Hybrid Jet and rocket idea I had (no oxidizer payload)

[Agent86]

So I was thinking about how much of a pain in the ass it can be to carry a rocket engine on my SSTO spaceplane as essentially a payload to high altitudes, then shutting down my jets and carrying THEM as a payload to orbit. I was wondering if an engine could be built that does both.

And to boot, what if instead of wasting thermal transfer potential (by bringing cryogenic hydrogen up to burnable temperatures using the jet nozzle as a heater (as in the SSME)) you could use it to generate your rocket fuel in-flight?

This may be hard to follow, since I am not a rocket scientist, or a teacher, but here goes:

On your ship you have:

- one large tank of liquid hydrogen. (boiling point 33K)

- two empty tanks, one for liquid air, and one for liquid nitrogen

- a mostly standard turbojet engine

At launch, your turbojet is sucking in air as it's oxidizer, and burning it with hydrogen to produce thrust. But on it's way to the engines, it passes through two heat exchange coils. The second coil wraps around the air intake, or air compressor, and it's cryogenic temperatures cool atmospheric air below it's boiling point (78K). Some gas air is fed to the engine, and some liquid air (blue) is bled off and stored in the previously empty tank. Since nitrogen (the main constituent of air) boils at a lower temperature than oxygen (90k), the liquified air will bleed off nitrogen gas resulting in:

- a slush of oxygen rich liquid and

- a stream of gas nitrogen (green)

The gas nitrogen (green) is passed through the FIRST coil of LH2, liquifying it. The liquid is then moved to a storage tank where it can later be used as RCS. Any excess can be vented to atmosphere or used as direct thrust.

When the air becomes too rarified for the air breathing engine, the valve to the combustion chamber closes, and the oxygen rich liquid is introduced, along with the remainder of the LH2 and used as a rocket to continue it's ascent into space and avoid a violent vehicle-ground interface event.

The benefits of this are:

- lighter takeoff weight, oxidizer and RCS are gathered mid-flight.

- only one engine to handle both jet and rocket configurations, further reducing weight and complexity.

- fully utilizing heat exchange potential from liquid hydrogen fuel transitioning to a higher temperature.

The only question is, how much of this is achievable using sound science, and how much of it is only attainable through some sort of black magic/witchcraft?

[andrewas]

The problem is that you now have to accelerate all that air to the velocity of your craft, so even if you can make it work, I'm not sure the savings in not having to haul the oxidiser all the way from the launch pad will outweigh the extra drag and the weight of the extra equipment.

SABRE is probably as far as its worthwhile to go in terms of air-breathing rocket engines.

[magnemoe]

The problem is that you now have to accelerate all that air to the velocity of your craft, so even if you can make it work, I'm not sure the savings in not having to haul the oxidiser all the way from the launch pad will outweigh the extra drag and the weight of the extra equipment.

SABRE is probably as far as its worthwhile to go in terms of air-breathing rocket engines.

SABRE use more hydrogen cooling the intake air than it burn in the rocket engine, this is burned in an extra ramjet engine mostly to do some good with it.

If you want to cool and store more air you will need to use more hydrogen, second you have to scale up the condenser and other parts for this so its probably not worth it.

[Nuke]

i dont really see any point to storing the liquified air on-board. in air breathing mode a sabre engine has a really good isp, so its no problem just carrying the required o2 during the atmospheric stage of the flight. so its better just to direct feed that air into the combustion chamber and do away with all the tankage, plumbing, pumps, etc for handling the liquid air, let alone the extra coolant (lh2) needed for the whole process. the sabre way seems to be more straightforward.

[softweir]

So I was thinking ...

You pretty much came up with HOTOL. That was a concept for a sub-orbital spaceplane that would have used liquid hydrogen and liquid oxygen, liquefying most of the oxygen from the atmosphere during the early stages of the flight. One of the suggestions was to start with mostly empty LOX tanks and fill them with excess O2.

Since nitrogen is the dominant gas (by mass) of air, you will have large amounts of it to deal with! Better, perhaps, to use the liquid or chilled N2 to pre-cool the incoming air so less liquid H2 is needed to liquefy the air, and you will have a more efficient engine that burns H2 and pure O2.

Also, you don't mention any ideas for coping with all the solid H2O and CO2 that will be formed. That's a pipe-blocking issue! The HOTOL engineers appeared to think they could deal with that issue, but as it never got beyond the concept stage we have no idea if their proposals could be made reliable.

[Themohawkninja]

i dont really see any point to storing the liquified air on-board. in air breathing mode a sabre engine has a really good isp, so its no problem just carrying the required o2 during the atmospheric stage of the flight. so its better just to direct feed that air into the combustion chamber and do away with all the tankage, plumbing, pumps, etc for handling the liquid air, let alone the extra coolant (lh2) needed for the whole process. the sabre way seems to be more straightforward.

When you are in space, you will need the oxidizer on board to run the engine. AFAIK, we have yet to develop an effective way of sucking in air, cooling in to liquid O2 (and filtering it for efficiency), and then storing it on board for later.

[Nuke]

Also, you don't mention any ideas for coping with all the solid H2O and CO2 that will be formed. That's a pipe-blocking issue! The HOTOL engineers appeared to think they could deal with that issue, but as it never got beyond the concept stage we have no idea if their proposals could be made reliable.

reaction engines has a solution to the icing problem, they are just keeping it proprietary for the time being. sabre will just feed it straight into the compresses and then to the combustion chamber, and i have a feeling their ice shedding process is done in the heat exchanger before the air reaches those parts of the engine.

i suppose you could store it at that point (this is completely ignoring the fact that sabre's heat exchanger doesn't liquify the air), but you still need enough to run the engine, so you are essentially asking the heat exchanger to do two jobs at once, and that uses more coolant. so your weight savings from not having an o2 tank are quickly eaten up by extra coolant and equipment. its air breathing mode isp is like 8x its vacuum mode isp, so you aren't loosing much carrying that o2 tank from runway to rocket mode.

Edited August 8, 2013 by Nuke

[Apotheosist]

Basically what you want is Reaction Engines SABRE. It does what you want, giving all the advantages you say, but they've been working on it for decades so their engine has been optimised, and all the silly ideas that seemed good at the beginning, they have ruled out. They're going to store liquid oxygen on board for the exoatmoshperic portion of the flight. They've solved the icing problem. ESA did a study on their engine, and found no technical reasons why it could not work.