# Could friction heat from aerobraking be used to power propulsion?

Aerobraking has always been thought of as a way to economically decelerate a spacecraft before landing or orbit insertion. I'm asking here if it could instead be used to accelerate a spacecraft.

Passing close by a massive planet, like Jupiter, a spacecraft seeking maximum speed as it heads outwards enjoys two benefits and one problem. The benefits are the gravitational assist and the Oberth effect of firing its engines at periapsis. The drawback is the resistance from the atmosphere if one gets too close.

Could the friction heat from aerobraking in an atmosphere be used to ignite a propulsion system, of any kind which transforms external heat to a fast ejection of mass? Rocket engines use oxidizers to chemically create heat that ejects hydrogen at high velocity. Could the oxygen, which represents the large majority of fuel mass, be replaced by the friction heat from passing through a planetary atmosphere? Somehow transferring the heat from the shield to some kind of hydrogen acceleration mechanism. It would come with the bonus of happening when the Oberth effect is the greatest. And would it be useful for igniting some kind of fission thermal short term booster?

The Chelyabinsk meteor produced an explosion of about 500 kilotons of TNT when it entered Earth's atmosphere. Could that kind of effect be harnessed and used to substantially accelerate a spacecraft through an atmosphere?

• Could the oxygen, which represents the large majority of fuel mass, be replaced by the friction heat from passing through a planetary atmosphere? are you really suggesting to substitute the oxidiser with thermal energy? Feb 4, 2016 at 14:01
• @Federico Well, I'm asking, not seriously suggesting. But yes. To use an atmosphere as a friction match to heat the hydrogen expelled to propell the spacecraft. Feb 4, 2016 at 14:03
• My problem with your question is your use of "combustion chamber": if you remove the oxidiser there is no combustion, no matter the amount of heat you pump in the system. Feb 4, 2016 at 14:05
• @Federico: In NTR engines there's no combustion either, just the propellant superheated by a nuclear reactor.
– SF.
Feb 4, 2016 at 16:03

Can it be done - I think yes. Does it make any sense - I think no.

The concept of a non-chemical thermal rocket is not new. Other types of such rockets would be a nuclear thermal rocket or a resistojet. Both work in principle by heating a fluid and then expelling it, just like the concept you suggest. The fluid can be anything, but hydrogen is best in terms of ISP.

So how would a use case of your rocket look like? Obviously, you would not try to use it to escape earth orbit, because the energy dissipating at re-entry is just the motion energy you put in your spacecraft in the first place. Furthermore, most of the energy is not transferred to the spacecraft but stays in the atmosphere (if I remember correctly, it was about 0.2% going into the spacecraft).

But, as you were saying, maybe your want to use this energy to fly a maneuver in the Jovian system. When you enter the system, you have escape velocity, so you want to shed some of that energy. In this case by aerobrakeing in Jupiter (may not be smart for other reasons), yes, you may encounter some heat, and that heat, in theory, can be used as energy for later course changes. You could, for example pressurize a lot of hydrogen, then use it to cool a heat shield, and then release the hydrogen for thrust.

This would be a very bad design. You would have to store a large amount of pressurized hydrogen somewhere, because you don't want to fire your thruster right away, while still in Jupiter. You want to have propulsion hours, or days, or years later. This makes your tanks massively heavy. And that is just the least of your problems.

In order to get any sort of decent ISP, you would need to heat your hydrogen to thousands of kelvin. Therefore, it would not be an effective coolant. Actually, you would somehow have to actively cool your storage tank, because there is no material in the world that could resist the temperatures needed for a gas to be an effective rocket propellant.

And even that ignores the largest flaw in this concept, that energy isn't even the largest concern in spacecraft propulsion, it's momentum. There are many sources of energy available, even in outer space. There are radioisotope thermoelectric generators, solar panels, chemical reactions, even nuclear reactors. The tricky part is to use this energy to expel a fluid backward at the greatest speed (read: ISP) possible, so according to the momentum balance, your space ship gets the most forward momentum. In this regard, there is nothing better than ion thrusters, and your concept would do worse than the simplest thruster, the resistojet. This is a thruster that does nothing but heat something (anything really - hydrogen, water, poop, you name it) electrically to expel it through a nozzle. So just like your idea, the resistojet also also heats up the propellant, but it doesn't have to store it and it is not dependent on a specific part of the mission in order to function.

