5 ms·

I am beginning to wonder if either: 1. This landing procedure is not viable. Restarting an engine in the middle of a rotation like that, fuel sloshing around

I am beginning to wonder if either: 1. This landing procedure is not viable. Restarting an engine in the middle of a rotation like that, fuel sloshing around is really hard. Just restarting an engine in zero G has always been tricky, because the fuel is floating around and doesn't have gravity pulling it to the pump. This case is even more dynamic, and you have to get it right immediately. or 2. If the raptor engine is too complex to be reliable. Then again Falcon9 also failed to land many many times early on, and it has become rather routine. Though still not routine enough that you would want to be a passenger riding one down. please nobody reply with "prototype" or "rapid iteration" we know, we have seen your posts 20,000 times here already :)_

rich_sasha 6y ago
I think they picked a very hard and complex task, and the hype around it is so high that everyone just assumes they will succeed. The procedure might be unworkable, the engine unreliable, or not reusable enough, tons of things. I think such an ambitious project is always a “maybe” until it’s done. But not in the general perception.
amelius 6y ago
> fuel sloshing around is really hard Can't the fuel be stabilized mechanically, e.g. as in a syringe?
- nickik 6y ago
  In theory yes, and if worse comes to worse that is an option. But its not what any rocket designer would want to do. Having a mechanical system inside of a deep cryo liquid methane is hugely complex and very heavy. Even if you put it outside, you still have deep cryo liquid inside and that would make it even heavier. Consider that 1kg extra weight on the second stage reduces payload to orbit by 1kg and payload to Mars by much more overall.
  - api 6y ago
    Is there some kind of material that remains elastic and strong at that temperature that could be used to create a bladder? I assume no, since that seems like something that would have already been done in place of helium pressurant if it were viable. All the rubbery or fabric-like things I know of that are stretchy like that become brittle at those temperatures.
    - musingsole 6y ago
      Not your line of thinking, but I believe ferrofluids were developed to provide control over fuel in zero-g (i.e. a material that is elastic and strong, but that gets mixed with the fuel instead of encasing it)
      - api 6y ago
        Doesn't it then get burned? Seems like it would deposit shit in the engine, which is bad especially for a reusable engine.
        musingsole 6y ago
        I believe it's meant to be fully filtered out of the fuel before burning (being magnetic, I think the filtering is a bit easier than some other non-reactive contaminate would be). Most of this is only half-recalled from someone's recent project that showed separating ferrofluid from some liquid on Earth...but I don't recall any other context :/
    - nickik 6y ago
      I don't know if such a material exists. The problem is SpaceX operates with Deep Cryo fuel (only company that does so as far as I know). They are literally going to 66K on the LOX and methane. Helium was just temporary as well so once the actually design system is back with SN15 it should be better already. The idea that Starship has is to use the fuel and oxidizer in its gas form. So the methane tank contains liquid methane pressurized by gaseous methane. The benefit of this is that if the engine ingests some of that gaseous methane its less of an issue in terms of engine thrust compared to helium and its possible more recoverable of a situation.
- hooper 6y ago
  I've heard of using a gas driven plunger in fuel tanks, but I can't remember the name of the vehicle that did this. Starship uses small "header tanks" that have a similar effect. Weight and reliability are big priorities in this kind of system.
  - dogma1138 6y ago
    Isn’t helium already used to push fuel into the pumps?
    - senectus1 6y ago
      I think that was part of the problem with the last launch. it started to burn some of the helium which caused issues.
    - hooper 6y ago
      I think it's more to maintain pressure in the tank, in cases where the remaining fuel doesn't vaporize sufficiently on its own (autogenous pressurization). Otherwise, I guess atmospheric pressure or compressive force from the engines/payload could crush the tank. In the case of Starship, the fuel is moved by turbo pumps, which are integrated with the engine, and are powered by burning some of the fuel (there's a diagram of this at https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor).
- Balgair 6y ago
  I thought they used surface-tension / capillary-action to move the fuel. Like with passive geosatellite heat distribution. Think a radiator, lots of tiny fins, but all inward pointing in a tube. Coat the fins so they are '-philic' to the liquid. Get creative with the geometries of the fins. Surface tension / capillary-action spread over a lot of the fins in the tube will move the liquid to a sink, like the combustion chamber.
nickik 6y ago
I think its a bit early to say its not viable. Header tanks are there precisely to solve this. The sloshing in the header tanks should manageable in a tank sized correctly, specially if you are willing add some extra baffling to the header tank. We should realize that after the first failure SpaceX already had built a couple of prototypes that were 'wrong'. So they had to basically do hacks to figure out how to fly them at all. I would assume with the next generation they have really thought about how to do the header tanks and redesigned them and how the pressurization of those work. They are seemingly learning about some interesting things about liquid methane and bubble formation in liquid methane. The baffles they had apparently caused a vacuum bubble in SN8.
evgen 6y ago
> Restarting an engine in the middle of a rotation like that, fuel sloshing around is really hard. Just restarting an engine in zero G has always been tricky, because the fuel is floating around and doesn't have gravity pulling it to the pump. I was re-watching a series of videos doing a walk-through if the Apollo guidance computer over the weekend and one of the cute 'yes, they thought of that' bits that was mentioned was that before the main firing sequence begins on the LEM descent they perform an 'ullage' burn using the RCS to settle fuel in the tanks.
vimacs2 6y ago
Well considering we already had a successful landing of sorts with SN10, I think this is a little premature. Yes, it exploded 8 minutes afterwards but it does not seem like that had much to do with the stresses of the landing manoeuvre. The raptor engine on the other hand does seem like something that they are far from mastering the production of. Being the first full flow staged combustion, methane fuelled orbital ready rocket means that there is less to directly compare to so I'm not surprised that they are having reliability issues with it. I am optimistic that they will eventually overcome these challenges but I'm less certain if they will be able to meet their projected schedule.
api 6y ago
Or (3) the first full-flow staged combustion engine to have ever really flown will require more iteration time to become reliable. Seems totally reasonable to me. It's a new design of a new type of engine.
ricw 6y ago
It took SpaceX 19 attempts before they landed their Falcon 9 rocket. By all means, they managed to land starship during their 3rd attempt, even if it blew up 8 minutes later due to a hard landing. Rather fail fast, than fail slow. SpaceX takes this to the next level of space hardware testing with their willingness to fail. Good on them. Ps: it is a prototype (i said it!). It's expected to fail at some point. Otherwise it would be production hardware...
- gameswithgo 6y ago
  19 attempts to land it from orbit, with many of those being onto boats. They had lots of "easy" success with low altitude test flights, and a booster isn't ever going to have people on it. this will. going to be tough, hope it works!
  - nickik 6y ago
    The first stage of the Falcon 9 does not go to Orbit. These test flights might be low altitude but the flip and landing is realistic.
    - perl4ever 6y ago
      I don't understand how the belly flop is even possible. I'm far from a rocket scientist, but I thought that rockets were kind of like soda cans - very strong on the long axis, but not at all perpendicular to that. If you make them strong in both directions, I wonder how that doesn't make them prohibitively heavy.
      - nickik 6y ago
        Well, sure that is generally what is the case. But of course the designer can design it as he likes depending on the situation. Generally, of course this is more true when the stages are empty. If the tanks are pressurized, as they are during the whole flight, they actually become very stable in both directions. Starship is specifically designed to survive a reentry (heating) into the atmosphere at an angle, just like the Spaceshuttle.
scottisbrave84 6y ago
Vacuum if over ingegr the follows
bryanlarsen 6y ago
It's only been a few months since the first landing attempt, so it's not surprising that it's not working yet. Number of attempts is not as relevant as time since time is needed to find and implement fixes.

