The Tyranny of the Rocket Equation (2012)
nasa.gov
nasa.gov
built on the success of a rocket-based program! no way would a country fund a vehicle that spews out a constant stream of nuclear bombs as their species' first foray into orbit!
That's not how the NERVA was supposed to work.
[0] https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propu...
Beamed propulsion throws out the rocket equation altogether, as do light sails.
Not an issue for flybys, but an issue if you want to get into an orbit.
False. Robert Forward has a technique for deceleration for interstellar lightsail craft, which is described in Rocheworld and one of his science books. Magsails using beamed particle propulsion can also exploit the interstellar medium to decelerate without onboard fuel.
Not an issue for flybys, but an issue if you want to get into an orbit.
Carrying a small rocket for a circularization burn is vastly cheaper than a 1st stage. This is covered in Jordin Kare's modular laser launch proposal, and this technique was also considered for Bull's light gas space gun. Those can produce launch economics approaching those of space elevators.
Sorry, but you're citing a science fiction author as proposing a credible method, yet don't even state what that method is.
Yes, I know Robert Forward used to work in aerospace, but his fiction writings are... fiction. He's been out of the industry since 2002 and a hand-wavy "he has a technique" doesn't strike me as being particularly credible.
(I was thinking of free-space skyhooks, exchanging momentum between inbound and outbound trips, but the ones for planets are also cool. Just harder when the planet is heavy like Earth and Venus.)
Chemical rockets are also much more efficient devices than is given credit for. A good, very optimized two stage rocket like SpaceX BFR (and the ITS concept before it) can get payloads to orbit using just ~300 Megajoules per kg (250MJ/kg in the more efficient tanker configuration). The specific kinetic energy of orbit is about 25MJ/kg (40MJ/kg if we include gravity and aero losses and changes in altitude), so rockets are total 10% efficient at converting chemical energy directly into orbital energy, and with an optimized, high pressure and variable mixture ratio hydrogen rocket engine architecture with some improvements in materials science, we could improve that to 20% or more. That beats the chemical to mechanical efficiency of the typical car.
That translates to between $1 and $10 per kg to orbit. That's 3 orders of magnitude less cost than today, if we can master reuse. So arguably, chemical rockets are perfectly fine for achieving orbit cheaply.
However, rramatically improving rockets might be like trying to improve the carriage to get a much faster personal transport, while keeping the horse.
It seems to be the case, that the cost of launching commercial payloads are still "low" enough compared to the cost of the payloads since there are few big budget ventures that tries to make lifting much cheaper through alternative methods.
I'm not saying that space elevators or other alternative lift devices are feasible, but we tend to get huge investments even into tech that seems absolutely implausible if there is a big market. The lack of these makes investments makes it plausible that the current launch costs are actually acceptable for all current, and expected near future commercial actors.
Space elevators, skyhooks, and launch loops are all big enough that it's an all-or-nothing deal. You can test individual components, but a whole end-to-end space elevator has to be exactly the distance between ground and geosync or else it just doesn't work. At best, you could build smaller versions of this stuff on the moon first and then scale up to Earth-size later, taking into account such factors as water, atmosphere, and the fact that the Earth is not tidally locked with the sun. But that requires hauling metric fucktons of parts and equipment to the moon, even if you're using lunar resources to build the space elevator itself, so there's no guarantee you end up ahead in the end.
The basic physics behind rockets is simple as all hell, and it still required von Braun to spend his entire adult life building progressively bigger and bigger rockets and refining the design details. The V2 only needed to be able to land nose-first 200 miles from where it was launched, but without that knowledge and experience, the Saturn V would have been impossible. That's the kind of iteration that's fundamentally impossible with these other launch systems.
Nuclear has amazing power ratios but detonating a nuclear warhead to get moving is somewhat of a problem when you're close to earth.
Fission Heat Engines are quite good but not good enough for takeoff. Ion engines suffer the same fate.
Almost no alternative propulsion method can replace takeoff boosters, thusly limiting the mass you can put into space. With limited mass you can only make engines that big. Or bigger if you assemble them in space which opens other cans of worms.
It's not really a 'near earth' issue. Fallout concerns can probably be mitigated (and Freeman Dyson came up with a few ideas in that direction). Research in this direction, however, got killed as part of the various test ban treaties.
