What is the reason for a LACE rocket to dump fuel?

Question

The concept of LACE sounds awesome, especially the part "fuel is hard to come by in the atmosphere but there's some oxidizer available".

The inefficiency due to the heat exchange requirements is understandable.

What I fail to grasp is the bolded statement:

By careful mechanical arrangement the liquid oxygen can be removed from the other parts of the air, notably water, nitrogen and carbon dioxide, at which point the liquid oxygen can be fed into the engine as usual. It will be seen that heat-exchanger limitations always cause this system to run with a hydrogen/air ratio much richer than stoichiometric with a consequent penalty in performance1 and thus some hydrogen is dumped overboard.

I suspect* it's somehow dumping the generated heat. A great answer would indicate why is this method considered most efficient currently (for a LACE vehicle).

_{* - didn't put in the effort to read around, sorry. I don't believe my silly self reading papers in advanced physics would be productive. Thus the question above: "please crunch those as to an 8 year old".}

_{As always on SE, accepting an answer in something the original asker isn's even remotely qualified in ... takes from weeks to never(thankfully!).}

Answer 1

The concept of LACE sounds awesome ...

You might want to rethink that. This is an old, old concept that has never seen the light of day, older than retirement age in several countries. There are good reasons this concept has not advanced past drawing board status.

One reason this concept never advanced past the "interesting idea" stage is because the latent heat of vaporization of oxygen is 214 joules/gram while that of hydrogen is 192.4 joules/gram. This means a lot of liquid hydrogen is needed to liquify gaseous oxygen. The ideal ratio for a typical LOX/LH2 rocket is about four parts oxidizer (by mass) to one part fuel.

Note that this 4:1 ratio is already strongly fuel rich; the stoichiometric ratio is 8:1. A less than 1:1 ratio is so far from stoichiometric that the result would be a warm gas thruster as opposed to a hot gas thruster. Forcing this ridiculously fuel rich mixture into the combustion chamber would be a bad idea. The second best thing to do would be to dump some of the extra hydrogen, for example, as auxiliary cold gas thrusters such as for roll control. The best thing to do is to dump the idea entirely. The concept is past retirement age, after all.

Answer 2

I suspect the reference to "stoichiometric" is an error, or at least poorly chosen. The optimum ratio for a rocket is fuel-rich rather than stoichiometric, but performance falls off on both sides of the optimum ratio. However, real world rockets run fuel rich to reduce average exhaust molecular mass, increasing exhaust velocity. For a rocket, which carries all its reaction mass, exhaust velocity directly determines its specific impulse.

In contrast, air breathers get most of their reaction mass from their environment, their onboard fuel contributes only a fraction to their exhaust mass flow rate. For an air breather, specific impulse can be increased by accelerating more air with lower exhaust velocity, exhaust molecular mass isn't as important, and they don't want to carry any reaction mass they don't need to, so they will indeed want to run close to stoichiometric, or even on the oxidizer-rich side of things.

In short: for a rocket, fuel is as important as part of the propellant as it is as something to combine with an oxidizer for energy. For an air-breathing engine, fuel is primarily something to be burned with air as an energy source, and unburned fuel is wasted fuel.

Back to LACE. LACE would have cooled and liquefied oxygen both for its immediate needs and to put into a tank for later rocket-powered flight, along with 4 times as much N2 which would then be used for cooling things and as reaction mass. Liquefying all that requires more hydrogen than it could reasonably make use of: there's not enough oxygen to burn it, and the vehile would be flying through a sea of reaction mass. So, you end up carrying a large amount of hydrogen that's only actually needed for cooling air.

In at least some LACE vehicle concepts, it was dumped into a ramjet to get some propulsive use out of it, but obviously, if that gave comparable performance there'd be no need for the whole LACE part of the system. This is just trying to find a secondary use for something you'd rather not carry in the first place.

Answer 3

The heat is being dumped into the fuel, almost always hydrogen.

The amount of fuel needed to absorb the heat is so much more than can be burnt fuel-rich to provide more-reaction mass.

(@russellborogoves answer-in-a-comment is incorrect for this reason)

the hydrogen is burnt as richly as is efficient, the remaining fuel is dumped, along with the inert gasses from the air. Typically this will be through an expansion nozzle as a rather warm cold gas thruster, but the trust is tiny compared to the actual engine.