rich_sasha 6y ago

I think they picked a very hard and complex task, and the hype around it is so high that everyone just assumes they will succeed. The procedure might be unworkable, the engine unreliable, or not reusable enough, tons of things. I think such an ambitious project is always a “maybe” until it’s done. But not in the general perception.

amelius 6y ago

> fuel sloshing around is really hard Can't the fuel be stabilized mechanically, e.g. as in a syringe?

nickik 6y ago
In theory yes, and if worse comes to worse that is an option. But its not what any rocket designer would want to do. Having a mechanical system inside of a deep cryo liquid methane is hugely complex and very heavy. Even if you put it outside, you still have deep cryo liquid inside and that would make it even heavier. Consider that 1kg extra weight on the second stage reduces payload to orbit by 1kg and payload to Mars by much more overall.
- api 6y ago
  Is there some kind of material that remains elastic and strong at that temperature that could be used to create a bladder? I assume no, since that seems like something that would have already been done in place of helium pressurant if it were viable. All the rubbery or fabric-like things I know of that are stretchy like that become brittle at those temperatures.
  - musingsole 6y ago
    Not your line of thinking, but I believe ferrofluids were developed to provide control over fuel in zero-g (i.e. a material that is elastic and strong, but that gets mixed with the fuel instead of encasing it)
    - api 6y ago
      Doesn't it then get burned? Seems like it would deposit shit in the engine, which is bad especially for a reusable engine.
      - musingsole 6y ago
        I believe it's meant to be fully filtered out of the fuel before burning (being magnetic, I think the filtering is a bit easier than some other non-reactive contaminate would be). Most of this is only half-recalled from someone's recent project that showed separating ferrofluid from some liquid on Earth...but I don't recall any other context :/
  - nickik 6y ago
    I don't know if such a material exists. The problem is SpaceX operates with Deep Cryo fuel (only company that does so as far as I know). They are literally going to 66K on the LOX and methane. Helium was just temporary as well so once the actually design system is back with SN15 it should be better already. The idea that Starship has is to use the fuel and oxidizer in its gas form. So the methane tank contains liquid methane pressurized by gaseous methane. The benefit of this is that if the engine ingests some of that gaseous methane its less of an issue in terms of engine thrust compared to helium and its possible more recoverable of a situation.
hooper 6y ago
I've heard of using a gas driven plunger in fuel tanks, but I can't remember the name of the vehicle that did this. Starship uses small "header tanks" that have a similar effect. Weight and reliability are big priorities in this kind of system.
- dogma1138 6y ago
  Isn’t helium already used to push fuel into the pumps?
  - senectus1 6y ago
    I think that was part of the problem with the last launch. it started to burn some of the helium which caused issues.
  - hooper 6y ago
    I think it's more to maintain pressure in the tank, in cases where the remaining fuel doesn't vaporize sufficiently on its own (autogenous pressurization). Otherwise, I guess atmospheric pressure or compressive force from the engines/payload could crush the tank. In the case of Starship, the fuel is moved by turbo pumps, which are integrated with the engine, and are powered by burning some of the fuel (there's a diagram of this at https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor).
Balgair 6y ago
I thought they used surface-tension / capillary-action to move the fuel. Like with passive geosatellite heat distribution. Think a radiator, lots of tiny fins, but all inward pointing in a tube. Coat the fins so they are '-philic' to the liquid. Get creative with the geometries of the fins. Surface tension / capillary-action spread over a lot of the fins in the tube will move the liquid to a sink, like the combustion chamber.

