Thruster for Mars mission breaks records | Hacker News Reader

Thruster for Mars mission breaks records

ns.umich.edu

141 pointsskolos119 comments

The hole of Space exploration is really hindered by the approach to nuclear power. Many missions that could fly with nuclear batteries don't use them. Rosetta would have been a far better mission with one.

On a larger scale we should have nuclear reactors in space. If we ever want a mars colony, nuclear is way, way better then solar. Elon Musk is gone try with solar but he really doesn't have any other options.

NASA is the one place there can actually be progress on this stuff, no private company can reasonable attempted this stuff because of regulation. We need more nuclear batteries, we need small reactors for space and we need nuclear thermal rockets engines.

One problem is that NASA is running out of plutonium [1] as the Pu-238 used to this point was largely a byproduct of the weapons program and not a lot of nuclear warheads have been built lately.

[1]: https://www.popsci.com/new-technique-could-double-plutonium-...

While this is true it is just another reflection of the backwards nuclear policy of NASA and the US government.

With different reactors, creation of more Pu-238 would not be a huge problem.

The issue is just that there seems to be very little interest in pushing forward any areas and as so often the case many of these technologies are mutually supporting.

I would really love a real push forward on Molten Salt reactors that would give you the ability to filter out a number of important substances that have a hole host of different uses. It would also go a long way in solving the nuclear waste problems.

Here is some exploration of what is contained in nuclear waste:

https://www.youtube.com/watch?v=rv-mFSoZOkE

There's a lot of that in Cumbria... about 120 Tonnes.

Spent nuclear fuel from reactors doesn't have usable quantities of the right plutonium isotope. RTGs use the uncommon Pu-238. Plutonium in spent nuclear fuel is mostly Pu-239, 240, 241, and 242.

Convert the warheads into batteries?

Warheads have 239, not 238.

Ah, I should have known that. Thanks

MIT astronautics faculty did a study which compares 13 different power generation methods for use on Mars [0].

They determined that: "For latitudes close to this optimum latitude (31" north), photo- voltaic power systems exhibit mass- and volume- based performance that is comparable to that of nuclear fission power systems. For global access for human Mars surface missions, nuclear power is required either in the form of fission reactors or radioisotope generators."

...further noting: "...solar-based Mars surface power systems should be seriously considered as an alternative to nuclear fission surface power."

The way I read the study, if you're happy with staying around the "near-equatorial north", you should use PV solar... if you want to site your colony somewhere else, you should use nuclear power.

[0]: http://systemarchitect.mit.edu/docs/cooper10.pdf

It's not so easy to make space nuclear power light and reliable. Most likely a nuclear powered spacecraft with electric thrusters would be a dog.

Nuclear thermal rockets aren't that great either, the temperature and thus the specific impulse is not very high, and they have high dry mass and would be really expensive to develop and hard to make reliable.

In space you have 1.3 kW per square meter sun power constantly without the atmosphere or clouds in the way, and solar cells improve constantly.

Nuclear thermal rockets have low specific impulse compared to electrical propulsion, but better ISP than any chemical rockets. They also provide orders of magnitude more thrust than electrical propulsion does. I don't think that they are in the sweet spot for uncrewed probe missions, but the combination of higher-than-chemical ISP and higher-than-electrical thrust seems like a good match for crewed missions to Mars or beyond.

Regarding the many comments in this discussion about accidents during initial launch: if nuclear thermal rockets are fueled with uranium 235, and don't reach criticality until after the risky chemical-booster ascent out of atmosphere, they should be quite safe. Uranium 235 has a 700 million year half life, hence very low radioactivity. The fission products from a live reactor of course include very-short-lived, very-high-radioactivity nuclides. But you can have nuclear power in space with extremely low risks to the terrestrial environment if you use it only for missions that leave Earth orbit, and turn on the reactor only after the risky initial ascent on chemical boosters.

That said, I agree that it would be expensive to develop nuclear thermal rockets. It's very expensive to test them on Earth, since the exhaust carries away some small amount of fission products and people don't just tolerate such radioactive pollution like back in the 1960s. You'd need some very expensive/elaborate test facility that can capture the hot, radioactive exhaust, or you'd need to do all the actual testing in space (also expensive/elaborate).

