Wiki SpaceX as a Company - General Discussion

[scaesare]

Well, with SpaceX, you get to approach a safe rocket, but then have to sit in it during propellant loading. I know of no accidents for a loaded rocket on the pad, but SpaceX had one accident on the pad during propellant loading. SpaceX has their approach because of the use of superchilled propellants, resulting in a "load and go" launch.

A Crew Dragon has a launch abort system which might get the capsule clear in case of an accident, but Starship presents some real challenges.

It was a throwaway comment for the lolz... but yeah. Still think it sends a funny message to travel in an explosive-resistant vehicle in order to climb aboard the bomb.. regardless of who makes it.

 

[ecarfan]

A Crew Dragon has a launch abort system which might get the capsule clear in case of an accident, but Starship presents some real challenges.

Yes, I’ve been wondering about that as well. Raptors can start producing a lot of thrust in a fraction of a second like the hypergolics on Dragon. Do you think it would be possible for a crewed Starship to keep its 3 sea level Raptors in a sort of “spin prime” condition during the first minute or so of flight; ready but not ignited?

 

[Ben W]

Yes, I’ve been wondering about that as well. Raptors can start producing a lot of thrust in a fraction of a second like the hypergolics on Dragon. Do you think it would be possible for a crewed Starship to keep its 3 sea level Raptors in a sort of “spin prime” condition during the first minute or so of flight; ready but not ignited?

For a launch pad abort, it would probably need the thrust of all nine Raptors. (Assuming Crew Starship will use the 9-Raptor configuration.) I’m not sure what happens when vacuum Raptors are fired at full power at sea level; maybe part of their bells could detach in such a scenario to make them more sea-level-like?

Even if this could allow Starship to get away from a low-altitude booster explosion or failure, it still sounds mega-dicey. Starship would then have to climb to a high enough altitude to burn off most of its fuel, perform a bellyflop, then perform the landing flip maneuver to be caught by the launch tower. And if this abort capability does exist, will NASA require it to be tested/proven? Now THAT will be an interesting flight test!

 

[ecarfan]

Starship would then have to climb to a high enough altitude to burn off most of its fuel, perform a bellyflop, then perform the landing flip maneuver to be caught by the launch tower.

So the ship would have to do a boost back burn and then a landing burn to hover for the chopsticks to do their job. Quite a thrilling experience for the passengers.

 

[JB47394]

One difficulty with the Raptors as the launch abort system is that the vehicle would have to be maintained at launch status. Probably off the header tanks. So they'd load those first, pressurize them, chill the engines, and then perform normal propellant load. They'd have to be careful about how many engines they fired, depending on the propellant load.

If anything happens during load of the header tanks, well...

Then there's the issue of thrust-weight ratio once fully prop-loaded. A Starship is going to mass perhaps 1500 tons by the time they're up to the nine-engine variant, and that's less than a 2:1 thrust:weight ratio. Is that a viable abort level?

I’m not sure what happens when vacuum Raptors are fired at full power at sea level; maybe part of their bells could detach in such a scenario to make them more sea-level-like?

They could go with an expanding nozzle. It would allow the engines to be fired as sea level or vacuum variants. Here's an image from the Pratt & Whitney XLR-129 engine - which was never built. There are other altitude-compensating nozzles, but I'm not sure any of them could be applied to Raptors.

To be honest, I'd go with a completely different structure for this Starship. I'd stack the crew module on top of a Starship thrust section, and include an abort system in that crew module. The only purpose to this vehicle is to get a lot of people to LEO. Once on orbit, they can transfer to a long duration Starship or any other vehicle appropriate to their mission.

So this vehicle becomes a LEO Shuttle. This variant would sacrifice some mass to the launch abort system, but who cares? The only purpose of the vehicle would be to get the crew to orbit safely.

Another alternative is to give each crewman an ejection capsule. Again, the only purpose of this vehicle is to get crew to LEO safely. They can generously apportion that 100-150 ton capacity to the capsules. Done right, those capsules could become a standard part of any orbital vehicle. A little delta-V (solid rocket motor, unpleasant but effective), a little life support, heat shield, cross-range and parachute landing. Or perhaps even a winged belly landing. Single use things. The downside is that if there are 100 capsules with 10 onboard systems each, that's 1000 points of failure that could take a life - or cause an emergency on their own. The design would have to be brain-dead simple and foolproof.

 

[Ben W]

One difficulty with the Raptors as the launch abort system is that the vehicle would have to be maintained at launch status. Probably off the header tanks. So they'd load those first, pressurize them, chill the engines, and then perform normal propellant load. They'd have to be careful about how many engines they fired, depending on the propellant load.

If anything happens during load of the header tanks, well...

My understanding is that the Falcon 9 launch explosion was due to an issue specific to the composite fuel tanks, which is a failure mode that may not apply to Starship. To the extent the Starship / SH fueling process is "safer" than Falcon 9, it may be a good enough compromise to chill the Starship engines starting just before launch, but not for the entire fueling duration.

Then there's the issue of thrust-weight ratio once fully prop-loaded. A Starship is going to mass perhaps 1500 tons by the time they're up to the nine-engine variant, and that's less than a 2:1 thrust:weight ratio. Is that a viable abort level?

