4 ms·

>What I'm wondering is, will they eventually carry enough fuel to both be able to do an abort, and then do a powered landing, if required? Even if it could, it

by snops 11y ago

>What I'm wondering is, will they eventually carry enough fuel to both be able to do an abort, and then do a powered landing, if required? Even if it could, its important to note that the abort system would only be used when something has gone drastically wrong, potentially an explosion, which means you can't garuntee Dragon will be undamaged. In that case, would you trust a complex powered landing, or a simpler parachute landing where you have triple redundancy? In addition, SpaceX launch on the coast and the rocket flies over the sea, so an abort may end up with a sea landing. I haven't seen any concepts from SpaceX showing a powered dragon landing at sea, and it would presumably require a pretty calm sea state, while parachutes would work in any weather. You could see similar problems with a powered landing on unfamiliar land, for instance trying to "land" on a tree canopy.

jessriedel 11y ago
Why would parachutes work better at sea? Just like a rocket, the parachute stops providing useful upwards force as soon as the capsule touches water/ground. The only thing I can think of is that it is probably easier to add landing air bags to a capsule descending by parachute than by rocket, but that's really only useful on land.
- Retric 11y ago
  Parachutes work better at sea than land; water becomes far softer as you reduce speeds. Also, a powered landing on a 30 foot wave seems like a rather complex problem not to mention buildings etc. Worse, powered landings have little redundancy where a sea landing with 2/3 of your parachutes is not a major issue. I suspect a larger issue is seawater is not something you want anywhere close to delicate equipment. But, for an emergency landing that's not much of a concern. Anyway, I suspect these are designed so if either the powered landing or the parachutes work the landing is survivable even if the craft is damaged.
  - simonh 11y ago
    The super-dracos are on the sides of the capsule, not the bottom, so should be able to provide thrust all the way down to surface contact on land and sea. Also the vehicle can suffer the loss of a thruster without compromising it's abort or landing capability. I don't really see that parachutes in 30ft waves are going to be all that much beter than anything else, but don't manned launches have fairly strict weather condition safety requirements? I'm pretty sure that would include sea states in any conceivable abort landing zone. One final advantage is that the vehicle can use the thrusters to manoeuver prior to landing, potentially avoiding problematic terrain. In theory you could probably designate a series of preferred abort destinations that are within the vehicle's operational envelope for different stages of the flight. So instead of ending up anywhere along a line, there would be a series of preferred landing zones along or near the line.
    - jessriedel 11y ago
      Thanks for the good point about manoeuverability when landing on bad terrain during an unplanned emergency. > don't manned launches have fairly strict weather condition safety requirements? I'm pretty sure that would include sea states in any conceivable abort landing zone. You might be right, but note that all the shuttle abort landings sites were on land at a handful of locations, in part because the shuttle was a glider (so it had a decent amount of unpowered range) and in part because landing the shuttle somewhere that wasn't planned was nearly impossible (since it needed such a huge flat expanse to land). Therefore, you only needed to have a good weather simultaneously at a few locations. For the Dragon, however, the lack of gliding ability and the ability to land anywhere in an emergency makes it infeasible to wait for simultaneously good weather everywhere, simply because it might end up anywhere along it's flight path in an emergency. So you probably need to be able to handle bad weather.
  - jessriedel 11y ago
    > Parachutes work better at sea than land; water becomes far softer as you reduce speeds. Sorry, my comment was ambiguous. I meant "Why would parachutes work better than rockets at sea?" not "Why would parachutes work better at sea than land?" > a powered landing on a 30 foot wave seems like a rather complex problem This is only an issue if you're greatly decelerating in the last 30 feet, since then that window is shifting up and down with the ocean. However, I'd guess that all the major deceleration has already happened by this point, and the rockets are just maintaining a slow steady descent speed for the last 30 (or 100) feet. (Would greatly appreciate anyone who can tell me I'm wrong here.) In this case, they function just like a parachute. > Worse, powered landings have little redundancy where a sea landing with 2/3 of your parachutes is not a major issue. As simonh mentions, the capsule has redundant thrusters, which again seems very analogous to a parachute.

by snops 11y ago

jessriedel 11y ago

Why would parachutes work better at sea? Just like a rocket, the parachute stops providing useful upwards force as soon as the capsule touches water/ground. The only thing I can think of is that it is probably easier to add landing air bags to a capsule descending by parachute than by rocket, but that's really only useful on land.

Retric 11y ago
Parachutes work better at sea than land; water becomes far softer as you reduce speeds. Also, a powered landing on a 30 foot wave seems like a rather complex problem not to mention buildings etc. Worse, powered landings have little redundancy where a sea landing with 2/3 of your parachutes is not a major issue. I suspect a larger issue is seawater is not something you want anywhere close to delicate equipment. But, for an emergency landing that's not much of a concern. Anyway, I suspect these are designed so if either the powered landing or the parachutes work the landing is survivable even if the craft is damaged.
- simonh 11y ago
  The super-dracos are on the sides of the capsule, not the bottom, so should be able to provide thrust all the way down to surface contact on land and sea. Also the vehicle can suffer the loss of a thruster without compromising it's abort or landing capability. I don't really see that parachutes in 30ft waves are going to be all that much beter than anything else, but don't manned launches have fairly strict weather condition safety requirements? I'm pretty sure that would include sea states in any conceivable abort landing zone. One final advantage is that the vehicle can use the thrusters to manoeuver prior to landing, potentially avoiding problematic terrain. In theory you could probably designate a series of preferred abort destinations that are within the vehicle's operational envelope for different stages of the flight. So instead of ending up anywhere along a line, there would be a series of preferred landing zones along or near the line.
  - jessriedel 11y ago
    Thanks for the good point about manoeuverability when landing on bad terrain during an unplanned emergency. > don't manned launches have fairly strict weather condition safety requirements? I'm pretty sure that would include sea states in any conceivable abort landing zone. You might be right, but note that all the shuttle abort landings sites were on land at a handful of locations, in part because the shuttle was a glider (so it had a decent amount of unpowered range) and in part because landing the shuttle somewhere that wasn't planned was nearly impossible (since it needed such a huge flat expanse to land). Therefore, you only needed to have a good weather simultaneously at a few locations. For the Dragon, however, the lack of gliding ability and the ability to land anywhere in an emergency makes it infeasible to wait for simultaneously good weather everywhere, simply because it might end up anywhere along it's flight path in an emergency. So you probably need to be able to handle bad weather.
- jessriedel 11y ago
  > Parachutes work better at sea than land; water becomes far softer as you reduce speeds. Sorry, my comment was ambiguous. I meant "Why would parachutes work better than rockets at sea?" not "Why would parachutes work better at sea than land?" > a powered landing on a 30 foot wave seems like a rather complex problem This is only an issue if you're greatly decelerating in the last 30 feet, since then that window is shifting up and down with the ocean. However, I'd guess that all the major deceleration has already happened by this point, and the rockets are just maintaining a slow steady descent speed for the last 30 (or 100) feet. (Would greatly appreciate anyone who can tell me I'm wrong here.) In this case, they function just like a parachute. > Worse, powered landings have little redundancy where a sea landing with 2/3 of your parachutes is not a major issue. As simonh mentions, the capsule has redundant thrusters, which again seems very analogous to a parachute.