Let's say you have a velocity requirement of 6 km/s.

A chemical hydrogen-oxygen rocket might have exhaust velocity of 4.5 km/s so mass ratio is exp(6/4.5) = 3.8.

A nuclear rocket with exhaust velocity 9 km/s gets a mass ratio 1.9.

Most likely the nuclear rocket uses hydrogen only, meaning the tank is likely bigger than the chemical rocket's! The thrust to weight ratio of a normalish solid nuclear engine is about ten times worse than a chemical one.

You do save on the oxygen mass, but that's it. It's probably bigger, heavier, more complex and definitely more expensive and dangerous than an ordinary chemical rocket.

For higher velocities, it starts making more sense...

Any regulations that would prevent the development of nuclear rocketry on Mars, after a base was established?

None at present. The 1967 Outer Space Treaty prohibits nuclear weapons in space, but does not restrict peaceful uses of nuclear energy. A Mars base that is developed enough not only to support permanent human habitation but to conduct nuclear rocketry R&D is so far from where we are now that I couldn't speculate about what sort of laws might then apply on Mars, though.

At Earth orbit, yes. Recall that the inverse square law applies for light emitted by the sun.

On Mars, solar power per square meter is a bit over 550W.

And you need about 200 sqm of solar panels to fully power each engine at Martian orbit, double that for the asteroid belt, double again for Jupiter.

Yes, and that's still pretty high.

We use solar cells at Jupiter distances nowadays (various reasons for that though).

That number sounds high. I make it about 300W for Earth.

http://heliosphan.org/solarenergy.html

1.4 KW is right: https://en.wikipedia.org/wiki/Solar_irradiance#Earth

It's an unusual methodology to begin by looking up the power output of the sun, which is worked out using the reverse of your calculation. Might you have erroneously divided out the entire of Earth's surface area (which is 4 times the area of a circle the size of the Earth)?

Yeh I forgot that's what I did, it was a while ago now, and I admit the average over entire surface area isn't a very useful number :)

Don't sweat it, it's a common mistake. IIRC some French physicists used the same surface area calculation to try to disprove global warming, for which they got the nickname "knights of the flat earth".

I should have clarified - this is raw irradiance, not the power you'd see from a photovoltaic cell.

Ahh, I forgot that my calc was based on an average for the Earth's surface - so including high latitudes and day/night. On that basis I figure our numbers broadly agree.

It's hard to miniaturize nuclear power. But if you're going to mars, you don't want miniature, you want economies of scale (both for descent/landing and for cruise). And this is what nuclear power provides in spades.

It's hard to do any kind of nuclear reactor in space. You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge radiators. They alone weigh a lot. You must pipe coolant through them. What about micrometeoroids? You probably have to have valves to shut off leaking radiators.

You don't have gravity so you don't have convection or separation of gas and liquid. Hard to test such systems on Earth.

You can't make repairs easily.

Solar cells start to look really good really fast.

Maybe on Mars surface they could make some sense, or on the Moon where the night lasts for two weeks. Though on the Moon one would preferably settle near the poles without such problems.

But even on the surface, just regular batteries look really good on the simplicity front, and fuel cells beyond that.

Internal combustion engines once look really complicated compared to horses too -- so many new problems to solve! -- but it was a forgone conclusion that cars would replace horses once the fundamental energy source could be harnesses.

That nuclear powered space travel is more economical and efficient at large scales isn't disputed by experts. The barriers reaching that scale are funding and political, not the engineering ones you've highlighted.

A more apt comparison might be coal fired steam. It was very unwieldy but replaced horses in some ways. Railroads and steam ships changed the world but still the technology was too heavy for personal mobility.

If you can invent any kind of technology at will, sure, it can be great. Then let's have light and reliable space nuclear power plants. At the moment we are very far from that.

It might have looked like a good choice forty years ago, but solar cells have improved a lot, as well as batteries.

We do have some newish things on the nuclear side that could help, like gas cooled reactors or supercritical power cycles. You need to get the temperature up (to make the radiators small) and the power density too.

