put a 200 lb person into orbit on a SpaceX rocket (yes, I know it’s not necessarily gasoline, but other fuels that are also either fossil fuels or create from burning fossil fuels). So a person would have to drive a Tesla for 120 years to save enough fuel for one spot on a SpaceX launch into orbit. Do environmental fuel costs only matter when you can save a few gallons on a two hour car trip, but not when you use 25,000 gallons for a two our jaunt into space and back?

Constructing machinery has huge environmental costs as well. The CO2 emitted in the construction of a car rivals that emitted by driving it afterwards, for example.

Getting into orbit requires a huge amount of energy. That's just physics, and there's no way around it. Spending lots of money on extremely complicated and efficient machinery to use less fuel getting to orbit does not mean you're more environmentally friendly.

Also, numbers matter. The environmental costs of space travel are completely insignificant, while cars are choking the planet, simply because there are billions of them. A tiny efficiency improvement applied to billions of cars will dwarf a gigantic efficiency improvement applied to a few rocket launches per year.

Good point about billions of cars having more impact than a few fuel-hoggy rocket launches.

I just hope that space-tourism doesn't catch on, because while you're correct that overproducing rockets to make them slightly more efficient does not make you more environmentally friendly, burning 25,000 gallons for a couple of recreational hours off the planet just because you have way too much money does make you an environmental monster.

"If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport."

I thought this was quite though-provoking from a Drake Equation standpoint. From what I've gleaned, the earth-like planets we know of seem to be a big bigger than Earth. Perhaps if there's civilization out there, they are hampered by the misfortune of being on a planet that's practically impossible to escape from. If they find it hard to put up a Hubble Telescope, perhaps they'll just not be as likely to bother.

My other remark is to the engineering. There's a quip that you have to be an engineer to make something that only just satisfies the requirements. Plenty of people build houses without much in the way of calculation. Even cars can be built by enthusiasts without degrees. This is what makes the space stuff such amazing engineering.

> From what I've gleaned, the earth-like planets we know of seem to be a big bigger than Earth.

This is entirely a selection effect. Big planets are easier to find, that's why we have mostly found big planets.

degrees are overrated. does john carmack have a degree?

Does he build rockets?

eer...yes?

https://en.wikipedia.org/wiki/John_Carmack#Armadillo_Aerospa...

Oh. I stand corrected, should have researched that before commenting.

This exchange made me grin :)

To add some value to my comment, here's a cool video from one of armadillo's test rockets: https://www.youtube.com/watch?v=9u0qlIoSSkQ

We're still very early in the development of spaceflight. If you compare with the car industry, we're at the early industrialized phase, where large companies carefully start to develop the necessary technology. Now, 100 years later, a lot is standardized and the knowledge is so common that everybody has the basics, and people can build cars in their back yards. It's the same development that happened with airplanes.

At its core, a rocket isn't more complicated than a car. The challenges are just different. My dearest hope is that eventually rocket components will be as commoditized as car parts, so people can build and maintain their own spacecraft. I want to see rockets held together with duck tape and spit, because that's the point where spaceflight is available to everybody and gravity stops becoming a hurdle.

A quick disclaimer: I have the greatest respect for rocket engineers. They are taking the first steps, the most difficult ones, and I don't believe for a second that they their work is easy. I just believe that eventually, they'll become obsolete to the majority of spacetravel :)

You might be interested in Copenhagen Suborbitals (copsub.com): A danish group trying to reach space in a DIY fashion financed by donations. It's quite impressive what they've already achieved with not that much more than duct tape!

That is amazing, thank you so much for linking it. I did not know about them, and it does indeed seem like they have made a lot of progress. I'll be following them from now :)

as the article says, space != orbit. i don't see how you can get 8 km/s delta-v in a garage.

edit: doesn't mean it's not interesting, but orbit is so much more challenging than just space it's not funny.

It's true that orbit is more challenging, but you have no hope of getting there if you can't do a suborbital jump first. Aiming for suborbital first might see them get a lot more funding if they succeed. You need to learn to crawl before you can run.

You could still do it with nuclear propulsion. Orion, if you absolutely have to go into space today and don't mind riding a string of atomic explosions.

