[panick21_]

> NASA nixed the idea of propulsive landing for Dragon 2 for this reason.

That is completely false. First of all, NASA didn't nix it, they just didn't make it a priority as it had little value from their perspective.

The reason it was not done is that para-shouts have to be in the design anyway for abort situations, so that was fixed.

So for SpaceX, the question was to likely delay the program, and take on a whole lot of extra engineering work that they were not actually getting paid for, remember fixed price contract.

They were only going to work on it if they really thought they needed it for something like Red Dragon. And then they could still add it later.

And one of the primary reason SpaceX thought that its to hard, is that they landing feet would have to have gone threw the heat shield. That would have made the whole heat-shield design massively more complex.

[wat10000]

Some dude who runs SpaceX seems to think the reason was safety. https://x.com/elonmusk/status/1104509345922838528

[somenameforme]

He said it was the difficulty in proving the safety. There's an informative article here. [1]

NASA likes parachutes because they've always used parachutes. SpaceX likes retropulsive landings because Mars is their goal, and Mars' atmosphere isn't dense enough for parachutes. It's also safer for the crew in nominal operation and enables a much higher degree of rapid reuse, relative to NASA's traditional operation of taking a salt water bath in the ocean.

So they could go through the [very reasonable] extensive costs and testing involved in proving the safety of the retropulsive landings, or just go old school, strap a few parachutes on and work on getting crew to the ISS (which was the goal at the time). They chose the latter and with the plan of getting back to retropulsive landings later, which they also did. Parachutes remain the main landing mechanism for the Crew Dragon, but it now also has retropulsive landing capabilities to be used in case of a chute failure.

[1] - https://www.nasaspaceflight.com/2024/10/dragon-propulsive-la...

[wat10000]

In what sense can it be called safe if you haven’t proven it?

[somenameforme]

They had repeated successful demos of it, but NASA kept adding on new requirements while implicitly signaling that they had no interest in approving the system, which would have made Boeing's lander look obsolete before it was ever finished. NASA's judgements are heavily influenced by external factors that make it quite difficult on outsiders, while enabling reckless behaviors for insiders.

For example NASA deemed the Boeing crew vessel safe after its pad abort test resulted in only 2 of 3 parachutes deploying and it suffering a propellant leak - all in beyond optimal conditions. They deemed it not only safe, but safe enough to completely skip the scheduled in-flight abort test. All of this is of course how you ended up with astronauts trapped on the ISS that had to be rescued by SpaceX.

For another contrast there after SpaceX did swap over to a simple parachute system, their pad-abort test went off flawlessly. NASA still required they do an in-flight abort. Granted, that's nothing to complain about, because that's exactly what NASA should do. But they also should have had Boeing completely redo their pad-abort test and damned sure do an in-flight abort as well. Safety culture at NASA is generally completely dysfunctional because of non-safety factors.

This is nothing new either. Both Space Shuttle disasters were 100% preventable, and not only in hindsight. Engineers brought up the exact causes of both explosions well before they happened, but the bureaucratic layer ignored them.

[wat10000]

I'm not entirely understanding your point. Are you saying they were able to demonstrate a better than 1-in-N probability of fatal mishap for the appropriate N (I believe about 300 for this case) and NASA just wouldn't accept it? Or they weren't, but ???

Shuttle is an excellent example of the sort of thing I'm talking about. It had no abort capability in the event that something went kaboom, and no realistic abort capability at all for large critical portions of launch. Their test pilot outright refused to test an abort because he didn't think it would be survivable. It never should have been human-rated, and it only was because NASA pretended it had an abort capability that wasn't really there.

Starship is even worse: not only does it have no realistic abort capability for most of the launch, it also has a very delicate landing procedure that requires a substantial amount of propellant to remain on board, no ability for the occupants to escape in the event that those propellants decide to mix in a place where they're not supposed to, and very limited ability to handle engine failures.

[somenameforme]

There is no exact and objective set of hoops one can jump through to prove a sufficiently complex (let alone novel) technology safe within a certain bounds, short of doing exactly what it will be doing over a large sample, which is often not economically feasible. In fact one of the first things that happened early on in the Apollo program is that mathematical risk modeling was completely scrapped. The results were always so pessimistic that NASA found it impossible to move forward with it!

So this leads to judgement calls from NASA that are opaque and, in practice, are not necessarily grounded in safety, as per your own example as well. NASA clearly did not want SpaceX doing propulsive landings and was making sure to dot all their i's and cross their t's with them, while simultaneously going YOLO with Boeing and actively greenlighting their vessel which clearly was not even remotely safe for a human. In this context, it's highly unlikely SpaceX could have convinced NASA to more forward with the propulsive landings, even if they were the safest thing ever invented.

[panick21_]

This literally doesn't disagree with what I said. I have no idea why you think it does.

[wat10000]

Reasons you gave: it wasn’t necessary, it would have been expensive, it would have added time and complexity.

Reason Musk gave: safety.

[panick21_]

Do you not understand anything about aerospace engineering? These concepts are not some independent variables that are unconnected with each other.

Maybe I have to spell it out for the slow ones.

it would have added time and complexity --> because proving the safety of additional system takes TIME.

it would have been expensive --> TIME is money

it wasn’t necessary --> The parachutes system was mandatory anyway.

[wat10000]

I don't understand much about aerospace engineering, but adding time, complexity, and expense sure sounds a lot like "extremely difficult to make safe" to me.

I guess people just want to argue. I have three different people replying to me effectively saying, "you're wrong, it's not because it's difficult to make it safe, it's because it was difficult to make it safe because of reasons X Y and Z."

[panick21_]

I explained what the issue was in technical terms, as I was following these issues in real time as it was developed and what communication from NASA and SpaceX was at the time. Then you made a smart-ass comment about how I was wrong and it was actually about safety. That wasn't incorrect if you want a one word answer but it also doesn't explain the 'why', and that's what I did.

[swores]

It's possible to disagree with people without being an asshole about it. It's even encouraged in this site's guidelines :)