Therefore, even if it was possible to recover some energy from aerobrakeing, I don't think that it can be a viable part of any propulsion system.

• To pick on one detail in your answer: " you don't want to fire your thruster right away, while still in Jupiter" Why not? The Oberth bonus is the greatest there. Feb 4, 2016 at 15:21
• @LocalFluff The point of aerobrakeing is to lose speed, not gain it. The energy on the heat shield is just part of your own motion energy. What you suggest would be like blowing into your own sail. Feb 4, 2016 at 15:24
• But it is also about exploiting (igniting) the chemical (or maybe fission nuclear) energy of the mass experiencing the atmospheric friction heating. The Chelyabinsk meteor of about 13 tons caused a blast of 500 kilotons of TNT. If a random meteoroid can convert its random rubble pile mass into 10,000 times its mass of high explosives by friction in an atmosphere, why couldn't a spacecraft benefit from a similar process? Feb 4, 2016 at 15:34
• @LocalFluff Again: The energy you can use is just what you put in. Less actually. 99,8% less. So what possible use would you get out of this? Feb 4, 2016 at 15:41
• Converting the chemical energy stored in the mass into propulsion as that mass is heated by friction. As in heating hydrogen without any need of oxygen. That's my basic idea. Feb 4, 2016 at 15:51

The fundamental problem is in law of conservation of energy. All the energy you might expend on propulsion obtained that way comes from one singular source: kinetic energy of your ship, braking against the atmosphere. You won't extract more energy than you're losing, since you can't collect it at any reasonable efficiency as it's generated.

You might help yourself with an oxidizer, or save up a certain percent of the descent energy for consecutive ascent if you want to enter Jupiter orbit and later depart it, but in the current form you've presented it, it's an aerodynamic perpetuum mobile. (just imagine instead of propelling yourself out of the atmosphere you enter an orbit through the atmosphere and somehow create all the heat without ever slowing down, just maintaining speed).

There might be a way though. Not now and not soon, possibly a couple centuries away. You can extract more energy from pure hydrogen than you spend, but you must find spend a lot, and there are no solids that can survive this order of temperatures involved.

Create a magnetic "air intake" - out of powerful magnetic field. Compress the incoming hydrogen into superheated plasma. Make it so hot that it undergoes nuclear fusion, turning into helium and expending a lot of energy in the process. In short, a Bussard Ramjet that is intended for gas giants instead of interplanetary medium. This would work, but the engineering task behind shaping such a magnetic field makes my head spin.

• Would not even an airburst of 13 tons of hydrogen as if it were 500,000 tons of TNT be able to help a spacecraft along? That's more mass than the largest container ship carries, chemically stored in a modest 17 meter sized meteoroid, suddenly ignited by atmospheric friction. Feb 4, 2016 at 16:25
• @LocalFluff: Where does the energy of that airburst come from? Trace it to the source: what gave it such explosive power? What happened to the hydrogen so that it changed from a common gas into such super-explosive?
– SF.
Feb 4, 2016 at 16:27
• The energy comes from the mass, like all rocket fuel does. But without the need of oxygen and other combustion processes. Where did the Chelyabinsk meteor get its 500 kilotons from? Feb 4, 2016 at 16:30
• @Rikki-Tikki-Tavi: Lots of cold asteroids clumping together into our Moon, and the lunar maria are essentially solidified basins of lava, the center of the Moon still liquid. What made the cold asteroids so hot?
– SF.
Feb 4, 2016 at 17:21
• ...and in the end let me use a car analogy: Can you take the energy from car brakes somehow to make the car go faster instead of slowing down when you brake?
– SF.
Feb 4, 2016 at 17:42