Nuclear rockets don't look much like the reactors used for power generation.

We are much closer to the tech for efficient nuclear rockets than we are to the tech for an (even partially) self-sustaining colony on Mars.

> You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge radiators.

This is what I thought. To my (admittedly fairly uninformed) knowledge, nuclear power is really nuclear steam engines, at least the ones I know of. As awesome as a space steam engine sounds, it doesn't seem too feasible for the reasons you presented.

Your intuition doesn't even agree with 1970s technology. Radioisotope thermoelectric generators developed then are still more efficient than modern battery power sources. They don't use steam and they have no moving parts.

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

They are actually extremely inefficient since they use a thermocouple. They only convert about 5% of the thermal energy to electricity, the other 95% of the heat must be rejected somewhere.

A steam turbine nuclear reactor is a Rankine cycle heat engine, their efficiency is above 40%.

The reason RTG are used is simplicity and no moving parts, but efficiency greatly suffers. If we could figure out more efficient thermocouple all sorts of opportunities open up, haven't seen much work in this area.

The efficiency I'm talking about here is power and thrust per weight, not absolute (percentage) efficiency in the thermodynamic sense. (Yes, the latter is important for worrying about heat dissipation, but the former is the key physical limitation for interplanetary travel.)

Does that scale to the level required here? Also, while it's not a steam engine, it is an engine based on heat transfer, so it's not entirely different, and a lot of the same problems apply. So, while my assumption doesn't hold, the requirements imposed by it may (and may be worse, RTGs can't vary their heat output in any way). Do we have numbers on how much heat a current fairly standard RTG generates (for what appears to be hundreds of Watts), and how hard that is to radiate in space?

Thermoelectric generators are at the moment not useful for primary propulsion use as the amount of energy generated is so small.

Though nowadays you could maybe improve them with advanced quantum technology, like long wavelength solar cells. The cell itself still has to be cooler than the source, so you still need radiators... https://en.wikipedia.org/wiki/Thermophotovoltaic

It only needs to be lighter then the massive amounts of solar panels that would be needed on mars.

People have done the calculation and nuclear would be way, way better then solar for a mars colony.

So the next step is to figure out how to build the power plant in space instead of lugging all that material up out of the Earth's gravity well.

"...1.3 kW per square meter sun power constantly..." at Earth's distance from the sun. At Mars it is .715 kW. Inverse square law and all that.

New plan! Colonize Venus!

(What's all this about cloud cover?)

Airship colonies, take advantage of that dense atmosphere and float over the clouds.

If you go high enough in Venus's atmosphere that the temperature and pressure are Earth-like you're getting 1.3 times the sunlight of Earth coming in from above and about 1.0 times reflected off the clouds below.

I think it's more like ~0.6kW/m^2 out at Mars

I think your 1.3 kW number assumes being near Earth.

The first time I was exposed to the concept of space-based industry was playing PC game Privateer as a kid. You land on these giant asteroids with attached mining bases... to transport ore to planets and refineries.

It never made sense to me. Why setup all of this massive capital investment for mere ore?

I think one of the very first space industries is going to be automated mining, refining, and production of nuclear fuel. Once freed from the widespread and totally irrational fear of nuclear technology here on Earth, the magnitude of advantage of nuclear power is stunning. Before the regulatory regime killed nuclear investment, it was well on its way to becoming cheaper than coal.

Cheap, abundant energy solves virtually every human problem, whether on earth or in space. A stable supply of space based nuclear fuel will lead to exponential growth in space based activity and greatly simplify colonization of our solar system... and beyond.

> Once freed from the widespread and totally irrational fear of nuclear technology here on Earth

Look, I'm pro nuclear, but I"m also really tired of seeing this straw man. Responsible management of nuclear power requires institutions that can act responsibly to manage risk on time scales of the order of centuries. Recent history shows that many of our institutions fall short of what's really needed to do that. An event like Fukushima was a statistical certainty, and warnings prior to the event that warned of exactly what would happen were ignored.

I don't think the answer to this is to turn our back on the potential of nuclear power. I do think we should be unflinchingly real about how hard and dangerous it is.