The common soda can, a marvel of mass production, is 94% soda and 6% can by mass. Compare that to the external tank for the Space Shuttle at 96% propellant and thus, 4% structure. The external tank, big enough inside to hold a barn dance, contains cryogenic fluids at 20 degrees above absolute zero (0 Kelvin), pressurized to 60 pounds per square inch, (for a tank this size, such pressure represents a huge amount of stored energy) and can withstand 3gs while pumping out propellant at 1.5 metric tons per second. The level of engineering knowledge behind such a device in our time is every bit as amazing and cutting-edge as the construction of the pyramids was for their time.

That's awesome, I had no idea.

> In the 1970’s, an experimental nuclear thermal rocket engine gave an energy equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of energy and hydrogen as the propellant.

That was intriguing, but didn't go into detail on why a nuclear thermal rocket hasn't been tried since. The obvious explanation is that there could be serious consequences if such a rocket exploded, spreading radioactive material, etc. And rockets tend to explode sometimes. However, it sounds like while that is certainly a concern, it is not as great of one as it might seem:

http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Risks

And indeed there is still work ongoing on such designs.

> And rockets tend to explode sometimes.

Chemical rockets -- the only ones we've ever actually used -- explode because that's what they're intended to do. The only difference between a successful rocket firing and a catastrophic rocket failure is the speed at which the explosion happens. A nuclear rocket engine has basically no risk of explosion; tearing itself apart at speed maybe, if the aerodynamics aren't done properly or there's a structural weakness. But that's about it.

Normal rocket operation is a burn, not a detonation, and there's a clear difference between those that's not just about speed.

Normally at least the first stage of the rocket will still be chemical, so the risk of explosion is still there. Fortunately the risk of spreading radioactive material appears limited regardless.

Pretty much any large rocket today has an explosive self-destruct mechanism to cover failure modes where the rocket goes in an unintended direction. So that's one reason a nuclear rocket might explode at low altitude.

Nuclear engines are not practical for lift-off. They can, however, be used for inter-planetary travel for a big enough payload.

There's a nice easy to read primer on nuclear rocketry here: http://www.lanl.gov/science/NSS/issue1_2011/story5full.shtml

I wonder if the treaty about nuclear weapons in higher atmosphere/space has something to do with it. A reactor is far from being a bomb, even if it explodes mid-air the explosion would essentially be hydrogen+oxygen (with maybe a high amount of radioactive plume, but not a proper nuclear detonation).

Nuclear thermal rockets also provide a very poor mass fraction of fuel because their propellant has very low density (liquid hydrogen) and the reactors are heavy. So their advantage is much small than it seems. And low thrust makes things even worse, they can be used only for upper stages/space tugs. In the end they were never used mainly because cost outweighed benefits. Imagine, nuclear fuel contains about millionx the energy per mass of chemical fuel, and yet exhaust velocity is only 2x higher, and effective combined ISP of the stage (velocity change vs mass fraction assuming dead weight of stage being zero, and with gravity/ballistic losses subtracted) is only about 50% higher than state of the art chemical systems, at vastly higher cost and risks involved. If i was Elon Musk i won't launch a nuclear rocket unless i had a really good liability insurance, and i was an insurer i'd say nah unless federal government backs me, and if i was uncle Sam i'd say nah, too. It simply doesn't worth it.

Also, problems with space access are mostly market size-related problems. Cheap access to space is possible, only requires a vast market to pay back the investments, which is simply not there.

More information than you require about different nuclear (and other) rocket designs.

http://www.projectrho.com/public_html/rocket/enginelist.php

People that have played different versions of Civilization will remember the Triremes.

They couldn't end a term in the ocean, only the coast. However, you could take a punt and travel over the ocean - with only the hope there was land on the other side.

Horribly expensive, but discovering new land or another civilization early could be transformative for the same.

This rocket equation is not shown, just it's consequences.

Try: https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

for the equation (ignoring gravity),

and: https://en.wikipedia.org/wiki/Delta-v_budget

for a diagram of the consequences.

Lest you despair of ever making space flight routine, a rocket is not the only way to get into orbit.

Virtually all of the needed velocity is tangent to the surface, not away from it. So you can accelerate the vehicle along the ground at least part of the way, and only then turn heavenward and burn fuel to get into orbit. With this boost you significantly reduce the amount of fuel needed.

