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Having read more of the many links I've now found this "Moving back at supersonic speeds that sent a sonic boom across Florida’s Space Coast, the first stage engines ignited to slow the booster down as it neared Landing Complex-1".
Assuming a constant thrust (with little change in total mass) the rocket is either:JohnD wrote:
I don't think that the landing Falcon would have taken off again if the rocket had kept on firing.
It's thrust , and the acceleration it caused counteracted the acceleration due to gravity, but not quite.
The result was a system that behaved like a much lighter vehicle, that came down slowly.
You're right. I'm right. Art's right. We're all just telling part of the story.JohnD wrote:Chris will correct me, neufer, but I don't think that the landing Falcon would have taken off again if the rocket had kept on firing.
It's thrust , and the acceleration it caused counteracted the acceleration due to gravity, but not quite.
The result was a system that behaved like a much lighter vehicle, that came down slowly.
Once it contacted Earth, the reaction of a solid surface provided a further acceleration that precisely counteracted gravity.
Continued rocket thrust (of the same amount) would have made gravity and the Earth's reaction less, but not enough to counteract gravity completely and launch the vehicle again.
John
Neal Armstrong had to do some hovering over the moon until he found a nice place to land (or run out of fuel).suicidejunkie wrote:
If the engine were able to throttle down to get anywhere near "just counteract gravity", then the landing would be MUCH easier, since the rocket could hover until it got things right (or ran out of fuel).
We were, of course, discussing the nature of landing rockets in general. That said, I doubt hovering would ever be a desirable option, nor that it would actually make things any easier. To make this cost effective, the system needs to land on the least possible amount of fuel. I imagine that the landing profile is designed with that in mind. And all profiles should be equally easy (but not necessarily equally robust) given that it's just an elementary Newtonian dynamics problem and basic control theory. As long as the rocket actually does what it's commanded to do by the computer (which is an engineering problem), the actual landing profile is pretty arbitrary.suicidejunkie wrote:If the engine were able to throttle down to get anywhere near "just counteract gravity", then the landing would be MUCH easier, since the rocket could hover until it got things right (or ran out of fuel).
^ That's the bit I really worried about. Almost compensating for gravity means not accelerating much, but that is not all that relevant; you're not fighting gravity so much as you're fighting your own momentum over these small time scales. It is the velocity which is very important; you need to fall (0 thrust to be efficient) and then stop precisely (max thrust = quickest to be efficient, but also dangerous!)... landing involves controlling the thrust such that gravity is almost, but not quite compensated for."
The dynamics may not all be strictly Newtonian, but they are simple.suicidejunkie wrote:I think it is quite fair to say the *option* to hover is quite desirable, since it allows you to correct for quite a lot of undesirable situations. (Low velocity at any altitude becomes survivable) Needing to use it, however would not be good, but that's how safety margin works and at least it doesn't cost you mass when not used. Things diverge a bit from a simple Newtonian dynamics problem when you have air and wind and many other variables you can't control, after all.
In this SpaceX update article, it's stated that a landing at 2g's is 5.5x more fuel efficient than landing at 1.1g's. Knowing the 1st stage length = 70 meters, I fairly accurately determined the decent profile by analyzing the video in 1-sec increments. The plot shows a simple (de)acceleration profile at 0.9g, or a nominal thrust of ~1.9g which is consistent with the article's higher-g, more efficient algorithm and the previously mentioned engine specs. In the video, the rocket is descending at ~65m/s when the rocket base first becomes visible (at 2 sec), and the simple acceleration model closely predicts zero velocity at 7.5 seconds elapsed time and -2.7m altitude. Given the simplistic analysis, I think the fit surprisingly good (for this discussion anyway) supporting the Newtonian model and successful higher-g controlled descent.Chris Peterson wrote:...To make this cost effective, the system needs to land on the least possible amount of fuel. I imagine that the landing profile is designed with that in mind.