Dangerous compared to what? What is your standard of measure for danger? What are the alternatives to provide vital energy for human flourishing?

What was the death toll from Fukushima? Zero. Everything at Fukushima went wrong, and yet only 8 out of 2400 workers were exposed to out-of-tolerate levels of radiation. None of the plant's neighbors got sick, and no discernible increase in the rate of cancer deaths is expected. Radiation is everywhere in the real world, it is a natural part of life, yet we treat it like any amount is a lethal danger, when in low doses it is not a meaningful threat.

Fukushima was a non-event, and yet it caused a movement to shut down all nuclear power. Fear of nuclear is totally irrational. The more we embrace nuclear power, the greater its benefits to human well being.

Nuclear is safer than every other form of large scale energy generation ever devised. The only concern with nuclear plants is a large release of radiation, which is unlikely due to the layers of shielding and containment. The scaremonger's goto example of chernobyl lacked such safety measures because the Soviets didn't care about human life.

Also worth reading is The Health Hazards of NOT Going Nuclear: https://www.amazon.com/Health-Hazards-Not-Going-Nuclear/dp/0...

I am very familiar with the details of Fukushima, the prevalence of natural radiation, and the health risks of alternatives.

I again believe it is entirely dishonest to call fear of nuclear power irrational.

Was it a mistake for Germany and Japan to shift generation away from nuclear? Yes IMO. But it's absurd to call an event that required mass evacuation harmless.

Fear is not binary. Yes of course fear of nuclear power is rational, but so if fear of cows, cars and a lot of other stuff.

The amount of fear that exists is competently out of proportion to the real problem.

Is there any uranium to be had in space?

Everything in the solar system is pretty much made out of the same stuff. There is uranium on Earth, the Moon, and Mars, so we can reasonably expect it to be present on other less massive objects in the solar system.

But you also have to consider that materials stratify based on their density and chemical affinity. The Goldschmidt classification of an element defines sort of chemical cliques for the elements.

Lithophile elements like to oxidize into minerals and float on top of any dense metal core. For instance, the dominant aluminum ore bauxite is frequently found close to the surface of the earth. To find those elements in space, you look for a rock-dominant (oxidized) asteroid.

Siderophile elements like to form solid solutions with iron, and may not oxidize to lighter minerals readily. Elements like gold and platinum are considered rare on Earth not because the planet does not have a great quantity of them, but because the vast majority of that mass is way down in the iron core, where we cannot yet reach it. The mass near the surface is only there due to quirks of geology. To find those elements in space, you look for a metals-dominant asteroid.

Chalcophile elements like sulfur, and on Earth their rarity (in terms of the amount we can dig up) is between lithophiles and siderophiles, because they tend to stratify above the iron-lovers and below the oxygen-lovers.

The only elements that may be difficult to find would be those that form volatile molecules, because they typically get gradually blown away by solar wind, or stripped away all at once by a catastrophic cosmic event.

Uranium and thorium are lithophile elements, so they shouldn't be too difficult to find on rocky, oxygen-heavy asteroids. Just look for low albedo and alpha particles from pitchblende. But beware that absent the ore-concentrating processes of large planets, you may have to process a lot of rock to get a usable quantity of fissile elements.

I know very little about this.

From what I've read, it is very unlikely that uranium ores will be found on asteroids, because concentrating uranium into economically mine-able quantities is believed to require long running thermal process in a planets core.

That would mean looking for uranium on larger bodies, like the moon.

https://www.space.com/6904-uranium-moon.html

I'd wager that uranium (as with all the heavier elements) is spread throughout the solar system as it is forged and spread in stars going supernovae/hypernovae

I freaking LOVED that game. Serious time suck though.

Isn't cooling a big problem for reactors in space? The ISS has massive radiators just for all the power solar panels put out.

Yes. I forget what specifically the energy gradient per square meter for radiators is, but it was surprisingly low. A third of a kilowatt comes to mind though not with any strong certainty.

Calculations here suggest 20 kW/m^2:

http://www.nss.org/settlement/nasa/spaceresvol2/thermalmanag...

