# “Hydrogen-breathing” jet engine?

Following question about flight on other planets an answer contained a quote:

Jupiter: Flight on Jupiter is unrealistic. Jupiter's gravity is much too strong. The power required to maintain flight is about 3x that of Earth making flight there highly unrealistic.

Fighter jet planes use engines that are much more than 3x the power required for flight. Sure that's wasteful and expensive, but not nearly impossible.

Moreover, what is expensive in terms of energy, is rocket engine that contains its own fuel and oxidizer. On Earth we commonly use jet engines that use atmospheric oxygen for oxidizer, and own fuel.

Now, what if we reverse that? If the engine were to fly in hydrogen (or methane for that matter!) atmosphere, it would need its own oxidizer but it could burn the atmospheric gas for power... or am I missing something? Would an aerobot on Jupiter, Neptune or any of the planets with enough burnable material for an atmosphere, using own oxidizer to burn it for propulsion, be viable?

• Oxygen weights far more than hydrogen... – PearsonArtPhoto Oct 4 '13 at 17:44
• I don't think you'd need 3x the power for flight on Jupiter. You need lift to counteract gravity, not necessarily power. Granted, you could triple your speed to get to the required lift, but that's only straightforward if tripling your speed doesn't make you go supersonic. Increasing the wing area seems a simpler way to gain lift. – Hobbes Nov 21 '13 at 15:31
• Now as I think of it, while chemical hydrogen-breather would be a short-term enterprise, something that wouldn't last long, a hydrogen-breathing nuclear thermal jet should be able to fly in circles all around Jupiter until something breaks. Possibly also until it scoops enough hydrogen to return to orbit. – SF. Mar 9 '17 at 16:57

Sure. That would save a fair bit of weight, avoiding the tankage, insulation, and cooling systems to maintain liquid hydrogen, or alternatively, the mass of the pressure tank for gaseous hydrogen. You'd need to bring the oxygen. You actually don't save much weight due to the hydrogen itself, which is only $1\over 9$ of the total propellant mass.

You can also consider bringing flourine instead of oxygen to burn with the hydrogen. Nasty stuff though.

• ...or possibly chlorine trifluoride, which is somewhat easier to handle than pure fluorine due to its higher boiling point, and which has actually been tried as a rocket fuel oxidizer. Still nasty stuff, though. – Ilmari Karonen Oct 4 '13 at 20:15
• "Ignition!" is both the most instructive and the most entertaining rocket science book I have read. If you can find a copy, read it. – Mark Adler Oct 4 '13 at 20:20

Hydrogen breathing jet engines

Since the gravity of of Jupiter is three time stronger than Earth so we need to create three time the lift that we create in earth to lift the craft. But hydrogen fueled aircraft are more powerful and we use then to attain hypersonic flight in Earth. The main problem with this is that materials at such high temperature would make the metals brittle. But creating lift in high Reynolds number flow is very hard and because lift coefficient is inversely proportionaly to Reynolds number.

Hydrogen breathing rocket engine

One can save the amount of materials such as insulation, tank but that it will be compensated to some extent by the following

• Instruments required to Increase of Hydrogen and simultaneously reducing the temperature of Hydrogen

• If you use Fluorine tho it may react with Hydrogen to produce Hydrogen Fluoride which is very toxic and burn to burn one mole of Hydrogen requires one mole of Fluorine (Oxygen requires one mole to burn two mole of Hydrogen )

• you need lot of thrust to take off which means we need to have solid booster and burn more hydrogen to accelerate .

• The exhaust velocity is directly proportional to the ratio of pressure inside and outside of chamber pressure . Since the atmospheric pressure of Jupiter is high, so the pressure of combustion must be very high so chamber must be heavy to withstand such high pressure

• Note that there is no place on Jupiter to "take off" from. The aircraft would be dropped out of an entry vehicle at high altitude, and could simply fall to its cruising altitude, accelerating in the fall to its cruising speed. – Mark Adler Oct 6 '13 at 16:51
• Didn't you get the mole number opposite? H2O - 2 atoms of hydrogen for one atom of oxygen, 2 moles of H2 for one mole of O2. Instead of starting, we have slowing down from orbital flight; and lift is also a function of atmosphere density: descend low enough and hydrogen will be quite dense. – SF. Oct 6 '13 at 20:43
• re: your last point: the durability of the combustion chamber must be proportional to difference between internal and external pressure, not just directly proportional to internal pressure. If the chamber can withstand producing 100 bar in vacuum, it can withstand producing 400 bar combustion pressure in 300 bar atmosphere. – SF. Nov 21 '13 at 15:38

Lift = Cl*0.5*(density)*(velocity)^2*Area. There are three variables that can be optimized to achieve the proper lift to remain in flight in Jupiter; density, velocity, and wing area. Increasing wing area adds weight and makes the vehicle more difficult to launch from earth. Maintaining a high velocity will require higher thrust and would expend more oxidizer on board, and add more drag. Finally flying in a lower altitude with higher density of Hydrogen will add more drag on the vehicle but will save on power requirements for compression before combustion. There is no true answer except for the optimization of all parameters. There's also design constraints of nozzle area ratios and operating pressures. As you increase the back pressure on the nozzle (at lower altitudes), it's more difficult to achieve fully expanded flow, since shocks may develop at the nozzle exit (assuming C-D nozzle)

The game changer for a mission like this would be to fly in a specific cloud type with a naturally high composition of oxydizer and engineer a way to extract it on board. Or, find a different power source all together (solar, thermal, radiation).

• I doubt there would be an easy way to develop an on-board oxidizer refinery, but I believe there should be a plenty currents going more or less radially, meaning gliding - like on Earth's thermals - should give some extra lift. Also, in the end, that doesn't need to be a jet engine. It could be an internal combustion engine for a propeller craft just as well. – SF. Jan 21 '16 at 18:25