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A recent comment in a discussion of satellite refueling mentioned the possiblity of a spacecraft dipping into the atmosphere from LEO to obtain oxygen. Similarly, the science fiction RPG Traveller proposes scooping hydrogen from gas giant atmospheres to refuel fusion-powered starships.

Are these techniques actually feasible? Specifically, I'm asking:

  • From stable LEO, is it possible to drop perigee into Earth's atmosphere, collect material, return to stable orbit, and filter the collection down to oxygen with a net gain in propellant mass, or will fighting drag use more propellant than is collected? Assume that the ship has high-thrust H2/O2 engines at around 450s Isp, and starts with a surplus of fuel to oxidize. Would the filtration need to be magically instantaneous (i.e. only oxygen is scooped), or could it still work if it scooped mixed atmosphere, rejecting the nitrogen, CO2, etc after regaining safe orbit?

  • From stable low orbit around a gas giant or ice giant -- Jupiter, Saturn, Uranus, or Neptune -- is it possible to skim the atmosphere, collect material, and regain orbit, again with a net gain of propellant mass? Again with H2/O2 engines, but this time the ship is assumed to start oxidizer-rich.

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  • $\begingroup$ Do you mean theoretically feasible, or doable with current technology? Anything is possible, what answer are you looking for/ $\endgroup$ – GdD Jan 13 '17 at 17:03
  • $\begingroup$ I'm looking for theoretical feasibility analysis, with practical engineering considerations secondary. And not everything is possible, as Blake Walsh's answer notes. $\endgroup$ – Russell Borogove Jan 13 '17 at 18:51
  • $\begingroup$ Whatever bonehead gave you that crazy idea, ehum, should have patched it up with saying that the drag created could actually be useful in itself for aerobraking, with the scooped up oxygen fuel component to be used for the landing afterwards. $\endgroup$ – LocalFluff Jan 13 '17 at 20:09
  • $\begingroup$ Related: space.stackexchange.com/questions/39439/… $\endgroup$ – Magic Octopus Urn Oct 18 at 18:59
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It is easy enough to analyze this in terms of conservation of momentum, we'll assume the velocity through the atmosphere to be 7.8km/s, that is LEO orbital velocity - in actuality it'll be a little different due to rotation of the Earth and eccentricity of the atmosphere-grazing orbit, but not different enough to change the conclusion. Now assume the ship scoops up 1kg of atmosphere; logically it would be necessary to fling that entire 1kg out the back at 7.8km/s to regain the velocity lost when catching it and that would leave the ship with no reaction mass gained, to actually gain reaction mass would require ejecting a portion of the captured gas out the back at faster than 7.8km/s.

According to the table of exhaust velocities a chemical rocket engine exhaust velocity maxes out at around 4.4km/s which is a serious problem because even if somehow all the oxygen and nitrogen can be used as oxidizer (it can't, though maybe some of the nitrogen can) and that the fuel is weightless, the exhaust velocity is still much too low to recover the lost momentum.

So this analysis would suggest that for chemical rockets an atmospheric scoop is a total non-starter, it's so far from something that would work we don't even need to consider all the other problems such as inefficiency. Nevertheless Nuclear or Electric propulsion could potentially reach the required exhaust velocities, assuming they can use nitrogen for reaction mass.

As for the gas giants, with orbital velocity being over 15km/s even for Neptune and Uranus the sheer exhaust velocity required would be a serious challenge even for electric propulsion. Now, if the atmosphere were pure hydrogen it could just about work with onboard oxidizer because the hydrogen is so light compared with the oxidizer - as such catching 1kg of hydrogen at 15km/s could be offset by ejecting 5kg of H₂O at 3km/s, so there is margin for velocity gain there. Unfortunately, the atmosphere is not pure hydrogen and also contains about 26% helium by mass which pretty much eliminates that margin unless you have some kind of magic filter. Also, you would be bleeding oxidizer like crazy making it an exercise of dubious usefulness.

