5 ms·

A question for those in the know: since the landing payload is much lighter, wouldn’t it be possible to use SuperDraco engines (the ones used for emergency esca

by jiofih 5y ago

A question for those in the know: since the landing payload is much lighter, wouldn’t it be possible to use SuperDraco engines (the ones used for emergency escape in the Dragon) attached to the outside to prevent RUD in case the main engines don’t light up?

walrus01 5y ago
I am no expert on these things but I think the starship has no tanks or system for the different, hypergolic fuels used for the superdraco and similar type of thrusters. "SuperDraco engines use a storable propellant mixture of monomethylhydrazine fuel and dinitrogen tetroxide oxidizer"
- rbanffy 5y ago
  Hypergolics are very toxic, corrosive and made of the most exquisite molecules of the purest evil. Dealing with them is expensive and complicated and drives cost up. It was one of the reasons why shuttles were so expensive to refly.
  - ncmncm 5y ago
    The new hypergolic fuel everybody is actively working on switching to is not (especially) toxic or corrosive. And has better specific impulse. Existing Dragons might never be switched over, because flight-qualifying a crewed vehicle over again is expensive. If they build any new Dragons, it would not be surprising if they still use the old fuel. But anything in design today will use the new fuel, because it's enough better.
    - CarVac 5y ago
      Are you thinking of the new viscous liquid monopropellants that replace hydrazine monopropellant, rather than hypergols? Hypergols are always highly reactive, by very nature of being hypergolic.
codeulike 5y ago
Better objective is just to keep testing and tweaking until the main engines do reliably light up Also, for landing the engines need to be steerable which I think the emergency Superdraco ones aren't.
- jiofih 5y ago
  When you’re carrying a hundred people inside on a weekend trip, what is “reliably”? 99%? five nines? Planes have a much less risky failure mode..
  - codeulike 5y ago
    Thats a good point. But I'm not sure that throwing extra engines into the scenario will help much. It takes very precise control to land a rocket. Those SuperDraco things are for going up fast, not coming down with sub-meter accuracy.
  - bkanber 5y ago
    I had an aerodynamics / rocket science professor in grad school who used to say 'planes are as reliable as toasters', and the reason for that is the literal 40 million flights per year. Planes can explode just as easily as rockets can. The reason planes have a much less risky failure mode is because we've flown them for billions of hours, not because we've added a ton of redundancies to them. In fact many of the structural 'factors of safety' in airplanes are only 1.5, as opposed to a FOS of 8 in your car. So again, it's not about just tacking on redundant systems, it's about getting the experience required to understand all the failure modes. The only times planes crash these days is when we encounter a new failure mode. Rocketry will get there as well.
deleted 5y ago
[deleted]
nickik 5y ago
Its a tradeoff between complexity and results. Think of it like this, every kg added to Starship is one kg less to Orbit. And maybe like 10-30kg less to Mars. For what you are talking about you likely need 16-32 SuperDracos, you need massive tank. A highly complex system of tubes along the vehicle and so on. You would reduce the payload to Orbit and return by a huge margin. On earth, for a system to be reusable AT ALL, is incredibly hard. Like seriously, super hard. Their margins are already very thin. I'm not even sure what you suggest is possible at all. And such a system would only prevent some very few cases. You already have engine redundancy. It wouldn't save you in many failure cases either. So its a gigantic increase in complexity for a slight reduction in potential failure. The extra complexity quite apart from the performance would also lead to many new failure most that must be considered. If in a full risk analysis this would actually be safer is questionable. If you want to increase safety adding a 4th Raptor would like by safer, as well as starting the flip earlier. P.S: The fuel would also be highly toxic and would make operations with humans much more complex and much more expensive.
- SECProto 5y ago
  > Think of it like this, every kg added to Starship is one kg less to Orbit. And maybe like 10-30kg less to Mars. With standard staging this would be correct. With starship it's a bit different: Every 1kg added to Starship is 1kg less [payload] to Orbit. And 1kg less to Mars. And maybe an extra tanker launch for refueling
  - nickik 5y ago
    That's exactly the thing. I will take more launches. So overall its still true. If you did the exact same flights, overall you would end up with many fewer kg landed on Mars. But of course that inefficiency is gone be buffered by more refueling. Still calculate the lost efficiency on the scale on a mars base and 1kg is a major. Not to mention the system suggest above.
dark_star 5y ago
SuperDracos use hypergolic propellants, which can't be sourced on Mars. So SpaceX isn't using them on Starship. The HLS moon lander version will have small methalox hot gas thrusters to avoid kicking up a cloud of debris, but these aren't needed right now, eventually they'll be used in place of the nitrogen RCS thrusters.