From the perspective of trying to reduce any country's desire for having a nuclear bomb program, Project Orion, and other nuclear solutions, is a no go. So even if you were to promise "We'll only start the nuclear component when we hit the moon" the proliferation concerns would still kill NASA's interest in pursuing it.
So yeah; it's not really a technical issue at it's core (though you definitely have technical issues to solve), but rather a political one (which certainly doesn't invalidate it).
There’s a lot of reason why they haven’t been used yet, but I think most the compelling argument against them is that they won’t be used until fuel costs become the competitive factor in launch prices.
https://space.stackexchange.com/questions/3004/why-arent-lin...
Nuclear rockets are quite the concept but have their obvious limitations. This video is worth checking out if you haven’t seen it yet.
But maybe there won't be one dramatic change that makes rocket travel cheaper. Maybe we keep iterating on rocket and rocket engine designs and eventually looking back it seems revolutionary. As composites get better and better more and more of rockets will be made from composites instead of the current material of choice, lithium aluminum alloy. Plus the increased use of oxygen rich combustion[3] with it's improved efficiency. Also it looks like Ch4/LOX might be more common in the future[4][5][6], which is good because Ch4/LOX has a higher ISP then RP-1 and also is significantly more dense then H2 propellant. There's also so called "slush" or "super chilled" propellants[7], Spacex has even been using it for awhile[8]. So maybe the future will be extremely light, reusable, oxygen rich, staged combustion, methane burning rockets with super chilled propellants?
[1]: http://www.astronautix.com/g/gnom.html
[2]: http://www.astronautix.com/p/pr-90.html
[3]: http://www.americaspace.com/2015/05/29/new-oxygen-preburner-...
[4]: https://spaceflightnow.com/2017/10/20/worlds-largest-methane...
[5]: https://en.wikipedia.org/wiki/Raptor_(rocket_engine_family)
[6]: http://www.russianspaceweb.com/soyuz5-lv-ptk.html
[7]: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197900...
[8]: http://www.businessinsider.com/spacex-deep-cryogenic-cryo-li...
Does Skylon fit the bill?
https://en.wikipedia.org/wiki/Skylon_(spacecraft)
It pushes the limits of materials and design. But I think a reusable 2-stage rocket—as boring as it sounds—will end up working better in the near term. It can be done with safe margins.
The challenge is to get from the Earth's surface, to orbit. Yes, there's oxygen along the way, but the speeds you have to go will wreck you if you stay in the atmosphere at all. So you get into vacuum ASAP.
Once you're in orbit, you have a lot more options. For example, you don't need a high-thrust engine—instead, high-efficiency is fine, even if it's low thrust. Etc.
Beam microwave or laser energy to ship
Launch loop
Sky hook
Launch fountain
Some company is even working on a launch slingshot.
I’d provide Wikipedia links but I’m on my palm pilot. Look them up though. Interesting stuff.
This is selling rocket science a little short, I think. The availabilty of gravity assists and aerobraking make it possible to achieve large variations in delta V necessary for a trip, as long as you can make the right tradeoffs elsewhere (travel time, etc).
See: https://en.wikipedia.org/wiki/Interplanetary_Transport_Netwo...
The problem is the same for all of the ideas like space-elevators, or asteroid mining, or spacetravel as a normal thing - given our current politics, there's no way you'd avoid occasional rock-droppage with catastrophic consequences for people still stuck on earth. It would just be too easy. If you start thinking along the lines towards nuclear-fueled rockets, the problem just gets worse.
Non-weapon rockets are high-energy, fiddly devices - and their dumb purely-destructive siblings are far simpler.
If launching rockets became cheap enough to put people on Mars, that would also mean they'd be cheap enough so even small states could afford ICBMs. Because rockets are ICBMs.
Lots of modern technologies do that. Like a child growing up—at what age do you use sharp knives? a lawnmower? a car?—humanity has matured ethically alongside our technology.
I agree we have to be careful not to Great Filter ourselves.
That's exactly the problem. I don't think we have at all. We do no serious study of ethics in our schooling, and frankly don't devote serious thought to the subject. It's very uncommon to meet somebody who has read even a single book about ethics. We're a society of meat-eaters who consider people who harm animals to be scum, a society that simultaneously considers the protection of children fundamental, and where the most likely age to die on a world scale is in childhood. These are reflections of a deep lack of ethical maturity.
Regardless of all of the inconsistency, I think it's hard to look at the times in which the world came to the brink of nuclear catastrophe in the cold war, and state confidently that this is a society with the maturity to hold nuclear weapons.