One problem with nuclear reactors on space craft is what happens if/when the craft re-enters the atomosphere? How do you stop the reactor from becoming a high-altitude dirty bomb?

Yeah, we should probably not do this in LEO, where orbits decay and half of the potential acceleration vectors lead back down to Earth. Fortunately, it takes a lot of energy and precision to de-orbit anything heavy that is in geosynchronous orbit or beyond.

If the fissile material could be acquired by asteroid mining, then it would never need to pass through the Earth's atmosphere. I know that still sounds way out there, but I think that uranium or thorium would actually be a great material for kicking off asteroid mining. A small amount will pay off in a big way, it is a very dangerous thing for humans to mine on Earth, and, like the parent mentioned, we really don't want to fly it around where it can fall on us.

No rocket has had 100% reliability. And you need to be pretty close to that if you want to make sure your nuclear-powered rocket won't explode either at take-off or when landing.

Even if rockets were as reliable as airplanes, it still wouldn't be good enough. We've had several major plane crashes just in the past decade. Now imagine if each of those made a nuclear explosion. Do you think that would be acceptable?

We don't have fission-powered rockets because the math works against them, not because of humans' "irrational fear of nuclear power."

There is very little point in using a fission rocket low in the atmosphere (wouldn't play to it's strengths, would not be much better than chemical rockets), but there is no problem in lifting fission rocket engines to orbit, because a fission reactor made out of U-235 is not significantly radioactive before it's turned on. So you can ship it into orbit in some configuration where it cannot turn on by accident (half of fuel removed and in storage or on a different rocket), and then only turn it on when you are in a safe orbit pointing away from earth.

Nuclear thermal rocket would be best for upper stages or mars-earth transfer vehicles.

Nuclear power to use on ion thrusts would only be used in Space.

what would’ve happened if challenger was carrying nuclear material? (i’m actually asking)

It would depend a great deal on what it was and how it was packaged. This seems to have some good discussion:

https://space.stackexchange.com/questions/16608/how-does-the...

The Earth would be immeasurably more radioactive than today.

The Earth would in fact be the same radioactive as today, assuming we had used stockpiled Pu.

It's the distribution that matters. A little bit of ocean floor over Florida would be much more radioactive than today if it was scattered with Pu-238.

Usually immeasurably means infinite, I'm guessing that's not what you meant?

Immeasurably means "incapable of being measured", which could refer to too small to measure or too large to measure, because English is fun like that. It does generally refer to the latter, though.

Doing the math on this 5.4 netwons in 108 kilowatts would give you a maximum exhaust velocity of 38 km/s which is in line with what you expect from a Hall effect thruster. You can get much higher exhaust velocities (and thus more fuel efficient engines) from gridded ion engine but that requires more power for a given thrust and the wear on the grid is a concern.

I'm not really sure why it's important that we have big hall thrusters, though? My impression was that you could just keep adding more of these to a spacecraft if you needed to.

It seems like the bigger ones are more efficient. Looking at the design I think they nested several hall effect thrusters which could improve focus or something.

Edit: http://pepl.engin.umich.edu/projects/NCHETs.html

This is a tradeoff that I don't think is widely enough understood. The faster the rocket's exhaust goes the less fuel and more energy you need to reach a given speed. This only makes sense if you have access to a lot of energy, and are concerned with conserving fuel.

102kW? How do they plan to provide power for that on a spaceship?

For Mars probably solar. The ISS's panels provide that much. Any time you're looking at electric propulsion your power plant is going to be a big fraction of your vehicle weight.

The current prototype weighs 500 pounds (226 kg) so it could just accelerate itself at 1/400 g. After 100 hours it could be moving at 8,500 m/s but that's not including cargo or solar panels.

The Voyagers do around twice that at the moment.

So, (very much ballpark, and ignoring all additional weight for fuel, etc.), would that mean this could reach three times their speed in a month, and, after that, catch up with one of them in 15 years or so?

If so, is it feasible to build one with enough fuel to accelerate for a year or so?

If so, where’s the billionaire willing to spend a few quid on a machine that goes out to photograph one of the Voyagers from close up while it zooms by, or even bring one of them back to earth before the century is over?

