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Comment by mongol | Hacker News Reader

What is the benfit of hot staging?

To expand on KennyBlanken's answer:

Rockets generally want their fuel to be sitting on the bottom of the tank, where the engines are. That's easy enough when the rocket is sitting on the ground and when the engines are firing, but once the rocket starts coasting/decelerating (e.g., when the engines turn off due to reaching the end of their burn) the fuel may drift away from the fuel intakes, resulting in the engines ingesting vapor/gas the next time the engines turn on. Rocket engines are designed with a pretty specific operating environment in mind, so ingesting vapor/gas instead of fuel usually leads to the engines expressing their displeasure in a very vocal fashion.

This poses a challenge for staging. The naive way to stage is to turn off the previous stage's engines then ignite the next stage's, but the time between the first set of engines turning off and the second set of engines reaching a sufficient thrust level to keep the fuel at the bottom of the tanks may be enough for the fuel to drift away from the fuel intakes, especially if staging occurs lower in the atmosphere or after an extended coast period.

One way of addressing this issue is to use "ullage thrusters" - small rockets that maintain a small amount of forwards acceleration during staging to keep the fuel at the bottom of the tanks. This is what the Saturn rocket did between the first/second stages.

Another way is to "hot stage" - ignite the new set of engines before the old ones cut out. This is what the Soyuz does (and is why its stages are connected with a lattice - to let the exhaust out), and is what Starship was trying out this time. This can be simpler than using ullage motors since there are fewer pieces, but also poses some additional challenges in that the first stage needs to survive the second stage's exhaust for long enough.

The last way is to use RCS thrusters for a period to settle the fuel. This was used by the Saturn third stage before trans-lunar injection, but can really only be used once you're in orbit.

It also makes the rocket more efficient, because any time spent coasting with no engines lit is time that gravity is acting on the rocket and slowing it down.

By always accelerating (with engines lit), hot staging improves the payload to orbit about 10%. So it's well worth doing.

Less gravity losses is another advantage, that's true.

I think it'd be interesting to see a breakdown of that 10% improvement number. Hot staging in and of itself resulting in a 10% additional payload capacity seems large enough that I feel it's rather odd that it isn't more commonly used. I'm curious how much of it is due to "direct" improvements from reduced gravity losses and how much is due to "indirect" improvements like (maybe?) not needing to save as much fuel for boostback.

A comparison against what a "Saturn-style" staging that uses ullage motors might achieve could make for a fun addition as well.

Well, the old staging method tried in the last launch was to rotate the entire vehicle stack and "fling" the ship off. That obviously has a performance penalty in that you're not going to be pointing in the correct direction when you come off. I don't know if the 10% was compared to that or compared to a hypothetical "straight" staging with pushers, though.

I think that was not intended, it's just that the control of the rocket was lost. The plan was just a normal stage seperation, just like on the Falcon.

Given how successful our Saturn package was, it seems curious to me that we wouldn’t just follow the ullage motor method.

One needs to put success into perspective. Saturn V only launched a very small amount of times and had some near misses in that time. How successful it would have been if it had flown 100+ times is a question.

Also, If you are not reusable you have much more margin to play with. SpaceX is optimizing this thing to an incredible amount. Liftoff thrust is 2x as much as Saturn V and they are aiming at 3x as much. Total payload to orbit is 2-3x larger while being reusable.

SpaceX could have done what they did on Falcon 9 (200+ successful launches in a row) but Hot-Staging like the Soyuz is also successful.

SpaceX optimizes for long term performance and operational simplicity. Ironically that leads to more Soviet way. The N1 would also have used hot staging.

Ullage motors have to be solid - this way they don't need ullage themselves to fire - or to be fed from separate tanks - this is another reasonably complex subsystem which needs to be refueled somehow between flights. If ullage motors are solid, that needs to be re-loaded between flights. So, overall the ullage system is a certain complexity to design, build, refuel, a weight to carry in flight - clearly some drawbacks.

Hot firing simplifies things in this regard.

Ullage motors as they were on Saturn V (and in most other cases) are solid fueled. Making them incompatible with reuse. SpaceX emphasize reusable systems even when not recoverable because that allows for testing of the exact flight hardware (eg mechanical separation systems over explosive bolts).

A reusable approach would involve some form of gas thruster, so might as well just try hot staging.

Hot-staging is amazing tech. It's odd how Russia is so good at rocket science. Until Falcon, they had the highest thrust-to-weight engine, Soyuz is insanely reliable at this point, and they generally just punch above their financial weight class.

10% increase in payload

https://space.stackexchange.com/questions/64713/how-is-hot-s...

What I don't see mentioned: at the end of the day, Starship is designed to be fully and rapidly reusable. The core / the most important part of that goal is obviously to recover the 1st stage (not throw it away) and to recover it safely and with minimum delays. That's why, once operational, almost all starships will have their booster return to launch site. To return to launch site, you need to keep some of the fuel in the booster to flip it, do a boostback burn and a landing burn ( starship boosters do not need entry burns but they will need landing burns that are not suicide burns ending with hoverslams like falcon does, but basically end almost at zero velocity, hivering as the tower arms close around it).

The primary reason for hot staging is that without it, they had to turn off all engines, mechanically "push" the ship away from the booster and then light the ship engines. Say this takes 5 or 10 seconds. During this time, the entire system is not accelerating anymore, but it's still screaming away from the launch site (because to go to orbit, you don't just go "up", you mostly go "sideways", that's why you often hear "vehicle pitching down range" during broadcasts) and to get the booster back to the launch site you then have to spend more fuel to get back to the launch site - you have to counteract all that time you spend moving away from the launch site.

With hot staging you get to the velocity you need to get the first stage to orbit sooner, because the system never stops "pushing", and there's less distance you need to cover to get the first stage back to the launch site. With means you need less fuel that remains in the first stage after stage separation, which means you can use more fuel during the first stage firing (and therefore put more mass to orbit).

Less time spent not firing engines, no need for separation hardware (e.g. hydraulic pushers), no need for ullage thrusters (settling fuel before lighting stage 2).

Maintaining velocity / acceleration.

Their previous method didn't work at all.

Fantastically cool explosion

You never stop accelerating. Escape faster. Less time fighting gravity.

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