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Comment by doomlaser | Hacker News Reader

I'm really interested in seeing how it will perform its "belly flop" landing. Seems like a high risk maneuver https://www.youtube.com/watch?v=DdTYMry7fq0#t=1m17s

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They've already done it[0], and it was a complete success until a propellant pressure issue resulted in lower than needed thrust, and a hard impact on the pad.

[0]: https://www.youtube.com/watch?v=_qwLHlVjRyw

It oriented almost perfectly. I'm truly amazed at what SpaceX has accomplished so far. Just need to stick that landing a few dozen more times. It's quite a reorientation.

Curious why does it need to wait so long to reorient? I thought it was going to crash and in last few hundred feet, it flipped! Can it orient straight - way up in the air like Falcon boosters?

Reorienting requires the use of main engines. You want to relight them only once, to ensure that the header tanks and the "downcomer" remain full. Additionally you want to burn off as much speed as possible with the bellyflop, so flipping vertical as late as possible. All combined this means combining the landing burn with the flip is best, as close to landing as possible.

It has a cute nickname, “Suicide Burn”.

Why is it efficient? [https://www.reddit.com/r/KerbalSpaceProgram/comments/2z534g/...]

Guessing we won't be using constant altitude landings in real life?

It actuallyoriented perfectly, but due to the flip, fuel pressure was lost leading an oxygen rich exhaust eating away the engine (becoming what's known as engine rich exhaust) before it was able to complete the landing burn. They should just have to fix the fuel pressure problem to have a complete success.

I'm very dumb so I'm asking: is "engine rich exhaust" a real term?

What do you mean by "real term"?

Real rocket science teams really use phrases like this, including this one. Is it a joke? Yeah, kind of. Is it what they would say if they had no sense of humour? No, probably not. Is it still a thing the experts actually say to actually talk about what's going on? Yeah, it is.

a few more examples:

"Rapid Unscheduled Disassembly" : explosive destruction of the rocket

"Lithobraking" : it's like aerobraking, but with less aero and more litho

Wow, that's surely one of the most amazing feats of human achievement I've ever witnessed - incredible!

On landing video, I see only 2 engines turn on instead of 3 - why is that? Was one engine malfunctioning, or was it the result of low pressure?

The rocket weighs quite a bit less at the end, having used most of its fuel, so only two engines are needed to land. The engines cannot be throttled down more than ~50% (as all rocket engines), so a third one would add too much thrust.

Falcon 9 does the same, relighting 3 engines (out of 9 total) for the boostback burn, 3 for the entry burn and then just 1 for landing.

For that flight the engines are even too powerful for the ascent (it wasn't fully fueled, I think) and as soon as they couldn't throttle any lower, an engine was turned off instead.

Thanks for the explanation! it this also the reason why Raptor is (relatively compared to like F-1) small and BFR uses so many of them?

That, and for redundancy. I've heard it's designed so that even if a significant number of engines malfunction, others can be used to compensate.

A complete resounding success until it burst into flames!

> “a hard impact on the pad.”

It’s called a RUD: Rapid Unscheduled Disassembly

I guess the counterargument is that as long as you bring it down in a "safe" area, you don't really need a 100% success rate.

If you can reuse it, great, if not, well, it's a rocket.

The belly flop is also how passenger variants will be landing so you do actually need a reliable program....

...eventually.

They didn’t launch humans on the Falcon 9 for the first 10 years either. Lots of time to iron out the kinks (though it will likely be much less than 10 years this time)

Humans aren’t landing propulsively on f9 either - if the landing fails it just costs money. The capsule lands with parachutes.

The Mars mission has a somewhat tighter timeline.

Aspirationally, yes, but it's not like they run out of money if they don't get to Mars that fast.

How many G’s will passenger experience when the ship transitions from the belly flop to the suicide burn?

That looks like an awful lot of G’s that this maneuver is pulling there.

I think it would depend on how close they can line up the centre of rotation and the passenger seating.

