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Comment by shkkmo | Hacker News Reader

That was the original end plan, but AFAIK it was mixed a couple of years ago.

“The reason we decided not to pursue (powered landings) heavily is it would have taken a tremendous amount of effort to qualify that for safety, particularly for crew transport,” Musk said. “And then there was a time when I thought that the Dragon approach to landing on Mars, where you’ve got a base heat shield and side-mounted thrusters, would be the right way to land on Mars, but now I’m pretty confident that is not the right way, and that there’s a far better approach.” [0]

[0] https://spaceflightnow.com/2017/07/19/propulsive-landings-ni...

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To be clear, propulsively landing after an abort was never planned -- you've already used up the fuel doing the abort. Plus, depending on when you abort, you could be half-way across the Atlantic.

Do you know if they had a path for redundancy for a propulsive abort? With 4 parachutes, they likely have the design able to handle a single 'chute failure. Not sure what would be involved with single failures on the propulsive abort but that may have been a large contributor to go with the required return fuel.

The abort is the redundancy... ;-) I'm not sure what specifically you are thinking about, but in general since an abort is already one critical fault away from the nominal case, I believe the requirements for redundancy are lower.

Just thinking this through, because I'm more familiar with airplanes than manned rockets:

The design has 4 parachutes, likely for packaging and design constraints, but also provide some level of redundancy.

My comment wasn't clear now that I re-read it, but if you use a propulsive landing after a propulsive abort, you'd need additional fuel. If you remove the parachutes, that weight could then be used for landing fuel, but fuel weight may still exceed that. I haven't run the numbers.

The aborted dragon, after leaving Falcon, is now a self-contained vehicle carrying multiple people. It is a redundant and failsafe safety part of the entire system, but it's hard for me to believe it would be designed with everything having to work perfectly after an abort in order to safely return the occupants. Guidance and Control, life support, propulsion...

I'm mainly wondering if you were aware of the systems they would have in place on a propulsive return.

It seems that the amount of fuel to return all the way downrange back to land on a propulsive abort would be more than the parachute weight, which forces a water landing. And since you already design the capsule for carrying those parachutes, you might as well use them on a normal landing as well. That cuts down on additional weight for an abort return. I also don't know that if you have a successful launch, that you have enough reserve from un-used launch abort fuel to then land the capsule without chutes. Seems unlikely, especially with needing failsafe/redundancy.

The Superdracos are powerful engines but they don't have anything near the delta-v to always return to land after an abort even if you fill the entire Dragon with propellant. So you'd always need the chutes anyway.

One additional issue with propulsive lsnding is that you need legs, and they need to penetrate the heat shield. This significantly complicates and weakens an absolutely critical part.

> The Superdracos are powerful engines but they don't have anything near the delta-v to always return to land after an abort even if you fill the entire Dragon with propellant. So you'd always need the chutes anyway.

Not sure that's true. The chutes bring the speed of the capsule down quite a bit, just a short burst of the dracos at the last few feet could allow it to softly land and wouldn't require much propellant. This is exactly what the Soyuz does. My guess is that this approach was rejected by NASA (not SpaceX) due to complexity rather than feasibility.

This is what's referred to in some circles as a "suicide burn"; in that you wait until the absolute last second, then ignite the engines at full throttle to zero out velocity. Assuming the engines work, it's very efficent as you're not accruing what's called "gravity drag" as the velocity goes to zero very rapidly and you're not fighting 9.8m/s acceleration.

If the engines don't start, or you don't get the predicted thrust out of them, well, that's why it's called a suicide burn.

> If the engines don't start, or you don't get the predicted thrust out of them, well, that's why it's called a suicide burn.

Fair enough, but this is exactly what the Soyuz has done for decades, and NASA (as well as Roscosmos) found the approach to be acceptable for human spaceflight. It may introduce complications, but it's clear that it's doable.

The propulsion is pairs of engines for redundancy, but, as people keep on saying, abort was always going to be parachutes.

Do you happen to know how long the crew is safe to be out in the middle of the Atlantic if it’s a particularly non optimal landing location?

Thats why you have the team who tracks the capsule by plane, they jump out with a raft that can survive several days on rough waters. The capsule is pretty sturdy I assume to stay in for some time.

Scott Manley's latest video ( https://www.youtube.com/watch?v=R-HOQrinzlY ) says the capsule should be able to survive in the water for at least 24 hours at minimum, but probably longer.

Okay that is pretty damned cool. Thanks!

The person I was responding to appeared to be talking about landings in general, but yes, you are correct.

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