3 ms·
The Merlin 1-D Vacuum on the Falcon upper stage is much more powerful than the RD-0124 used on the Soyuz third stage (934 kN vs 294 kN), but uses a single huge
by mhandley 6y ago
The Merlin 1-D Vacuum on the Falcon upper stage is much more powerful than the RD-0124 used on the Soyuz third stage (934 kN vs 294 kN), but uses a single huge engine bell, whereas the Soyuz has four smaller engine bells. I would imagine this might make the Soyuz a bit smoother.
The Falcon also stages really early (this allows you to land the first stage), so the 2nd stage MVac engine is more powerful than is really ideal. As fuel burns away, I understand the MVac has to throttle down a lot to keep g-loads under control. I believe it can throttle down to something like 35%. Rocket combustion tends to get less stable at lower throttle settings, so this probably is part of what they're feeling.
- sandworm101 6y agoIirc, Soyuz also hot stages. So no dramatic zero-g cutoffs between stages.
- KuiN 6y agoYou're right. The first stage (4 conical shaped boosters hanging off the sides) and the second stage (central core booster) all fire together to lift-off. When the side-boosters are depleted they jettison (and perform the Korolev cross) while the second stage keeps firing. The third stage also starts before the second stage is completely depleted; this is why there's an "open grate" visible in photos of the rocket (about half way down the stack, see [1]), gives the exhaust somewhere to go before the previous stage is jettisoned. Hot staging is advantageous as you don't need extra ullage motors to deal with propellant being in the wrong place inside the tanks in microgravity. The Soyuz (and other R-7 variants) are brilliantly engineered; There's a reason they're still flying nearly 70 years on. [1] https://upload.wikimedia.org/wikipedia/commons/thumb/9/9a/Soyuz_TMA-9_launch.jpg/1200px-Soyuz_TMA-9_launch.jpg https://upload.wikimedia.org/wikipedia/commons/thumb/9/9a/So...
- Already__Taken 6y agoYou see a lot of that whole-system level engineering in Russian things imo.
- Udik 6y agoWhy is there a G buildup? I would love imagine acceleration to be fairly constant, not to increase with time (at least once out of the atmosphere).
- torpfactory 6y agoSame thrust but less mass as the fuel burns up.
- Udik 6y agoAh, right!
- m4rtink 6y agoAlso rocket engines are (IIRC) generally less efficient at lower than full thrust, so you want to keep firing them at max as long as possible. On the other hand you also don't want to turn your crew into pancakes or even make your upper stage look like a squashed can. You have a couple options: - throttle the engine - design payload weight, propellant load and trajectory to make g loads bearable till burnout And one more thing related to stage 2 burnout - precise orbit insertion. Rocket engines used to be pretty imprecise at shutdown, delivering a slightly lower/higher impulse at shutdown at random. Due to this many older rockets have also some small & precise engines to correct the orbit insertion once the main one shuts down - many Chinese rockets have that. As far as I know, the Falcon 9 upper stage engine (Merlin 1D Vacuum) is precise enough so this is not needed.