by jiofih 5y ago

walrus01 5y ago

I am no expert on these things but I think the starship has no tanks or system for the different, hypergolic fuels used for the superdraco and similar type of thrusters. "SuperDraco engines use a storable propellant mixture of monomethylhydrazine fuel and dinitrogen tetroxide oxidizer"

rbanffy 5y ago
Hypergolics are very toxic, corrosive and made of the most exquisite molecules of the purest evil. Dealing with them is expensive and complicated and drives cost up. It was one of the reasons why shuttles were so expensive to refly.
- ncmncm 5y ago
  The new hypergolic fuel everybody is actively working on switching to is not (especially) toxic or corrosive. And has better specific impulse. Existing Dragons might never be switched over, because flight-qualifying a crewed vehicle over again is expensive. If they build any new Dragons, it would not be surprising if they still use the old fuel. But anything in design today will use the new fuel, because it's enough better.
  - CarVac 5y ago
    Are you thinking of the new viscous liquid monopropellants that replace hydrazine monopropellant, rather than hypergols? Hypergols are always highly reactive, by very nature of being hypergolic.

codeulike 5y ago

Better objective is just to keep testing and tweaking until the main engines do reliably light up Also, for landing the engines need to be steerable which I think the emergency Superdraco ones aren't.

jiofih 5y ago
When you’re carrying a hundred people inside on a weekend trip, what is “reliably”? 99%? five nines? Planes have a much less risky failure mode..
- codeulike 5y ago
  Thats a good point. But I'm not sure that throwing extra engines into the scenario will help much. It takes very precise control to land a rocket. Those SuperDraco things are for going up fast, not coming down with sub-meter accuracy.
- bkanber 5y ago
  I had an aerodynamics / rocket science professor in grad school who used to say 'planes are as reliable as toasters', and the reason for that is the literal 40 million flights per year. Planes can explode just as easily as rockets can. The reason planes have a much less risky failure mode is because we've flown them for billions of hours, not because we've added a ton of redundancies to them. In fact many of the structural 'factors of safety' in airplanes are only 1.5, as opposed to a FOS of 8 in your car. So again, it's not about just tacking on redundant systems, it's about getting the experience required to understand all the failure modes. The only times planes crash these days is when we encounter a new failure mode. Rocketry will get there as well.

deleted 5y ago

[deleted]

nickik 5y ago

Its a tradeoff between complexity and results. Think of it like this, every kg added to Starship is one kg less to Orbit. And maybe like 10-30kg less to Mars. For what you are talking about you likely need 16-32 SuperDracos, you need massive tank. A highly complex system of tubes along the vehicle and so on. You would reduce the payload to Orbit and return by a huge margin. On earth, for a system to be reusable AT ALL, is incredibly hard. Like seriously, super hard. Their margins are already very thin. I'm not even sure what you suggest is possible at all. And such a system would only prevent some very few cases. You already have engine redundancy. It wouldn't save you in many failure cases either. So its a gigantic increase in complexity for a slight reduction in potential failure. The extra complexity quite apart from the performance would also lead to many new failure most that must be considered. If in a full risk analysis this would actually be safer is questionable. If you want to increase safety adding a 4th Raptor would like by safer, as well as starting the flip earlier. P.S: The fuel would also be highly toxic and would make operations with humans much more complex and much more expensive.

SECProto 5y ago
> Think of it like this, every kg added to Starship is one kg less to Orbit. And maybe like 10-30kg less to Mars. With standard staging this would be correct. With starship it's a bit different: Every 1kg added to Starship is 1kg less [payload] to Orbit. And 1kg less to Mars. And maybe an extra tanker launch for refueling
- nickik 5y ago
  That's exactly the thing. I will take more launches. So overall its still true. If you did the exact same flights, overall you would end up with many fewer kg landed on Mars. But of course that inefficiency is gone be buffered by more refueling. Still calculate the lost efficiency on the scale on a mars base and 1kg is a major. Not to mention the system suggest above.

dark_star 5y ago

SuperDracos use hypergolic propellants, which can't be sourced on Mars. So SpaceX isn't using them on Starship. The HLS moon lander version will have small methalox hot gas thrusters to avoid kicking up a cloud of debris, but these aren't needed right now, eventually they'll be used in place of the nitrogen RCS thrusters.