There are many way of doing this. You can have magnetic propulsion (very futuristic and powerful), you can have a simple motor on the vehicle, powered by contact with rails on the ground (but motors have a limit of how fast they can work). You could have fuel "guns" fired into the back of the vehicle as it passes them, which would then capture the container and burn them as a normal rocket would. This has the benefit of not requiring a large rail, just fuel stations that the rocket would pass over.

These are just some basic ideas, there are many more.

Given that earth is a ball, aren't you always pointed heavenward if you are between 0-180 degrees. So you might as well start of by pointing directly heavenward, i.e 90 degrees. Plus, turning requires acceleration you cannot just capitalise on the velocity you have built up otherwise you would violate the 1st law of thermodynamics.

You don't want to turn. To orbit you need to have velocity tangent to the earth (i.e. along the ground), not directly up.

You only turn ever so slightly heavenward, mainly to avoid air resistance.

I'm still lost. Isn't being tangential along the equator less advantageous than being perpendicular at the poles. When you start tangential, as soon as you cover a distance equivalent to the radius of the earth, you are then perpendicular (maybe not directly so but there is no practical difference as far as the gravity well is concerned).

When going tangentially, you'll maintain tangentiality until you're going so fast that gravity isn't pulling you down fast enough to keep you pressed against the track you're on (because the Earth's surface curves downwards like the widdle spheroid it is). At that point, you're in low earth orbit (though you might want to fire the rockets a little bit to give yourself an orbit that doesn't intersect the planet). The whole point is that you gain your velocity while you're pushing off the earth, instead of your own propellant. That way, you don't have to carry so much propellant.

> When you start tangential, as soon as you cover a distance equivalent to the radius of the earth, you are then perpendicular (maybe not directly so but there is no practical difference as far as the gravity well is concerned).

If you were assuming the tangential take-off would continue going a straight line instead of curving in an orbit, I still don't get what you mean -- after covering one Earth radius, you'd be traveling at a 45-degree inclination relative to the surface of the planet, not perpendicularly.

I see it this way. You have to accelerate the mass to the escape velocity. You also have to achieve a net effect of being in orbit (some distance x above surface of earth). The most direct vector to that distance is perpendicular. The two combined should give you the minimal energy requirement. Any engineering (and aerodynamics) creativity cannot give you anything better.

> being in orbit (some distance x above surface of earth). The most direct vector to that distance is perpendicular.

That's not what being in orbit is. In fact that's the opposite of being in orbit. To be in orbit you need to move parallel (tangent) to the surface of the earth, not perpendicular.

The distance about the surface is entirely for air resistance, and has nothing to do with being in orbit.

i'll spare everybody else the snark here

Zero feelings involved, just analysis before numerical computation. The concepts need to make sense first.

Okay. In this scenario where you accelerate upward then turn 90 degrees and accelerate horizontally, are you talking about this being powered by a rocket or by a track?

@SamReidHughes

> ... accelerate upward then turn 90 degrees and accelerate horizontally

I am not for such a scenario at all. The point of the upward (upward only and not considering the atmosphere) acceleration to the desired location is to give the lower bound of the energy requirements. This is the baseline (baseline-1) and the rocket equation is as simple as possible.

In reality with an atmosphere and to put the object in orbit, the aerodynamics change and the energy requirement increases beyond the above baseline-1.

If you "accelerate upward then turn 90 degrees and accelerate horizontally", you can calculate an energy requirement for that and it is easy. Only two vectors involved. That should give some limit (call it baseline-2). We should expect to do better than baseline-2, how better? A calculation using the diagonal of the vectors involved in baseline-2 should give us baseline-3.

We shouldn't do better than baseline-3. Our launch designs and ingenuity should have an energy requirement between baseline-2 (this is bad, we are not thinking) and baseline-3 (this is maybe closer to ideal).

The rockets and shuttles do "pitch-over manoeuvres" to turn the straight upward acceleration into an elliptical acceleration.

* Note, I have not addressed the complications of the variations in atmospheric drag, but if it varies close to linearly along the vertical cross-sectional then how I think about it above does not change unless there is some other oversight.

I don't get what idea you're communicating here. You seem to be saying that the best option for rockets is to accelerate upwards, out of the atmosphere, gradually transitioning to accelerating horizontally (if this is what you mean by baseline-3). Well okay, that's what we do, when using rockets.