Even more intuitively: it's about distance. Flying from Los Angeles to Salt Lake City requires less fuel than flying from Los Angeles to New York City. Flying from Los Angeles all the way around the world and landing again in Los Angeles takes even more fuel.
I've always felt uneasy about this, as it seems to imply that the only way to put things in orbit is via rockets.
For example, there is the idea of a space crane (assuming it can be built) that is simply a very, very tall building, assembled one chunk at a time, and that once built can simply lift things in orbit.
Would a space crane be slave to the same equations?
The tallest structure ever built by humans is a little more than half a mile tall.
A space elevator to geostationary orbit (required for a circular orbit with no rockets) would have to be 22,000 miles tall.
Low earth orbit is a lot closer, what is considered LEO is pretty wide, but the ISS is at about 250 miles.
If you get an crane ride up 250 miles, you aren't orbiting. You haven't even done half of the work, to be in orbit at that altitude means you have to accelerate to several miles per second.
How orbital mechanics works is not very intuitive.
We don't really know how to reliably manufacture materials that can sustain the loads required for something like this.
"doesn't the hypothetical space elevator break the tyranny of the rocket equation?" (which you did answer).
Something more feasible would be a Skyhook[1] which only provides part of the energy needed to get to space but which due to the exponential nature of the rocket equation could make a huge difference while being conceivable to build.
But again, this is still a materials science problem.
No it would not, it has its own cross to bear, tensile strength.
If you start from the ground, then as the crane gets taller, the forces on its outer edges are the torque applied when a lateral force pushes against it (think wind). As with a stubborn nut, the longer the wrench handle the more force you can apply to the nut. If you think of the crane as your wrench handle, the taller it gets, the more force it will exert on its mounts when air moves past it. (this it true of sky scrapers as well).
Eventually you move past the point where there is anything that can stay vertical (steels rip and bend, carbon fiber breaks, Etc.) There is a working solution, a pyramid, but to build a pyramid that reached low earth orbit requires a base that is larger than the state of Kansas and will sink down to bedrock. There is an entertaining discussion of this from 2011 here: https://www.wired.com/2011/07/does-the-slope-of-a-pyramid-re... We know from the Himalayas you can stack up a lot of rock into a really really tall mountain, but its really really a lot. This might occur naturally on some planet, Olympus Mons on Mars is 16 miles high (about 85,000 ft or almost 26 km) that is 25% of the way up to space (assuming the 100km is your line for 'space'). Eventually you hit the limit which is that gravity prevents you from having a feature stick out too far from a sphere.
Another way to do this is to put a weight in geosynchronous orbit so it stays over the same spot all the time. If you start building a cable downward and adding mass further out you can keep the "effective" mass at the geosyncronous point. Eventually, (materials science not withstanding) the part you are lowering down to the planet reaches the surface and voila you have your jack and the beanstalk type beanstalk.
As fantastical as that sounds, it should actually work, if, and it is a big if, you can make a material that can withstand the tensile force of being both pulled up from the counter weight, and down by gravity. Once built you could attach a vehicle that would ride up and down the cable moving things from ground level into geosynchronous orbit, with only the energy needed to climb up the cable. These "space elevators" are not precluded by physics so they show up in science fiction stories.
But once you get away from building structures you are left with either chemical rockets or some how pushing against the Earth. Creative ideas there are balloons that take you part of the way up, and electrostatic systems that use an electric field to push against the charge on the Earth. These ideas start off far fetched and move right into crazysauce fairly quickly.
Also, the economic implications of the equation don't matter so much when you have a reusable rocket, and the BFR is supposed to be able to reuse both stages.
NASA source is from 2012, before BFR was fleshed out.
I like space, lots of neat things to learn, but the discussion always seems to get a little religious and doomsday-ish, and I find it less than genuine. Like have people thought about where the energy to get massive numbers of people off the planet will come from, and how that will leave the planet in even worse shape for those that can't leave?
It is folly to think technology can fix everything, and I sometimes wonder if it has actually "fixed" anything, aside from helping to enable overpopulation.
A billion people is a PWh. That's 0.5% of the earth's annual energy use. It's a blip on the radar, especially since it will subsequently reduce annual consumption in the 10% range.
plus where are you going to send %10 of the population where they will be immune to human nature, whatever that is.