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Until now most information out of SpaceX has suggested it needs roughly 30% propellant left in the first stage of Falcon 9 to attempt a retrieval. Numbers are
by jpgvm 12y ago
Until now most information out of SpaceX has suggested it needs roughly 30% propellant left in the first stage of Falcon 9 to attempt a retrieval.
Numbers are not yet known for the 2nd stage retrieval or Falcon Heavy. One could probably assume a similar number for the first stage but possibly a bit less given the Falcon Heavy's higher thrust/weight ratio.
- deleted 12y ago[deleted]
- grecy 12y agoIs SpaceX's long term plan to also have 2nd stage retrieval? (I didn't know that)
- jccooper 12y agoYes; in fact they have videos showing the whole stack re-entering and doing a propulsive landing (one at a time, of course.) The second stage will need a re-entry heat shield, but otherwise can land like the first. Tricky part is getting enough margin in the second stage for the landing fuel and legs and heat shield. Usually the top stage is the one you optimize most heavily. So plausible, but tricky. Good news is that the second stage is much cheaper than the first, at least on the F9. Eight fewer engines, for one.
- davidwhodge 12y agoCool. Where might one find those videos? I can't seem to locate them.
- ejr 12y agoTheir website has been changed quite a bit and old linked videos are hard to find. I found a copy uploaded to YouTube in 2011 : https://www.youtube.com/watch?v=sSF81yjVbJE https://www.youtube.com/watch?v=sSF81yjVbJE Edit: Found the original copy on the Web Archive http://web.archive.org/web/20110615000000*/http://www.spacex.com/assets/video/spacex-rtls-green.mp4 http://web.archive.org/web/20110615000000*/http://www.spacex...
- kashkhan 12y ago30% sounds a bit extreme, especially since it is unloaded. I think they could probably do with 5% or less since the vehicle is unloaded (all the propellant is burnt out) and burns for only a few seconds for soft landing, with most of the braking from peak velocity provided by aero braking.
- ejr 12y agoThere's deceleration involved as well. Here's a very nice video of the shuttle boosters on their way down to give you an idea of the velocities involved. https://www.youtube.com/watch?v=2aCOyOvOw5c https://www.youtube.com/watch?v=2aCOyOvOw5c An enormous amount of inertia to overcome so 30% doesn't sound so unlikely.
- rspeed 12y agoThere are a total of three burns: 1. Alter the path so the parabolic arc goes back to the launch site. 2. Braking burn during descent to minimize atmospheric stress. 3. Landing burn (AKA "suicide burn") to slow the vehicle enough for the landing legs to absorb the impact.
- ejr 12y agoIndeed. Combined, these all burn quite a large volume, but #1 is the most costly in terms of fuel, I think. It's essentially a reversal of the flight path.
- rspeed 12y agoNot quite reversal since it doesn't negate the vertical motion, but yeah. Definitely the most significant of the three burns. Though it's not quite that simple. Based on the trajectory of last week's launch, they're actually going to use a much steeper trajectory during the first stage launch. The result is that the first stage uses significantly less propellant to make that first burn (since its downrange velocity is much smaller), but it cuts into the payload capacity quite a bit by forcing the second stage to do more work.
- m_mueller 12y agoFrom what I've read they need 30% additional fuel at launch (as in a 30% bigger first stage) - that's different than needing 30% left in the rocket after separation. The reason being that to have X [kg] of fuel at Y [m] altitude you'll need (1 + Z(Y)) * X [kg] of fuel at launch, with Z being some function of Y. My gut tells me they'll need 15-20% max for retrieval, the 30% + is just to get it up there. Source: Play KSP.
- jpgvm 12y agoAhh good catch, thanks for the additional insight.