Doing that with a track would be expensive because the track would have to be built hundreds of miles high over all of its length. It would be cheaper to build most of it lower, and maybe accept that we'll have to handle the air resistance somehow. If we build a track that doesn't go out of the atmosphere, we could still use it to build up a lot of speed and then turn the rocket upwards before the thing is self-powered. If we do build a track that goes out of the atmosphere, we'd still want to get as much ground-level acceleration as we can.

Maybe it's more practical to build the track on the Moon, where there's no atmosphere.

The scenario for baseline-3 is just a conceptual tool(pythogras theorem) to establish a bound. It still doesn't get you into orbit.

What I was grappling with, is. I presumed @ars (parent) knew he was talking and in expressing my contention it would get addressed with a little bit more information in what I was missing. I now see some emphasis in his explanation and additional links.

The key is was that tangential acceleration opens up none fuel based acceleration mechanisms i.e change to the type of energy and the quantity (you accelerate less fuel to burn up the fuel).

It's escape velocity, not escape speed - and it's orbital velocity that matters here, we're not escaping entirely. If you're in the same place, travelling at the same speed, but pointed down, you're not in orbit, right? It's the same if you're pointed straight up. You need to be at an orbital altitude and travelling at the right speed in the right direction.

(well, any combination of speed, direction, and position will be an 'orbit' in some sense, in that there's a conic section you're on that you would follow if you were in freefall. But if you're sitting still over a planet then it's the degenerate ellipse that's a straight line down to the planet's core).

So in the absence of atmosphere the ideal ascent trajectory would look pretty much like a Hohmann transfer orbit: you'd accelerate horizontally until you were in orbit at surface level, and then you'd do the minimum energy transfer from that orbit to a higher orbit. In reality it's worth getting to altitude where the air is thinner before turning horizontal, but even so, the vast majority of a rocket's acceleration is horizontal, not vertical.

You can do the maths, but if you want to really understand these things, play Kerbal Space Program. Seriously.

escape speed is technically correct. you need to go fast enough, direction doesn't really matter.

Some directions may require more speed than others.

Or perhaps you are having a bad problem and will not go to space today.

> You only turn ever so slightly heavenward, mainly to avoid air resistance.

And then you turn Earthward, to avoid crashing into the Earth.

You are always pointed "up", but as long as you don’t accelerate too much, earth’s gravity keeps you on a circular trajectory. This is especially true if you fix the vehicle to the ground using e.g. rails. It would then be conceivable to accelerate to a high speed on ground-level rails (which is considered “free” as you can get energy delivered through the rails) until your speed is such that gravity alone does not hold you down any more and then unlock from the rails and fly off.

I did not do the maths.

You would have to build up a lot more speed if all you are going to do is decelerate once you "take-off". Also noteworthy, once you get to the escape velocity you would require an additional force just to keep you circular.

Staitght up is also wrong if you want a circular orbit.

You can still have a rocket on your vehicle to accelerate further after take-off, but you certainly need to accelerate less if you already at a certain velocity than if you are stationary. Since the “certain velocity” is free by assumption (i.e. supplied by ground-based electricty generators or something similar), I don’t see how you could be off worse. But I still didn’t do the maths fully.

Air friction, perhaps? There is more air in a tangent line.

You don't get to orbit by going up far enough, if you did you'd just end up falling back to earth.

You get to orbit by going fast enough, so that acceleration due to gravity acts perpendicular to your velocity and so acts to change your direction and pulls you around the earth.

What really helped me understand why you do not want to go up is the following thought experiment:

Imagine you are standing on Mt Everest (and the atmosphere has magically disappeared), and you would like to shoot a bullet once all the way around the earth, how would you have to fire it? It seems pretty intuitive that shooting it up into the air won't do the trick. If you want to shoot around the earth, you need to fire in parallel to the earths surface, with a sufficiently high muzzle velocity. And really an orbit is nothing else than a shot around the earth.

Yep, you just need the ground to fall away from the bullet as quickly as the bullet is falling towards the ground.

Turning requires acceleration, but you can convert velocity direction with reasonable efficiency by pushing on something (like rails or magnetic field in a tube or just road).

https://what-if.xkcd.com/58/

If you go straight up, you can never attain orbit.