I would imagine it could be. Perhaps Crew Starship will intentionally have less fuel or payload in order to achieve a higher T:W ratio for abort, at least for the first several crewed flights? The bulkhead cap that provides protection to the booster during hot staging may also provide temporary protection for Starship in case of a booster malfunction/explosion.

They could go with an expanding nozzle. It would allow the engines to be fired as sea level or vacuum variants. Here's an image from the Pratt & Whitney XLR-129 engine - which was never built. There are other altitude-compensating nozzles, but I'm not sure any of them could be applied to Raptors.

That's really interesting! Or maybe Crew Starship will go with a compromise approach like the "short bell" Falcon upper stage seen on some flights? A small enough bell to be functional (though not optimal) at sea level, while large enough to provide some advantages in vacuum. Maybe that's how they plan to fit nine of them into the upper stage?

To be honest, I'd go with a completely different structure for this Starship. I'd stack the crew module on top of a Starship thrust section, and include an abort system in that crew module. The only purpose to this vehicle is to get a lot of people to LEO. Once on orbit, they can transfer to a long duration Starship or any other vehicle appropriate to their mission.

So this vehicle becomes a LEO Shuttle. This variant would sacrifice some mass to the launch abort system, but who cares? The only purpose of the vehicle would be to get the crew to orbit safely.

This would be really difficult to design I think, while retaining full reusability of both Starship and capsule. Do you envision that the capsule would separate from Starship on every flight, or only in emergencies? To have fast-abort capability, the capsule would need a bunch of external thrusters pointing roughly back at Starship (to get the craft away as quickly as possible), and it might be hard to geometrically do this without having the capsule/Starship interface go through Starship's heat shield. It's a tough problem.

Another alternative is to give each crewman an ejection capsule. Again, the only purpose of this vehicle is to get crew to LEO safely. They can generously apportion that 100-150 ton capacity to the capsules. Done right, those capsules could become a standard part of any orbital vehicle. A little delta-V (solid rocket motor, unpleasant but effective), a little life support, heat shield, cross-range and parachute landing. Or perhaps even a winged belly landing. Single use things. The downside is that if there are 100 capsules with 10 onboard systems each, that's 1000 points of failure that could take a life - or cause an emergency on their own. The design would have to be brain-dead simple and foolproof.

I can't imagine this being the best design; it may compromise the vehicle's external structure to have 100 individual "ejection ports", and how do you prevent 100 independent capsules from slamming into each other when simultaneously ejected from an exploding rocket? (Noted that any survivors is probably a better outcome than no survivors.)

In any case, I do expect that SpaceX's ultimate design for crewed Starship (which should have 99.999% reliability/survivability on launch, especially if EP2P becomes a thing and transports hundreds of thousands of passengers yearly) may be quite different than for payload Starship (99.5% reliability probably good enough).

 

[scaesare]

They could go with an expanding nozzle. It would allow the engines to be fired as sea level or vacuum variants. Here's an image from the Pratt & Whitney XLR-129 engine - which was never built. There are other altitude-compensating nozzles, but I'm not sure any of them could be applied to Raptors.

Oh cool.. I had thought about something like that... didn't know it had been explored.... would be cool to see that work...

 

[scaesare]

My understanding is that the Falcon 9 launch explosion was due to an issue specific to the composite fuel tanks, which is a failure mode that may not apply to Starship. To the extent the Starship / SH fueling process is "safer" than Falcon 9, it may be a good enough compromise to chill the Starship engines starting just before launch, but not for the entire fueling duration.

Specifically, it's believed that liquid (or potentially solid O2 particles) got in to the voids under the carbon fibers of a COPV inside the LOX tank... as the helium tank was pressurized the metal walls of the tank were pushed out, pushing the O2 against the carbon fibers, causing enough pressure to generate heat or a spark that set off the explosion.

I remember thinking how bizarre that was in those temps...

 

[JB47394]

Do you envision that the capsule would separate from Starship on every flight, or only in emergencies?

Only for emergencies.

To have fast-abort capability, the capsule would need a bunch of external thrusters pointing roughly back at Starship (to get the craft away as quickly as possible), and it might be hard to geometrically do this without having the capsule/Starship interface go through Starship's heat shield. It's a tough problem.

Do the same thing as with Crew Dragon; place engines around the base and point them down the side of the stage below. I'm not trying to avoid heft in this system because the goal is safe ascent. We don't really care if we aren't being efficient because people aren't cargo.

how do you prevent 100 independent capsules from slamming into each other when simultaneously ejected from an exploding rocket?

They'd be angled out and up, with each tier slightly delayed in firing (i.e. milliseconds). I'm picturing a kind of expanding petal arrangement, repeating for each tier. If they hit each other, it shouldn't be particularly high energy because they have a low relative velocity. Besides, there's no reason why they couldn't have onboard radars and guidance to keep the capsules away from each other. Note that the tower side of the vehicle would have no capsules.

My greatest concern would be loss of structural integrity as the capsules try to leave, but if the vehicle is losing structural integrity, trying to use it to keep the crew safe seems unwise.

 

[Eka A]

My daughter is a contract manager on top secret government contracts and is familiar with this subject. She says that as SpaceX holds top secret contracts, they are not allowed to hire non citizen, non green card employees. It is written into those contracts. This is either one hand not knowing what the other is doing or politics. Her company, a large contractor, is subject to the same requirements.

I've had a few DoD contracts, and this was the way it was. I was a small contractor with less than 15 employees.