So, getting up to 25.5 km/s with a rocket with an exhaust velocity of 30 km/s means you need the wet weight to be 2.33 times the dry weight. Last I checked [1] you can get 77 W/kg with photovoltaics so you need you'll need 1402 kg of those in addition to the 226 kg of engine for a total of 1626 kg. Round that up to 2200 kg for structure and tankage and so on for a total we mass of 5126 kg where 2926 kg of that is fuel. You're using that at 5.4/30000=.00018 kg/s for a total of 16255555 seconds of acceleration or about six months. So six months instead of one but basically yes.

[1]http://hopefullyintersting.blogspot.com/2015/03/rockets-elec...

EDIT: Of course, as it accelerates it'll get further away from the sun leading to less power from the solar panels and a slower acceleration.

Xenon is apparently pretty expensive, like $1,200/kg. Just running it for 100 hours at 0.00018 kg/s will cost $76,800 for the propellant. Plus 10,200 kWh of electricity!

That's cheap! The launch'll set you back about $100m so another $3.5m for propellant isn't going to be worth worrying about. At some point we'll be wanting to start finding more fuel for our rockets at our destinations. There are ways we know how to do this for oxygen, hydrogen, and methane but getting more xenon on Mars or Titan just isn't doable. So at that point I expect people to switch over to Argon which doesn't ionize quite as easily but can be found all over the solar system.

Or we could set up a Xenon distillery at Jupiter, where it's more abundant :)

I don't envy whoever would have to haul it up that gravity well.

For a deep space probe that fuel cost is not a showstopper. That's well down in the weeds compared to the cost of launching it to orbit and manning the tracking stations and buying airtime on the Deep Space Network.

Before doing more estimates, it's probably worth adding in the mass of a plausible payload i.e. the scientific instruments that are the purpose of the spacecraft.

Solar cells. Electric propulsion is potentially good for a cargo carrier, where you don't care so much about the extra acceleration time.

What's the mass consumption/ejection rate for this thing? What's the power consumption rate? Wondering if a non-solar power source (batteries, nuclear, etc) could cannibalize its waste fuel as ejection material.

One of the best things about these thrusters is how they look exactly like a science fiction space thruster should. This would not look out of place on a Star Destroyer or a Federation Constitution class cruiser.

Isn't corrosion an issue with hall thrusters, especially at high power and long-duration firings? At least that was one of the selling points of VASIMR over ion thrusters.

how does this compare to Space-X Raptor engines ?

Raptor is a conventional chemical rocket engine. The thrust is much higher (of order 1 million N vs 5N) but the specific impulse is much worse so your ability to reach speeds is limited by the amount of propellant you need to carry.

Typically hall effect thrusters have an ISP of 1600 while Raptors have an ISP of 375 in a vacuum. In a cost benefit analysis this is what makes electric propulsion sensible for long term missions. Otherwise their low thrust and heavy weight (Thruster and electrical generator) would make them unappealing.

Url changed from https://newatlas.com/x3-hall-thruster-test-record-nasa/51869..., which points to this.

Weren't we still unsure how the thruster actually worked? As in where the thrust was coming from?

No, that's the EM drive. This engine is less or more a particle accelerator shooting high velocity noble gas. It's like a rocket (less or more), but instead of using a lot of slow matter, it use a little, but very fast, matter. It still requires a supply of matter to trow from the back of the spacecraft, it just uses it in a way to minimize the amount required (by trading it by silly amount of power).

It's based on the classic `F=m*a` Newtonian second law. But uses high `a` instead of high `m`.

The Hall and Ion engines just replace chemical energy with electricity.

Edit: Added the `F=ma` part. Edit2: Added that it still requires matter.

But you still need to give it a supply of noble gas, correct?

So how many AU per unit of noble gas do you reach at terminal velocity of the thing?

Also what are the manufacturing limitations for noble supply - given all the talk about us running out of helium.

Aside: it would be ironic if they had helium balloons at the launch party of some such rocket...

Finally, what is the state of the noble gas when consumed by this engine in a full vacuum of space? Is it destroyed into a state that is unrecoverable?

> terminal velocity of the thing?

What?