I wonder if you could build a particle accelerator of sorts. A toroidal tube in which the vehicle is placed, and magnetically accelerated to launch velocity. Would just need some way to let the vehicle out; some kind of track switch. Such a setup would not require a large area to bring the spacecraft up to speed, and could be vacuum sealed to negate air resistance.

https://en.wikipedia.org/wiki/Launch_loop

https://en.wikipedia.org/wiki/Mass_driver

A long time ago I did the math for putting one of these on Mt Kilimanjaro, with another few thousand or so feet of height magically added. If you build up any significant speed in an evacuated tunnel, even at those very high heights you still face an incredible pressure wave as you exit.

Starting from a ground propulsion is a priori a bad idea due to friction forces in the atmosphere. An almost plausible design though, would be to have a first stage taking advantage of the atmosphere (i.e. using jet engines with oxygen from the environment).

To build one's intuition about space exploration, Kerbal Space Program is an excellent start.

It's amazing how quickly and clearly KSP makes this line from the article quite evident:

>> The giant leap for mankind is not the first step on the Moon, but in attaining Earth orbit.

I really enjoyed this. It lays out in some very accessible ways, the challenges of getting into space. The recoverability of the Falcon9 will cut its costs dramatically as it reduces the cost of the launch by several tens of millions of $. I'll mention on-orbit refueling as well since you don't need recoverability per-se if you can refuel in orbit. Then your Mars lander / Crew Module can launch with enough fuel to get to Low Earth orbit, refuel, and then head out to Mars.

Air launched (Skylon, Pegasus, Et alia) are also interesting, laser boosting (using lasers to add energy during the initial launch) would also help. As Elon points out though, a multi-gigawatt laser for boost to orbit is impractical both from a construction standpoint and a diplomatic stability standpoint.

If the quantity of water on the moon is accurate, then it should be possible to create a 'refinery' on the Moon which could more easily get material into Earth orbit. We'll see though if we can get a group established there.

I had hoped to visit the Moon at some point (I was assured by NASA in my youth that would be able to :-)) but I don't expect that to come to pass unless something amazing changes.

If you do the math, air-launch does not save much in fuel costs, while it adds a great deal of complexity to the launch system, and you do not save much in terms of reduced energy requirements. Most of the rocket fuel and oxidizer are used to increase velocity; comparatively little is used to gain (the first 30k') altitude or lost to atmospheric drag. The main benefit to air-launch is reduced range safety costs, and ease of scheduling, as the launch vehicle can be taken out over an unoccupied stretch of ocean, and range rental/lease costs are basically eliminated.

An interesting paper that is doing that math : http://mae.engr.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pd...

Why not assemble some rockets in space?

Get the pieces up there as efficiently as possible, and then assemble a more efficient rocket that's not designed to leave the gravity well.

If you skip the middle step, that's called staging.

It's really expensive to try and keep a permanent base in space.

If anyone is interested, these researchers from Russia have proposed a way to accelerate a spaceship while in flight – firing a ground-based laser up its backside

http://www.osa.org/en-us/about_osa/newsroom/news_releases/20...

Well that seems less preposterous than my own idea of using lasers to accelerate a ring of air, generating a plasma circuit that would pull the craft up magnetically. (Version 2 would have a series of plasma rings that the craft would fly through like a coil gun.)

The soda can analogy is horrifyingly informative. I wonder what kind of efficiencies could be obtained with semi-science-fiction fuels like the Orion nuclear-explosion propulsion system (which is different from a nuclear rocket) or even straight-up antimatter.

en.wikipedia.org/wiki/Project_Orion_a(nuclear_propulsion)

If you've got a poor mind for Math but want to understand rocket science on a more intuitive level, check out Kerbal Space Program. It's the most educational game I've ever played that still retains being fun.

I usually avoid video games because I feel like I'm wasting time, but I make an exception for KSP.

Fascinating read. Are there more blog-type posts like this from NASA? I'd be interested in reading more but I can't seem to find them from the main page and can't find links to these posts...

>Currently, all our human rated rocket engines use chemical reactions (combustion of a fuel and oxidizer) to produce the energy.

Yes, however, for completeness: an explanation of why we must limit designs to chemical rockets ought to include an explanation of why the dozen or so fusion projects underway around the world will all fail, i.e. let's inject some rational optimism. Note that the Apollo programme began before its tech was ready.

