Some thoughts on the Proton rocket and its engines

[wb99999999]

Apparently the Proton rocket have some of the highest TWR engine in the world. Its first stage RD-253/275 engine is the second highest TWR in any rocket engine in the world, only beaten by SpaceX's brand new(that is, compared to the Cold War era design used in Proton engines) Merlin engines. The second stage engine, RD-0210, also has a very high TWR of more than 100.

From my point of view, these engines are extremely powerful (per mass) and highly reliable, yet relatively simple and inexpensive. I understand that hydrolox engines like the SSME and J-2 can only achieve about 60 to 70 TWR because the complexity involved with the very low density hydrogen fuel and the huge difference in density between LH2 and LOX; however in the Kerolox front, it still took some radical design choices and technological innovations to produce engines of similar TWR, NK-15 and Merlin respectively.

I speculate that hypergolic propellant must played a role in this situation. Having your fuel and oxidizer spontaneously combust must simplified a good portion of the machineries in the engine to reduce mass/increase power significantly...but I have no proof for this. Can you guys give me some insight on this?

[K^2]

Not an expert, but I would guess that the biggest simplification is in the pump mechanism. These rockets use your basic fuel-rich staged combustion setup, and hypergolics make it almost laughably simple. No starter mechanism required. You just open the valve, the tank pressure forces a bit of fuel and oxidizer into pre-burner, and that's enough to start spinning the turbine, ramping up the pressure at which the fuel and oxidizer are fed until you have proper flow into the main combustion chamber.

The other simplification is the fact that both fuel and oxidizer are liquids at room temperature with just a touch of pressure in the tanks. This means you don't need to have any kind of insulation or low-temperature materials involved. All in all, nitrogen tetroxide with some kind of hydrazine fuel allow for very simple, very light engines.

The drawbacks, of course, are that both are quite toxic, and between that and aforementioned hypergolic quality, failures tend to be extra dangerous even by rocketry standards.

[Kryten]

Are you sure the T/W ratios you're comparing are like-to-like? e.g. the standard RD-180 ratio includes the gimbal mechanism, and the very high one for Merlin does not. I would not expect RD-253/275/276 to have higher T/W than RD-180 in a like-to-like comparison, given the higher chamber pressure on the latter.

8 hours ago, K^2 said:

These rockets use your basic fuel-rich staged combustion setup, and hypergolics make it almost laughably simple.

The engines on Proton are ORSC, not fuel-rich.

[sevenperforce]

Another issue is the propellant density. If you take an engine capable of running on multiple propellants (like an NTR or some variable-mixture tripropellant engines), increasing the density of the propellants while maintaining the same fuel flow increases thrust and decreases specific impulse. Hypergolic fuels are much higher-density than hydrocarbon fuels, and nitrogen tetraoxide is slightly higher-density than LOX, IIRC.

[kerbiloid]

On 12.05.2017 at 5:50 PM, Kryten said:

standard RD-180 ratio includes the gimbal mechanism

Proton first stage 253s are gimballed, too. This allowed to replace the original 4+4 with modern 6.