Assume you have a thing: rocket, satellite, probe, etc, and you’re pushing it to mars or whatever, and you have an engine that, based on the comment states in consumer noble gasses as fuel, one would assume you have to provide the noble gas, correct?

So after you falcon9 the thing into orbit, and push it along, and it then uses this engine to go the distance, will it constantly increase or hit a max terminal velocity. In either case, would that not tell you how much fuel to give it; which comes down to “if it is powered by helium and we are supposedly running out of helium, how much helium would it use vs how much helium is available?”

Is that not a sound question?

The second part of the question is “if we give it one ton of fuel, how far can it get?”

Pretty simple MPG question in my mind...

There are a lot of variable not accounted for, like the mass of the ship(and cargo) you are going to attach it to. In theory it can keep accelerating as long as it has fuel, and the maximum speed would be related to the speed of the exhaust. At very high velocity we would have to account for friction with the interstellar medium, space isn't entirely empty just the density is very (very) low.

In the real world you probably have a destination in mind... and maybe want to come back. For one way mission, accelerate till half your fuel is gone, perhaps coast for a bit, then decelerate with the remaining fuel (maybe keep some reserves for maneuvering).

So basically, step two of getting to Mars is to setup interstellar refulling stations.

Once you get to mars, or any destination, you need fuel to get back/other things...

So assuming we have an engine that can deliver us back and forth in a reasonable time, then we need to think about deploying intermediate refueling drones... and then drones to refill them... and then how to manufacture and deliver that fuel to the various nodes...

And if we are running out of helium, on earth, we need to find the most harvestable noble gas we can in the solar system...

What are the atmospheres of the other planets made up of, specifically Jupiter, how could we slurp off its atmosphere to Bush is around the solar system?

Assuming other energy sources don’t pan out

Interstellar refueling stations don't make any real sense because you spend as much fuel rendezvousing with the refueling station as you'd get back.

Ideally, you find fuel at your destination, or you have to take enough with you to get back. Or you make plans to stay...

The parameter you are looking for is delta-V, the total change in velocity that the craft can accelerate to. It relates to inverse specific thrust, which is the parameter discussed earlier, comparing slow heavy exhaust to fast light exhaust.

https://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet#Delta-v...

You are right. The answer is, it depends. The others just downvoted you because of the ”terminal velocity” thing.

Guess it’s better than reaching terminal volatility on twitter as we have seen how those experiments have been going...

Not everyone is an aerospace investment banker so don’t be a dick.

Trying to learn here, not be belittled.

There were a lot of questions your comment could imply. I couldn’t think of a simpler way to request clarification. Didn’t mean to offend.

On Earth we’re used to power scaling with maximum speed because of drag. In space, there is no drag. The weakest engine can propel something to close to the speed of light given the time and energy.

There is a measure called specific impulse. It asks how long an engine would hover if fuelled with a take-off weight ratio of one in Earth’s gravity (ignoring the mass of structure, tanks, et cetera). I don’t have an answer for this engine, but ion thrusters clock in around 30,000 seconds while most rockets are between 250 and 500.

Terminal velocity relates to the interaction between drag and an object in free fall.

That's not what specific impulse is... Or at least, I've never heard it explained that way. Certainly no chemical thruster has a specific impulse in the thousands. The SSME was the most efficient chemical rocket for a long time, with an Isp of ~450 seconds.

Specific impulse is directly proportional to exhaust velocity, and the units of "seconds" come dividing the velocity (m/s) by the acceleration of gravity (m/s^2) leaving 'seconds'.

Pre-coffee brain fart. Should be 250 to 500. The physical analogy is, within approximation, an accurate way to think about isp.

Yes, ok I understand your confusion to my comment as I had forgotten that terminal velocity applies to atmospheric based objects?

But I haven’t thought about specific impulse much, so I am very naïve (but curious)

So, what, if any, does the mass of the object being pushed by an engine with the 5.4 Newton’s of energy have of the ability of the engine to push it?

If you push a 1-ton thing with 5.4, and a 100-ton thing with 5.4 Newtons will they reach mars at the same time? And what will be the fuel consumption diff?

