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Comment by whoisthemachine | Hacker News Reader

While the tried and true method of splashing down in the ocean seems like a reasonable first effort for landing the dragon spacecraft, watching the difficulty the boats have in reaching the spacecraft (the hosts of the stream mentioned it takes them 2 hours!), and the difficulty the SpaceX team had in getting reasonable weather conditions for this test, it seems to me that a propulsive land-based landing would still be a reasonable future improvement to pursue.

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Boeing's Starliner lands on solid ground after a normal mission while SpaceX's Dragon lands in the ocean. But that's not relevant to an in-flight abort. Any abort at this stage in the launch (whether Boeing or SpaceX) has to come down over water. That's simply where the trajectory of the launch goes.

As I replied elsewhere, for an abort, a water landing definitely makes sense (and is likely the only reasonable option). I was merely commenting on how drawn out the recovery process is after splashdown, which I would assume would be less time-consuming with a land landing.

In the introduction of the space-x launch broadcast, they mention that in real aborts, military will be involved. With helicopters, rescue personnel will be dropped of.

Also there, it still takes a while for boats to arrive.

Your odds of hitting something hard are much higher on land. Or smooshing someone.

Was crew dragons plan back with normal propulsive landings to always fly back to ground and just load more fuel?

My understanding is, that the Nasa didn't like the idea of propulsive landing. As the dragon capsule is purely for supporting the ISS, SpaceX has no incentive to spend money on anything they are not using for the Nasa missions. For everything else, they are developing the Starship. If that program goes as planned, the dragon capsule will be used only for a short time, I guess.

> If that program goes as planned, the dragon capsule will be used only for a short time, I guess.

I can't see Starship being approved for Nasa use by 2024 even if it were flying today.

I believe SpaceX isn't working with NASA to get Starship human rated by them. I think they are trying to do it 100% private, partially so they don't need to prove it to NASA's standards.

Why not? It didn't take that long to certify Dragon, and most of the delays were on SpaceX (like blowing up the capsule they were originally intending to use for this test).

That would require significantly more fuel and a different design, likely limiting the Dragon's crew payload.

This abort happened after the rocket reached max Q (highest dynamic pressure) when it was flying East (and still upward, but to get to orbit you must fly sideways over water). Returning West to get back over land would be fuel prohibitive on a fully loaded crew capsule.

The reusable falcon 9 stages have almost nothing in them when they fly back to land.

See trajectories: https://i.redd.it/xaisqxao5ef01.png

For an abort, a water landing definitely makes sense. Perhaps there is no way to avoid the tough weather requirements for the downrange abort. The lengthy recovery time still seems due to the water landing, however. I wonder if it will be a 2 hour recovery time during normal water landing?

I still wonder how much more difficult it is to get proper weather conditions over open sea vs. land when preparing to land during a normal flight. I'd imagine strong winds become stronger over sea than over land, and of course, the ability for ships to reach you in choppy seas becomes much more difficult.

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...

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.

I think you misunderstood the announcer - the recovery takes 2 hours, but that's not how long it takes the boats to get to the capsule at its splashdown location.

The boats get there pretty quickly.

The time required is for safety checks and so on - there's some pretty nasty materials involved in those thruster engines, and all that stuff has to be checked first.

In addition to this. The Air Force has personnel that can jump out of the plane with flotation, medical equipment and food in order to survive for an extended period of time on the ocean while it may take a few hours to get fast boats and recovery teams there.

Interesting, so given this do you think that a normal landing would not have as long of a recovery time, given that it won't use those thruster engines?

For a normal landing, there’s an extra step of hoisting the entire capsule onto the ship before opening the hatch.

it seems to me that a propulsive land-based landing would still be a reasonable future improvement to pursue.

They're working on a second stage that can carry a few more astronauts and land itself, it's a bit bigger though ;)

https://www.spacex.com/starship

Yes, I'm well aware of that! Hopefully, they get that flying so soon that pursuing more advanced landing methods for the dragon is not needed :)

NASA didn't require it. There was an early plan at SpaceX to have Crew Dragon land propulsivly, but they didn't pursue it in favor of developing Starship and SuperHeavy faster. That's what Musk said in an interview some time ago.

I remember that. It still seems to make sense to research propulsive landings and gain NASA's approval for them eventually (while maintaining the well-proven water landing capability) if it would cut costs significantly.

From what I remember, after Dragon2 is approved by NASA, all development efforts of SpaceX will be focused on Starship and Super Heavy only.

Dragon2 is no use in the efforts to build a city on Mars. And even for a Moon colony, its too small. Its just there to earn money from flying people to the ISS and back. And for that its already good enough.

Losing science return cargo from a landing failure during testing is a no-no.

> watching the difficulty the boats have in reaching the spacecraft (the hosts of the stream mentioned it takes them 2 hours!)

From my understanding, the host meant that getting the dragon and crew back to the shore would take 2 hours, not the recovery boat to get to them. They could probably even have helicopters around the area which could fly and pick the dragon up.

Interestingly, in the press conference, Musk mentioned that SpaceX would like to try to catch spacecraft, which I would think would reduce some of the water landing costs.

If you were the astronaut and it was an emergency that triggered the abort (e.g. the launch vehicle blew up like the Falcon 9 just did), would you really prefer the Dragon tried to land instead of at sea?

Keep in mind it wouldn't be able to choose its landing site either.

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