Rocket Throttle

[dimovski]

Hello guys, could anyone please, pretty please, tell me how much the RD-108 sustainer engine of the Soyuz was throttled down at liftoff?That would be really helpful for me!

[Kryten]

I can't find a single source that says the RD-107/8 is even capable of throttling.

[dimovski]

Really?Because Im 99% sure the sustainer was throttled down at liftoff and achieved 100% throttle after booster seperation?Or am I wrong?

[Banbite]

I don't think lifter engines are capable of throttling, just adds complexity. The only engines I know which has this capability is the ones on landers where the control is needed.

Edited May 28, 2013 by Banbite

[Rune]

A lot of engines have some amount of available throttle (even if it can only be done by physically changing parts in the engine with it disassembled). The thing is, deep throttle (going substantially below 70-80% max thrust) is very difficult and pretty much awful for efficiency (can't really change the expansion ratio, etc..), so it is pretty rare. I believe the SSME's could go from 70% to 110% of rated thrust, so you can make yourself an idea.

In the specific case of the Soyuz booster, I think you are confusing T/W ratio with throttle. See, that is why it uses boosters: at launch, the core stage can't lift its own weight and that of the payload. During the burn, however, it gets lighter, and so by the time the boosters run out of fuel, it has a T/W greater than one. It's a good trick to manage accelerations without throttling or turning off engines. Sometimes, accelerations at the end of the burn can be dangerously high. Falcon 9, for example, turn off several engines prior to second stage ignition, and the Mvac can throttle quite a bit.

Rune. I might be a little fuzzy on the exact numbers, but a quick wiki away are all the explanations you might ever want.

[dimovski]

Thanks, I tried doing an exact rescale in mass (fuel,empty,etc.),thrust etc. of Soyuz parts, and after Block B,V,G,D seperate, its TWR is 1...thats kinda bad if youre still not in the 30-40km height range...

[Rune]

Thanks, I tried doing an exact rescale in mass (fuel,empty,etc.),thrust etc. of Soyuz parts, and after Block B,V,G,D seperate, its TWR is 1...thats kinda bad if youre still not in the 30-40km height range...

I think that is why it's called sustainer engine. Have in mind than real-life rockets often lift off at ridiculously low T/W (the Saturn V was 1.18). Earth's atmosphere is also thicker (more time on ascent before turning), and unlike in KSP, RL rockets actually can create lift with their bodies.

Rune. And have much higher fuel fractions, too.

[magnemoe]

A lot of engines have some amount of available throttle (even if it can only be done by physically changing parts in the engine with it disassembled). The thing is, deep throttle (going substantially below 70-80% max thrust) is very difficult and pretty much awful for efficiency (can't really change the expansion ratio, etc..), so it is pretty rare. I believe the SSME's could go from 70% to 110% of rated thrust, so you can make yourself an idea.

In the specific case of the Soyuz booster, I think you are confusing T/W ratio with throttle. See, that is why it uses boosters: at launch, the core stage can't lift its own weight and that of the payload. During the burn, however, it gets lighter, and so by the time the boosters run out of fuel, it has a T/W greater than one. It's a good trick to manage accelerations without throttling or turning off engines. Sometimes, accelerations at the end of the burn can be dangerously high. Falcon 9, for example, turn off several engines prior to second stage ignition, and the Mvac can throttle quite a bit.

Rune. I might be a little fuzzy on the exact numbers, but a quick wiki away are all the explanations you might ever want.

Makes sense, has done the same in KSP and buckets of trashcans to get up to terminal speed fast with low TWR launchers.

[TheShadow1138]

I feel I should also mention that as the atmospheric pressure decreases with altitude the thrust the engine provides will increase as the exhaust gases will expand more at altitude than at sea level. This is why engine thrust or ISP numbers are usually given at sea level and in a vacuum. Also, throttling of a kind at least can be achieved by altering propellant ratios, as was done on the Saturn V second (S-II) stage during its burn.