> ability of the engine to push it

Acceleration. For equal mass a more powerful engine would make the object go faster, faster.

> If you push a 1-ton thing with 5.4, and a 100-ton thing with 5.4 Newtons will they reach mars at the same time?

    Force = Mass * Acceleration
    Acceleration = Force / Mass
    a: 5.4 / 1000 =    0.0054m/ss
    b: 5.4 / 100 000 = 0.000054m/ss

As the space-crow flies, the 100 ton thing would take longer - it's accelerating 100 times slower. You also have to factor in the mass of the fuel, both starting weight and consumption. As the engine depletes the fuel reserves it gets lighter and therefore accelerates faster (increased Jerk[1]). So if the 100 ton engine was a chemical engine the situation might be different.

The following would be the same time: "assuming reactionless drives, if you push a 1-ton thing with 5.4N, and a 100-ton thing with 540N."

[1]: https://en.wikipedia.org/wiki/Jerk_(physics)

Awesome, so the next question is to know the fuel consumption per engine vs the acceleration of the mass... basically their efficiency.

So if we smack 4 engines on 100 tons, assuming the same fuel... how far do we get.

I’d love to get these numbers.

But I noticed you had a stated m/s - so is there a constant of Newtons of thrust to acceleration of 1ton of mass in space?

(Sorry I don’t know how to word that question better)

I dont know why - but I was under some weird impression that mass is less affected in space due to lack of drag.... Stupid thing to think.

I guess that this is why the slingshot method is so important.

> mass is less affected in space due to lack of drag

Your intuition is closer to correct than many might think. It's harder to move massive things than non-massive things in space. This is because of inertia. Inertia may be more fundamental to physics than mass [1], and could be thought of as matter "dragging" against the gluon (and possibly other) fields [2].

[1] https://www.princeton.edu/~pear/pdfs/2001-inertial-mass-quan...

[2] http://web.mit.edu/physics/news/physicsatmit/physicsatmit_03...

> Stupid thing to think.

There's a difference between stupid and unaware. We live in an environment that is vastly different to space. You have to know quite a bit of Newtonian mechanics before it really starts making intuitive sense. Asking questions is how you get there.

Living in the small town of Newton NC, my car broke down and I took it into the exclusive dealership and shop I got it from - the only place I know - but It wouldn’t budge after I got there. Completely motionless.

I asked the guy to fix it and he exclaimed “oh, sorry can’t help you there - all of these cars are dead. We can’t work on any of these! It will tend to just stay!”

Just then, another car of the exact same model and color drove out of the lot, right by us!

I said “what! That’s the same freaking car! Same color and body style and everything!”

And he said, well you can find another person to try to work on it, but that body tends to stay in motion, this one... not so much...

Clearly I don’t know enough Newtonian mechanics!

My apologies,

Appreciate the response.

Do you have to provide it with a fuel load of noble gas that it will consume?

Assume I give 1 ton of noble gas fuel in whatever format that is, how far and fast will it get before the fuel is consumed?

Ah, got it. So when we think of “rocket fuel” we’re conflating two things: energy source and propellant. Kerosene and oxidiser gives you both heat and combustion products for the heat to fling out of your nozzle.

Hall effect thrusters decouple energy and propellant. The noble gas is the propellant. It’s stuff you’re throwing. The energy, however, must come from elsewhere, e.g. solar panels or a nuclear reactor.

There is an aerospace term called specific impulse [1]. It measures engine efficiency. Ion thrusters are about as efficient as the turbofans on a modern jetliner. Those, in turn, are about 12x more efficient than the Space Shuttle’s solid-fuel boosters and like 7x better than cryogenic, i.e. hydrogen-oxygen, fuelled engines.

[1] https://en.m.wikipedia.org/wiki/Specific_impulse

Apologies then for my other reaction.

I was a reactive gas...

probabbly a little less

You are confusing it with EmDrive [1].

[1] https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster

I think you're thinking of EmDrive https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster

From what I gather this is a pretty standard electric propulsion system: some inert gas is ionized and accelerated with an electric field. Gas escapes through the back, motor gets pushed forward.

Pretty much like any rocket.

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