The capsule containing passengers will separate during re-entry and land with parachutes.

This suicide burn is just to try to recover the rocket to save money.

This is the first I’ve heard of this description of a separate crew module.

This sounds even crazier than the bellyflop-swivel-to-suicide-burn (BSTSB).

So the Starship system is some type of 3 piece mechanism? Where the 3rd piece is the human module?

And they’re going to land this with parachutes? Where? In the ocean?

Won’t this kill the reusability of the plan, for quick turn around time?

Granted, if the BSTSB is to only be used for transporting inorganic materials, like fuel, then G-forces does not matter for human usage.

Edit: Disregard this, it's wrong entirely.

The fuel used to attempt the landing the instead be used to launch.

That's true, but a suicide burn uses a _lot_ less propellant than you'd think. The atmosphere does most of the work.

My guess would be it will still have a launch escape system for the passenger variant. So if the belly flop doesn't look right or if it doesn't stick the landing it just ejects the crew.

Well Everyday Astronaut did a long article and YouTube video about why starship won’t have a launch abort system. [1] I don’t believe I’ve watched the video but I remembered it exists so I thought I’d share.

https://everydayastronaut.com/starship-abort/

Where would you put a 100 person LES? I don’t think a solid rocket escape system would work in the belly flop position anyways since you wouldn’t gain any altitude, you’d actually be making it more dangerous since you’d be increasing horizontal velocity. Airliners don’t have ejection seats or parachutes.

A launch escape system is not necessarily safer than without one.

In theory you’re right, in reality Space Shuttle.

IIRC never fired in actual missions; did not help avoid any of the two fatal accidents.

All of standalone LES activation ever in the history of human spaceflight had been successful in saving all occupants' lives.

Space Shuttle failed twice on the other hand, each time killing all aboard, and in neither cases of it its integral abort systems/abort modes did help at all.

So in theory the lack of capsule-only LES do not necessarily mean safer rocket, in the history and in the statistics it does.

Statistics say you shouldn’t call your reusable ship a word beginning with ‘C’

In 1963 a LES killed someone, in 1983 it saved lives.

> In 1963 a LES killed someone

Which manned abort in 1963 is that specifically? Why are you trying to skew discussion into some direction?

Note: LES stands for Launch Escape System, usually a small solid rocket that activates in case of a failure during ascent to take the manned capsule away from exploding rocket under it, perhaps equivalent to airbags in a car.

IIRC this is the case when Soyuz LES activated on the pad before an unmanned test flight, killing some of the ground crew.

So not in flight. Not relevant.

People gets hurt by airbags all the time trying to service them. That doesn’t tell if it’s worth having them in actual situations they’re designed for, nor whether the situations are worth preparing for.

It is relevant. If (giving an extreme example to make the point), statistically two humans were killed servicing airbags for every life saved by airbags, cars wouldn’t have airbags (with possible exceptions for cars of the super rich and super important such as the POTUS)

Also, an escape system adds weight to a rocket. That weight could be used for other stuff that makes the rest of the system more reliable (slightly stronger fuel tanks, redundant pumps, whatever). Designing the escape system also takes resources away that could be used to improve the safety of the primary system.

For rockets, it’s not easy to decide what’s the best choice; we launch too few of them, and the ones we launch change too often to gather reliable statistics. Getting real-world data on the reliability of escape systems definitely is hard, as it requires numerous rocket failures or elaborate test setups (a full test must include high speed and an explosion that may throw material through the engine of the escape capsule)

> All of standalone LES activation ever in the history of human spaceflight had been successful in saving all occupants' lives.

> Space Shuttle failed twice on the other hand, each time killing all aboard, and in neither cases of it its integral abort systems/abort modes did help at all.

> So in theory the lack of capsule-only LES do not necessarily mean safer rocket, in the history and in the statistics it does.

Is this supposed to be a recursion joke? Sorry if I'm not getting it.

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