Hacker News Reader

Comment by CarVac | Hacker News Reader

A full-flow staged combustion engine has more efficiency due to both propellants being fully vaporized before they mix, rather than a liquid-liquid mixture (as in a gas generator engine) or a liquid-gas mixture (as in a normal staged combustion engine).

The seals are between the turbine side (hot gas) and compressor side (liquid fuel) of the turbopump. In a full-flow staged combustion engine, you don't have to worry about accidentally getting a little of the hot gas into the liquid propellant, because that would be mixing fuel with fuel-rich gas or oxygen with oxygen-rich gas. But in a normal staged combustion engine, you have to carefully seal off the oxygen from the fuel-rich gas (in a fuel-rich staged combustion engine) or the oxygen-rich gas from the fuel (in an oxygen-rich staged combustion engine.

View on HN

No, this is all just wrong. The efficiency is because you dont throw a load of enthalpy overboard as in a gas generator. The phase is _exremely_ second order by comparison.

You’re all over this thread being wrong and confused.

Source: am a rocket engine designer.

I'm not clear what the efficiency gains are here at all over a SSME style staged combustion cycle. In both designs all propellants go through the chamber. Comparing to a gas generator cycle there are gains, sure.

It seems like the major win here is running oxygen rich on the oxygen side and fuel rich on the fuel side to reduce the chance of a leak destroying the whole vehicle.

Am I missing something?

> I'm not clear what the efficiency gains are here at all over a SSME style staged combustion cycle.

You have higher pressure in the chamber with full-flow approach. Since the mass flow is the same, and Isp is higher, thanks to higher pressure, you're winning.

And you have higher pressure because you supply more power to pumps.

Reducing risks to destroy the vehicle due to leaks is, I agree, very secondary benefit. May be even tertiary - how about possibility to optimize turbine frequencies for both propellant flows without losses on gears?

But... you don't.

The turbines in any kind of staged or full flow system run at a pressure higher than the chamber is at... I don't remember the numbers offhand but I want to say the SSME's preburners were at 8000ish psi, with a drop to 4000ish through the turbines, and finally to 3000 in the main combustion chamber. The SSME was only partially preburned, and so the flow through the turbine has to expand over a higher pressure ratio, but I still don't see a fundamental difference.

Let me explain. The principal advantage of full flow system over regular staged combustion is higher Isp of the engine.

Higher Isp of the engine is achieved because the pressure in the chamber of full flow engine is higher, than chamber pressure of staged combustion engine.

The pressure in the chamber is higher because pumps produce higher pressure.

Pumps can produce higher pressure, because with full flow pumps have more power driving them than with staged combustion. Power here is roughly equal to volume flow (m^3 / s) multiplied by pressure drop (N / m^2). Full flow scheme gives you more volume flow, since all the propellant is used to turn turbines.

So even though all the propellant in both cases is burned in the chamber, Isp is higher with full flow scheme. Rocket engine maximizes momentum of the exhaust, not the energy, which is conserved.

I was under the impression that the question was regarding FFSC versus non-FF staged combustion.

You say the main benefit is just increased pressure?

does the mass of the exhaust fuels count as well? whats the percentage of thrust being lost in a gas generator?

It definitely counts. The effect it has on exhaust velocity is maybe 5-10%, which is a big deal when you consider that has an exponential effect on your mass fraction, i.e the percentage of total lift-off mass that is fuel vs machinery and payload.

Reply on news.ycombinator.com