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Comment by arijun | Hacker News Reader

It's too bad NASA didn't let SpaceX test propulsive landing with the cargo Dragons. Without that, SpaceX scrapped the feature from the Dragon program as a whole. It seems like they might have been able to get more reliability with that than the parachutes. And even if not, if I was sitting in a capsule that had a history of parachute failures, I'd feel a whole lot safer if there was a backup propulsive system ready to kick in in the event of a chute failure.

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They would still have had to work on parachutes because a Pad Abort uses up all the propellant... leading to a parachute landing just off the beach at the Cape.

They didn't scrap it exactly, they just relegated its function to use as an emergency propulsive abort system. But the Dragon 2 still has the super dracos... and one of them exploded, causing the loss of a test vehicle, a few months ago, which has resulted in timeline setbacks to the program. It doesn't seem like reliability has been a walk in the park there either.

The super draco didn't explode fyi, it was the fueling system.

An interesting distinction, like saying "the engine didn't explode, the gas tank did" but one wouldn't be there except for the other. I suppose for properly fixing the problem it's important to say one vs the other, but in general conversation it could come off as a pedantic?

Well, yes. Pedantic as it may be, the fueling systems seem to be a recurring theme for SpaceX, not their motors. Remember Amos-6?

Both failures may have had something to do with "fuel", but they aren't in any way related.

The Crew Dragon abort test anomaly was caused by a problematic valve leaking oxidizer "backwards" into the helium tubes which are used to pressurize the system right before ignition. The second they pressurized, it sent a huge amount of oxidizer into the engine where there should have only been helium at the time which caused the explosion.

The AMOS-6 anomaly was caused by LOX pooling and working it's way into the liner of a COPV inside the LOX tank. The LOX got into the fibers, and a small amount of friction ignited the oxygen and caused the "boom".

They may seem similar in that they are both related to the oxidizers getting somewhere they shouldn't and igniting, but a rocket is pretty much just a tube of carefully controlled fuel and oxidizer and a nozzle to shoot it out.

Of course most problems are probably going to result in fuel or oxidizer getting somewhere it shouldn't, and when that happens they are going to explode. For laypeople the exact reason why the fuel and oxidizer mixed at the wrong time isn't really important, but don't fall into the trap of thinking that the 2 are related.

Well an engine explosion and a gas tank explosion are very difference failure modes, and in rockets, pedantics matter.

If I remember correctly, SpaceX totally could have gone for propulsive landing, but opted not to because certification would have been a lot more work.

I think it basically came down to NASA not wanting them to test it on NASA flights.

SpaceX would have had to do their own launches and landings to verify the propulsive landing system, but they wanted to test it out on "production" cargo missions they were already doing for NASA.

Wouldn't dragon need the rockets to land on Mars? Parachutes aren't gonna do jack in Mars atmosphere.

I thought that was the bigger reason for the retro rockets.

SpaceX has no plans to send Dragon to mars

SpaceX had plans to send Dragon to mars, that were cancelled at about the same time propulsive landing was cancelled for commercial crew. It's likely that cancelling it for commercial crew also cancelled the mars plans.

I think they cancelled it because it was a hair brained scheme from the get go. You need a lot of juice to not only land on mars, but then to propulse back off of it.

Red Dragon was always going to be one way, as a lander for some notional science payload. It was still a bit harebrained because it wasn't clear how to deploy that payload, whether rover or other, through the hatch. (I think I once saw a slide that had a subsurface drill deployed from inside the capsule by drilling through the heat shield.)

If propulsive landing was a harebrained scheme, we'd have never had the lunar landings.

It takes ~3,000 m/s of dV to slow down, and do a powered landing on the Moon.

It takes takes ~6.400 m/s of dV to slow down, and do a powered landing on Mars.

Thanks to the tyranny of the rocket equation, it takes ~5x more fuel to do a powered landing on Mars, than it does on the Moon. For the entire mission. That's five Saturn V rockets, if you want to get a similar payload to Mars and back.

Shaving most of the dV requirements of the 'land on Mars' leg of the journey will reduce that requirement, from ~5x to ~2x.

> It takes ~3,000 m/s of dV to slow down, and do a powered landing on the Moon.

With generous reserves, I guess. The speed on a low Moon orbit is about 1700 m/s, Apollo's LEMs had about (less than?) 2500 m/s delta-V for landing.

> It takes takes ~6.400 m/s of dV to slow down, and do a powered landing on Mars.

Seems too generous. Escape velocity for Mars is 5.027 km/s, so low orbital speed is about 3.6 km/s; more importantly, there is atmosphere - which maybe too thin to land safely on parachutes, but it definitely is going to reduce speed from orbital to some extent.

As I understand the numbers something like 99.9% of that 6400 m/s dV is done via aerobreaking. Which makes it actually much easier to land on mars than on the moon energy wise.

The falcon heavy and dragon were almost definitely up to the task, given that SpaceX was for a short period of time actively looking for customers.

Source on the 99.9% is this talk (about starship, but I doubt red dragon would have been substantially different in terms of delta v): https://www.youtube.com/watch?v=bysu8XN5OfY

>99% is possible for the Starship because it has a lot of pitch control, and can fly a trajectory in the atmosphere where it maximizes lift for as long as it can.

A capsule doing a simpler landing generally needs to get rid of about 400m/s using other methods, based on NASA's past missions.

Propulsive landing on Mars doesn't take anywhere near that much. The atmosphere is thick enough that you can bleed 90% of your energy with a heat shield.

Propulsive landing is awesome if the surface can stand up to it. Propulsive landing on mars with only a crew capsule is a little bonkers.

SpaceX pivoted to building an entirely new rocket for Mars called Starship.

That's not true, Curiosity used a parachute for part of its landing - it just needs to be pretty large to have an effect in atmosphere so thin:

https://www.space.com/16878-mars-rover-landing-sky-crane-gui...

It seems like the minimal possible complexity of a sheet of fabric is always going to be higher than the minimal complexity of an engine simply because the physics of cloth are much harder to simulate accurate so you will never be able to find all the edge cases. Yes engines are initially more complex to build, but their minimum complexity once they are well understood should be lower -- even with the code needed to run them. You can't exact attach a debugger to the universe.

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