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  • $\begingroup$ You've got m/s in a bunch of places where you mean km/s, but good analysis. $\endgroup$ – Russell Borogove Jan 13 '17 at 18:52
  • $\begingroup$ It WOULD be feasible for scooping hydrogen for electric propulsion though! $\endgroup$ – SF. Jan 13 '17 at 19:25
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    $\begingroup$ So it's practical for science fictional fusion torch-ships but not much else. $\endgroup$ – Russell Borogove Jan 13 '17 at 20:42
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    $\begingroup$ I was using the term loosely to mean "with high enough exhaust velocity to be profligate with propellant" ;) $\endgroup$ – Russell Borogove Jan 14 '17 at 4:12
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    $\begingroup$ This analysis seems to assume you bring all the gas to ship-rest in a tank, reject the associated heat, then combust into rocket exhaust. That’s not necessarily needed. For example, a scramjet approach burns external gas without bringing it to rest, which lets the ship burn many kg for each onboarded. $\endgroup$ – Bob Jacobsen Apr 12 '18 at 1:04
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It can be made to work, but it would be quite different from the typical SF depiction.

As Blake points out, you can't gain anything from mass that is already moving faster than rocket exhaust. However, you do not have to collect the gas at orbital velocity. In the extreme case you could slow to a complete stop and refuel while landed, as long as your fuel tank is big enough to allow a single stage to orbit.

There are serious plans to refuel that way on Mars (~5 km/s) using current technology, so nothing impossible there.

Earth (~10 km/s) is close to the practical limit for a single stage - enough that we've never actually built one, but it could be done if there weren't better options available.

Assuming that refilling the tank takes less than a year, there should be some point between orbit and the ground where you don't have excessive drag, but also don't have to empty the tank to get back to orbit.

Nuclear rockets

Using chemical rockets is probably not going to work - I prefer to avoid atmospheres composed of perfectly mixed rocket fuel and oxidizer - so I'll start with a nuclear thermal rocket that has similar performance to H2/O2 but will burn anything that can be sprayed onto hot rocks. That doesn't exist, but seems reasonable with current technology.

I'm also going to assume separate fuel gathering ships rather than a small add on to a larger ship - burning 90% of the collected fuel to return to orbit only gets you a full tank if you can make ten trips.

Mars gets closest to basic atmosphere skimming - orbital velocity is lower than exhaust velocity, so you don't need to slow down to gain a small amount of fuel on each pass through the atmosphere. Completely filling the tank may be possible.

Earth requires slowing to at least 4 km/s below orbital velocity. You'll need a very fast pump, as you have to either produce enough thrust to hover or fill the tanks in the few minutes before you hit the ground or thick atmosphere. Getting back into orbit will use up at least half of the collected fuel. Starting with enough fuel to return to orbit isn't really an option, so the pumps will need to be very reliable as well as fast.

Jupiter requires slowing by 10 km/s, so there is really no margin for error. Each trip into the atmosphere uses up 99% of the collected fuel - you need hundreds of flights to fill the tank, and abort to orbit isn't even a theoretical possibility.

Chemical rockets

Fully fueling a chemical rocket this way probably isn't going to happen. If you do find all the necessary elements in one place, it is probably as water. The energy to process that into rocket fuel probably means a nuclear reactor, which you may as well just use more directly.

Collecting only the heavier oxidizer is more reasonable, but probably not worth it in most cases - even if Jupiter had a pure oxygen atmosphere you would be using 99 tons of hydrogen to get 8 tons of oxygen.

On Earth you might be able to come out ahead - Starting with 100 tons of hydrogen you could collect 800 tons of oxygen and return to orbit with 50 tons of hydrogen and 400 tons of oxygen.

Engineering challenges

The rocket itself is relatively easy - several suitable designs already exist, although nothing flown.

To store the collected fuel, you need something about the size and weight of the space shuttle external tank, which is easy enough, but it also needs to be able to survive reentry, possibly hundreds of times, without adding too much mass to get back to orbit.

To fit a useful amount of air in the tank, you probably need to liquify it. A high pressure tank may also work, but is likely heavier. Either option generates a lot of heat. That isn't usually a problem, but you are trying to do this while flying at mach 10 attached to both a nuclear reactor and a rocket engine.

Would it ever be worth doing?

On Earth, it might happen if sufficiently reliable technology was already used for something else, but otherwise there are better options. Using a falcon style ground based first stage gets you most of the benefits with none of the risk.

An Earth-like planet without infrastructure could make sense, but you are probably better off landing - use a rocket that can reach orbit empty or act as the first stage for a large load of fuel.

Venus actually makes it a look like a decent option - mid air fuel production is just as dangerous as anywhere else, but the surface environment manages to be even more unpleasant.

