I wonder what's really preventing them from deploying the landing legs earlier. That would save fuel in theory, but maybe it's a question of aerodynamic stability even assuming the legs would stay rigid, but maybe there is a risk that the legs would bend and oscillate.
I think the legs could burn with rocket exhaust if exposed too soon.
Quote from: Jcc on 01/08/2016 12:10 pmI wonder what's really preventing them from deploying the landing legs earlier. That would save fuel in theory, but maybe it's a question of aerodynamic stability even assuming the legs would stay rigid, but maybe there is a risk that the legs would bend and oscillate.I think the legs could burn with rocket exhaust if exposed too soon.
I think a likely required part of the landing burn is correction of targeting errors from using just the grid fins. Unless the grid fins are pinpoint accurate, you want to start the burn early so the rocket has time to adjust for the final targeted location. Even then, upper-layer winds might push the stage off-target and the grid fins might not be able to compensate as much as the engine can. This might be a limiting factor in starting the burn later, however many engines are used.
Quote from: macpacheco on 01/08/2016 12:35 pmI think the legs could burn with rocket exhaust if exposed too soon.If the deploy mechanism would be changed to allow this they could do a partial deploy first. Make it look like an arrowhead. It would provide drag and keep the legs away from the flames. It should cause less stability issues too.
Yes, we saw how badly the metal Grashopper legs got smoked. SpaceX obviously wants to reuse these F9 composite legs which will be more easily damaged by heat than the metal GH legs, and the obvious way to preserve them is to minimize time on the barbecue grill.
I thought we understood the legs required deceleration force to deploy...
Quote from: abaddon on 01/08/2016 02:34 pmI think a likely required part of the landing burn is correction of targeting errors from using just the grid fins. Unless the grid fins are pinpoint accurate, you want to start the burn early so the rocket has time to adjust for the final targeted location. Even then, upper-layer winds might push the stage off-target and the grid fins might not be able to compensate as much as the engine can. This might be a limiting factor in starting the burn later, however many engines are used.That's an unintuitive aspect of landing.The faster the vertical speed, the less winds will affect you (less time exposed to them).
Finally you're assuming sideways control margins are tight. That might be right or very wrong.
And cold gas thrusters add more control authority.
The fact SpaceX allows for 50mph winds for land landings suggest there are plenty of margins.
But more interestingly, how useful is a fuel savings of about 2000kg? or even just 600kg? Is there any reason to expect that SpaceX will attempt to land more aggressively and really push the limits?
It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn. In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point. I think this would be difficult to argue against.
Quote from: abaddon on 01/08/2016 03:10 pm It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn. In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point. I think this would be difficult to argue against. Actually, it's extremely easy to argue against this. Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades. It's well proven technology.