Is a full-flow staged Hydrolox engine possible?

[Pthigrivi]

I was watching this excellent video by Everyday Astronaut and it got me thinking,.. is full-flow stage combustion possible with Hydrolox? Is it just more expensive because of all the complex seals? I know there are other considerations like tank insulation and size, but just from the standpoint of the engine itself is this possible? I couldn't find much in my brief google search.
 

 

[Beamer]

The Rocketdyne Integrated Powerhead Demonstrator was meant to be further developed into a hydrolox engine AFAIK (1990s project). It never got to that stage since the govt cut funding but to the best of my knowledge there are no principle objections to it being possible, and they did complete and run the powerhead at full capacity.

It's just a combination of 2 things that are 'hard' compared to their alternatives (hydrogen storage and a full flow cycle) so it probably requires a hefty R&D investment to make it work.

 

[intelliCom]

7 minutes ago, Beamer said:

since the govt cut funding

A tale as old as time. If one of the things that the gov cut actually got brought into reality, we'd be much better off for it; NTRs, Venturestar, possibly even a full flow hydrolox engine.

But nah, we're perpetually in a state where NASA keeps being treated as a jobs program.

[Beamer]

29 minutes ago, intelliCom said:

A tale as old as time. If one of the things that the gov cut actually got brought into reality, we'd be much better off for it; NTRs, Venturestar, possibly even a full flow hydrolox engine.

But nah, we're perpetually in a state where NASA keeps being treated as a jobs program.

While true, this was a USAF/NASA collaboration, so the bigger part of the funding probably came from the defense budget, as it tends to go with such projects. Since it was a cancelled project there's not all that much information to be found about it, but I suspect the USAF's involvement might mean it was considered as an option for orbital cruise missiles or perhaps defensive rockets as part of... what was it called, the space shield thing that Reagan had in mind. It might simply have been cancelled because of shifting defense priorities or international treaties regarding the use of space for military operations.

So in this case it wasn't necessarily a bad thing the project itself got cancelled. It's always nice if NASA can hitch a ride on the defense budget, but the risk is that priorities for that sort of funding are not necessarily in line with space exploration and science so might dry up quickly when the global political landscape changes. I'd rather see them give more money directly to NASA (and perhaps NASA spending it a bit more wisely, but yeah, that's not going to happen as long as pork gets involved).

 

[intelliCom]

2 minutes ago, Beamer said:

It's always nice if NASA can hitch a ride on the defense budget, but the risk is that priorities for that sort of funding are not necessarily in line with space exploration and science

The Moon landings weren't about space exploration from the perspective of the US government either. The only time a scientist actually went to the Moon was the very last mission; Apollo 17.
To the government, it was about demonstrating the US's superiority over the Soviet Union to avoid too many countries becoming Soviet territories. That and thinly-veiled demonstrations of their ICBM technology.

But anyway, back to the main subject of this post: the technicalities of a full-flow hydrolox engine. Just how possible is it, and if it is possible, is it worth doing over conventional methods? To my own not-an-engineer knowledge, it might be possible but not particularly useful over a less complex, more conventional system.

[Beamer]

33 minutes ago, intelliCom said:

The Moon landings weren't about space exploration from the perspective of the US government either. The only time a scientist actually went to the Moon was the very last mission; Apollo 17.
To the government, it was about demonstrating the US's superiority over the Soviet Union to avoid too many countries becoming Soviet territories. That and thinly-veiled demonstrations of their ICBM technology.

But anyway, back to the main subject of this post: the technicalities of a full-flow hydrolox engine. Just how possible is it, and if it is possible, is it worth doing over conventional methods? To my own not-an-engineer knowledge, it might be possible but not particularly useful over a less complex, more conventional system.

A full flow cycle generally leads to better efficiency when done well, and better durability which is important for re-use of course. It's more complex because it requires a twin shaft turbine design, which means more points of failure (but some fuel or oxygen rich preburners use twin shaft turbines too). The efficiency comes in large part from the fact that your combustion chamber is running on a (hot) gas-gas reaction. I would guess that means the efficiency gains might be a lot lower for hydrogen than for any other fuel. Making hydrogen react efficiently with oxygen is just a lot easier than for most other rocket fuels.

This goes way beyond engineering though, it involves things like chemistry and other such magics It might mean a higher chamber pressure and/or temperature with different material requirements. It might simply be prohibitively expensive. I'm just an armchair rocket scientist, for these questions I think you need a real one

 

[RyanRising]

2 hours ago, intelliCom said:

But anyway, back to the main subject of this post: the technicalities of a full-flow hydrolox engine. Just how possible is it, and if it is possible, is it worth doing over conventional methods? To my own not-an-engineer knowledge, it might be possible but not particularly useful over a less complex, more conventional system.

The powerhead of the rocket engine is the part that differs with FFSC, all the other parts of the engine have no important differences because of it. And the powerhead of a full-flow staged combustion engine as the IPD was brought up to full power operation and otherwise succesffuly tested (https://spaceref.com/press-release/new-rocket-engine-of-the-future-goes-to-mainstage/). So it's very possible, just as much as with methane. In fact, it might be less complex to make a FFSC hydrolox engine than a dual-shaft fuel-rich staged combustion engine like the SSME because it eliminates the need for a complicated seal between a hydrogen-rich preburner turbine and the pump it drives, full of oxygen.

[wumpus]

What is missing from the Shuttle engines from that description?  I was pretty sure the were.  Something about it being relatively easier than kerolox.

[sevenperforce]

The RS-25 is very close to being a FFSC engine. After all, it has two different preburners and two different turbopumps.

The only difference is that both preburners are fuel-rich, because hydrogen's heat capacity and specific energy are both just so much greater than oxygen that running both preburners fuel-rich makes more sense.

But sure -- if you replaced the RS-25's fuel-rich preburner attached to the oxidizer turbopump with an oxygen-rich preburner, you could do it. It just wouldn't be quite as efficient because hydrogen just works better than oxygen.

 

[DDE]

17 hours ago, wumpus said:

What is missing from the Shuttle engines from that description?  I was pretty sure the were.  Something about it being relatively easier than kerolox.

 

16 hours ago, sevenperforce said:

The only difference is that both preburners are fuel-rich, because hydrogen's heat capacity and specific energy are both just so much greater than oxygen that running both preburners fuel-rich makes more sense.

That, and the US at the time didn't have much experience with oxidizer-rich cycles - Soviet achievements in that department were a bit shocking to them in the 1990s, and SSME had to be built with what was available to the US in the 1970s.

The bigger question is whether the increase in performance is worth a separate second GG/PB. RD-0120 shows that you don't need such a set-up for SSME-like performance - although it had its own surprises in the form of a concentric dual pump shaft.