Jupiter needs either a lot of flights or a lot of ships. If you have sufficient local infrastructure to support a hundred ships, you may as well build a space elevator or something. Without that, entering the atmosphere of a gas giant is reserved for the truly insane starship captain who somehow missed all the moons and ring system full of easily accessible water ice.

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  • $\begingroup$ NTR generally has at least double chemical ISP, but lower TWR, so you should consider factoring that into your calculations, and probably also listing those out. $\endgroup$ – Nathan Tuggy Jan 14 '17 at 4:15
  • $\begingroup$ @NathanTuggy True, though this is engineering rather than physics, so exact numbers are tricky - I'm extrapolating enough that doubling is a rounding error. Suboptimal propellant would lower ISP, and I suspect that a large part of the thrust difference is lack of interest - optimizing thrust over ISP only makes sense for first stages, and it's been a while since anyone considered that a good idea. Chemical equivalent performance seemed a reasonable default assumption given that the important property is not needing any specific chemical. $\endgroup$ – Quentin Clarkson Jan 14 '17 at 6:31
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    $\begingroup$ @BlakeWalsh Yes, there is a big difference between "it could work" and the best available option. Titan would work better than anywhere else, but the same nuclear jet you need for the skimming can also give you a free ride to the surface, where you can use a very ordinary pump to load liquid methane, and liquefying nitrogen only takes about ten degrees of cooling. $\endgroup$ – Quentin Clarkson Jan 14 '17 at 12:34
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ESA is currently (2018) studying a quite similar project. In short, it does not use oxygen but any gas in upper atmosphere to feed an ion thruster instead of flying with a tank of gas to feed this thruster.

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As others have noted, exhaust velocity needs to be higher than orbital velocity. Some ballpark velocities for low, circular orbits around various bodies:

Venus 7.2 km/s
Earth 7.8 km/s
Mars 3.5 km/s
Jupiter 42 km/s
Saturn 25 km/s
Uranus 15 km/s
Neptune 17 km/s

Lox/hydrogen's 4.4 km/s exhaust velocity is about as high as you're going to get with chemical propellents. I understand 30 km/s is doable with ion. But it's difficult to imagine ion engines providing enough thrust to maintain an orbit that passes through a body's upper atmosphere.

However

One of my favorite science fiction day dreams has been vertical elevators anchored at Phobos. Relevant to this question is my look at a lower Phobos tether.

The foot of a 5800 kilometer tether descending from Phobos would skim through Mars' upper atmosphere during Phobos' periapsis. Tether foot speed relative to surrounding atmosphere would be around .6 km/s. About mach 2. The Concorde would routinely do that through a much thicker atmosphere.

A tether foot passing through Mars upper atmosphere might harvest CO2 and argon. The harvesting might also turn turbines, providing power. Yes, this would subtract from Phobos' momentum. But at 1.1 e16 kilograms, Phobos is a momentum bank we could draw on for millennia with little effect.

At this time I don't think such a tether is plausible using existing materials like Xylon. But given bucky tubes, the benefits of such a tether might out weigh the costs.

Yes, this is somewhat far fetched science fiction. But if we talking about skimming atmosphere from ice giants we're already in the realm of highly unlikely. A Phobos tether foot skimmer is most plausible skimming scheme I can think of.

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To onboard 1kg of gas requires many kg of exhaust at lower-than-orbital speed.

The key to doing that is for the ship to burn atmosphere while going by, rather than bringing it to rest relative to the ship first which requires speeding it up in the wrong direction.

Instead, use a scramjet approach: enter image description here

Burn as much as needed, pulling off a small amount to keep.

Can such an engine work at high enough speed? Most of the literature is in Mach units, not km/sec (and gas heating effects, which change the speed of sound, make it all a bit confusing), but there seems to be confidence in Mach 10 (of about 25 needed), speculation about 17, and some approximate arguments that thrust/weight makes 25 improbable. But ideas do tend to get better with time....

And if you’re willing to throw a stage away, a scramjet first and rocket second could get you back to orbit.

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  • $\begingroup$ Why throw the stage away, why not a single stage with both engine types? $\endgroup$ – lijat Apr 12 '18 at 18:04
  • $\begingroup$ @lijat “why not ... both engine types?” Could be. The thrust-to-weight issues with the high temperatures of high-speed air breathers are hard, but maybe technology will advance to where that weight could be carried. $\endgroup$ – Bob Jacobsen Apr 12 '18 at 21:31

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