The fusion projects have been running for a long time with little success. Apollo was built on scaling tech that already worked (1940s rocketry could reach space although not achieve orbit)

If you consider exponential progress since 1970 or so to be "little success," you're right. Fusion has a very high threshold before it becomes useful, but we've come a long way, and we're not that far from the breakeven point now.

NASA is currently working with John Slough's company on a fusion rocket for interplanetary travel.

http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII...

http://www.washington.edu/news/2013/04/04/rocket-powered-by-...

To address my claim you would need an explanation of why that trend will continue, including for the several new projects. Btw, Apollo used a novel alloy.

Fusion devices aren't rockets and are thus, not subject to the Tyranny of the rocket equation.

Anything that pushes stuff out from a small hole in order to accelerate in the opposite direction is a rocket, by definition, and subject to the rocket equation. If a chemical reaction is used to create the energy to push the stuff out, it's a chemical rocket. If a nuclear reactor is used to heat the stuff so it's pushed out, it's a nuclear thermal rocket. If a nuclear reactor or solar panels are used to generate electricity that's used to push the stuff out, it's a nuclear electric or solar electric rocket respectively.

How about http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi... ?

The rocket equation applies to all momentum machines which carry its own "fuel", that is, the mass to eject.

Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.

is there a form of transport that is _not_ limited by the rocket equation?

Solar sails, bussard ramjets, space elevators.

Solar Sail, Laser Sail, Electric Sail, Magnetic Sail, Mini-magnetospheric plasma sail, MagBeam, Plasma Magnet Sail, Photon Drive, Bussard Ramjet, Ram-Augmented Interstellar Rocket.

Basically any spacecraft that does not carry its own propellant. Most of these have either pathetic thrust or are way beyond our current state-of-the-art.

Agreed, but to be pedantic, it applies to all momentum machines which carry its own "propellant", not "fuel".

For instance, in a NERVA rocket, the "fuel" is the uranium rods in the reactor, the "propellant" is the liquid hydrogen that is heated and shot out the exhaust bell.

The rocket equation still applies, but the propellent velocity is so high that you don't a large mass fraction.

I was addressing the statement 'Currently, all our human rated rocket engines use chemical reactions', and I agreed with it. But it's not good enough if you consider the lengthy design cycle of propulsion systems. We ought to be creating additional designs which assume that fusion sources of various sizes and masses will be available.

How much does your fusion plant weigh?

We ought to design for a range of masses.

What about firing the rocket from Jules Verne's space gun[0]? Now, I don't mean an actual gun what with the high g-forces and so, but only some machinery that gives the rocket high initial speed. Maybe a super sonic evacuated tube maglev that ends with a ramp? Put it on Mt. Everest for less air resistance.

[0] http://en.wikipedia.org/wiki/Space_gun

You still get a massive hit from the atmosphere when you exit the tunnel if you've built up any significant speed. Air pressure up there is still a third of air pressure at sea level.

So this is something I've always wondered about.

Can anyone explain the equation for getting to an earth sun Lagrange point?

Since were already on earth orbiting the sun it would seem we already have the correct orbital velocity to hang out at a Lagrange point. So we could really approach these points at any speed no? Is there potential to use less fuel than you'd need to reach an earth orbit?

By "orbital velocity", I assume you mean orbital velocity relative to the sun. The need for this clarification should be the first indication that we do not already have the necessary orbital velocity (as there is no particular reason to use the sun as the reference other than the fact that it is the more massive body). This diagram [0] shows the Earth-Sun Lagrange points. As you can see, they all have a slightly different orbital radius around the sun, and therefore will have different orbital velocities (larger radius=slow velocity). Additionally, (most) of these points have a different direction, which would means a different velocity. (However, we can change direction for free simply by orbiting, and as long as our orbit is different from Earth's we will change our direction relative to Earth as well).

The bigger issue is 'escaping' Earth's gravity well[1]. The most efficient way to do this is through an orbit. To see this, consider the effect of gravitational drag. That is to say, the speed lost due to the force of gravity. If you are in orbit, the net gravitational drag is 0 [2]. Suppose you are at point P of an orbit, travelling at speed V. If you accelerate with X delta-V at this point in the orbit, your new orbit will still contain point P, and you will pass through P with velocity (V+X), indicating that there was 0 gravitational drag. Intuitively, this is because you are accelerating perpendicular to the force of gravity.