nickik 6y ago

I think its a bit early to say its not viable. Header tanks are there precisely to solve this. The sloshing in the header tanks should manageable in a tank sized correctly, specially if you are willing add some extra baffling to the header tank. We should realize that after the first failure SpaceX already had built a couple of prototypes that were 'wrong'. So they had to basically do hacks to figure out how to fly them at all. I would assume with the next generation they have really thought about how to do the header tanks and redesigned them and how the pressurization of those work. They are seemingly learning about some interesting things about liquid methane and bubble formation in liquid methane. The baffles they had apparently caused a vacuum bubble in SN8.

evgen 6y ago

> Restarting an engine in the middle of a rotation like that, fuel sloshing around is really hard. Just restarting an engine in zero G has always been tricky, because the fuel is floating around and doesn't have gravity pulling it to the pump. I was re-watching a series of videos doing a walk-through if the Apollo guidance computer over the weekend and one of the cute 'yes, they thought of that' bits that was mentioned was that before the main firing sequence begins on the LEM descent they perform an 'ullage' burn using the RCS to settle fuel in the tanks.

vimacs2 6y ago

Well considering we already had a successful landing of sorts with SN10, I think this is a little premature. Yes, it exploded 8 minutes afterwards but it does not seem like that had much to do with the stresses of the landing manoeuvre. The raptor engine on the other hand does seem like something that they are far from mastering the production of. Being the first full flow staged combustion, methane fuelled orbital ready rocket means that there is less to directly compare to so I'm not surprised that they are having reliability issues with it. I am optimistic that they will eventually overcome these challenges but I'm less certain if they will be able to meet their projected schedule.

api 6y ago

Or (3) the first full-flow staged combustion engine to have ever really flown will require more iteration time to become reliable. Seems totally reasonable to me. It's a new design of a new type of engine.

ricw 6y ago

It took SpaceX 19 attempts before they landed their Falcon 9 rocket. By all means, they managed to land starship during their 3rd attempt, even if it blew up 8 minutes later due to a hard landing. Rather fail fast, than fail slow. SpaceX takes this to the next level of space hardware testing with their willingness to fail. Good on them. Ps: it is a prototype (i said it!). It's expected to fail at some point. Otherwise it would be production hardware...

gameswithgo 6y ago
19 attempts to land it from orbit, with many of those being onto boats. They had lots of "easy" success with low altitude test flights, and a booster isn't ever going to have people on it. this will. going to be tough, hope it works!
- nickik 6y ago
  The first stage of the Falcon 9 does not go to Orbit. These test flights might be low altitude but the flip and landing is realistic.
  - perl4ever 6y ago
    I don't understand how the belly flop is even possible. I'm far from a rocket scientist, but I thought that rockets were kind of like soda cans - very strong on the long axis, but not at all perpendicular to that. If you make them strong in both directions, I wonder how that doesn't make them prohibitively heavy.
    - nickik 6y ago
      Well, sure that is generally what is the case. But of course the designer can design it as he likes depending on the situation. Generally, of course this is more true when the stages are empty. If the tanks are pressurized, as they are during the whole flight, they actually become very stable in both directions. Starship is specifically designed to survive a reentry (heating) into the atmosphere at an angle, just like the Spaceshuttle.

scottisbrave84 6y ago

Vacuum if over ingegr the follows

bryanlarsen 6y ago

It's only been a few months since the first landing attempt, so it's not surprising that it's not working yet. Number of attempts is not as relevant as time since time is needed to find and implement fixes.