Another way to look at this is to remember that there is no particular reason to prefer using the Sun as a reference point, in which case we can see that Lagrange points are also in Earth orbit.

However, your idea does point to a (theoretical) way to reduce fuel usage. When calculating the fuel needed to establish an orbit, we assume that Earth is the only massive body. However, we can (in theory) use the gravitational force of the sun to reduce the needed fuel to reach Earth orbit (in a way that has nothing to do with Lagrange points). However, the effect is not significant enough to be worth talking about.

[0] http://upload.wikimedia.org/wikipedia/commons/e/ee/Lagrange_... [1] Of course, by definition of Lagrange points, we are not actually escaping [2] Unless the orbit is circular, gravity will change your speed, but the acceleration/deceleration will balance out.

I found the Slingatron to be a particularly novel approach for launching small amounts of unmanned mass to LEO - I am curious to know what came of their concept.

https://www.kickstarter.com/projects/391496725/the-slingatro...

"If a vehicle is less than 10% propellant, [c]hanges to its structure are readily done without engineering analysis; you simple weld on another hunk of steel to reinforce the frame according to what your intuition might say."

This is why you don't let cabinet makers build ships.

The point is that they can actually. A cabinet maker could build a fairly large boat at least (maybe not a ship...) entirely by hand and mostly using very simple rules of thumb. It probably wouldn't perform very well but it would float and be able to sail around.

Amateurs build small boats that cross the Atlantic and even the Pacific all the time, precisely because ship building is easier than building rockets.

When I say it's easier, that doesn't mean that naval architects aren't as smart as rocket engineers, but the ratio of engineering effort to performance is much more favourable.

I read this story[1] once, although I don't remember where:

Someone somewhere needed a large number of cargo vessels built quickly (think WWII liberty ships, but those weren't made of wood), so they brought in a bunch of cabinet makers to bolster their shipwrights. It worked great, until the ships built by normal woodworkers saw significant wave action, at which time they broke up and sank. The punchline being that, on land, rigidity is the primary constraint; if you build it not to be floppy, it will be plenty strong. At sea (and especially in aerospace), strength is primary; if you build it to be rigid, it will be too heavy and if you build it to be not-heavy, it will probably fall apart.

An amateur can build a fairly large boat (although usually to plans by an actual naval architect), and a small boat can make it across an ocean, but if you're serious about schlepping things around, the design constraints for ships aren't much looser than those in aerospace.

[1] Maybe this, although that's not the cover I remember: http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/03...

This might be interesting for those that haven't come across it yet http://en.wikipedia.org/wiki/Space_elevator

"If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport."

Don't tell hollywood.

what about the feasibility of injecting energy on the ground (say a maglev rail based launch) such input wouldnt be contributing to the rocket's weight and therefore would reduce the needed fuel consumption quite a bit I would imagine.

just a noob question: why transport all the fuel, and not start from a cannon/catapult, something that make you start with some speed?

edit: it's not clear, I was talking about partially using a cannon, not about a complete ballistic launch.

The moon is a harsh mistress. So it the amount of G the human and most cargo can withstand.

If you were in a vacuum, that could be a wonderfully efficient way of converting mechanical movement into kinetic energy. We have to deal with atmospheric friction, though, and the places where the cannon would launch from (the surface) has the thickest part of the atmosphere, so the most friction.

what about vacuuming the cannon? like those ping pong ball cannons?

What happens when you leave the cannon? Either the cannon exit is near ground level, in which case you still have the air resistance problems; or it's not, in which case you have the problems of building very a tall cannon.

you don't have to exit the cannon at high speed, the v^2 drag will kill you. You're just trying to put some kinetic energy in the rocket in a way that doesn't make you carry it. you don't have to put all the energy in it that way, the current system works, it's just a tentative improvement.

But your answer makes me feel you've not seen the ping pong ball cannons, because they have very good results with a very low mass.

You would also have to vacuum all the space between the cannon and space, and at that point you have a space elevator you can just climb up.

That's why some people have thought about sending a fuel factory to the Moon, refining the fuel there, and using mass drivers to send it to Earth orbit for serious interplanetary missions.

How hard would be building small size space fouintain model?

the rocket equation with fusion engine expelling alpha-particles at 0.05c doesn't look that bad, and at least it is good enough for Solar system colonization and for sending a probe to the closest star.

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