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> if he honestly believes that SES-9 required 100,000 pounds of fuel to make its landing attempt, should he really be part of the upper management of a rocket c

> if he honestly believes that SES-9 required 100,000 pounds of fuel to make its landing attempt, should he really be part of the upper management of a rocket company? What is the correct amount of fuel used to make the landing attempt? I just googled and didn't quickly find it, though I did find this speculation at StackExchange[0] talking about reserving 15% of the fuel, or 66 tons, for the landing. 66 tons is 132,000 pounds, so 100,000 pounds seems a reasonable rounding - what am I missing? (I don't know anything about rockets.) [0] http://space.stackexchange.com/questions/4466/how-is-spacex-going-to-land-their-falcon-first-stage http://space.stackexchange.com/questions/4466/how-is-spacex-...

mikeash 11y ago
First, a quick overview on how the landing works. After the first stage separates, it executes a "boostback burn," using the engines to get it moving toward the landing site. Sometime later, it fires the engines again as part of the "reentry burn," which is used to soften the blow of hitting the atmosphere at Mach Whatever. Apparently the rocket exhaust acts a little bit like an ad hoc heat shield, plus it helps slow the thing down. Finally, right as it approaches the landing site, it fires the center engine a final time to, hopefully, bring the rocket to a stop at zero altitude. The amount of fuel used varies a lot, because they can land in different places. At one extreme, you have a landing at the launch site, which uses the most fuel in order to get the first stage turned around and coming back. Barge landings use less fuel, because they can be positioned under the flight path so the rocket doesn't have to maneuver as much. SES-9 was at the other extreme. It reserved the bare minimum of fuel for the landing attempt because of its heavy, high-performance payload. That meant no maneuvering boostback burn, no or minimal reentry burn, and the landing burn itself was done with three engines instead of the typical one. That saves fuel because the rocket decelerates faster and spends less time fighting gravity, but of course the control challenge is magnified, which is probably why it went all explodey. Putting some numbers on it, I found some estimates here: https://www.reddit.com/r/spacex/comments/2vuy1v/how_much_fuel_do_the_boost_back_reentry_and/ https://www.reddit.com/r/spacex/comments/2vuy1v/how_much_fue... According to that, landing at the launch site would require about 38 tons of fuel, and a (normal) barge landing would require about 20 tons. SES-9's barge landing would have been even less. I believe the 15% from your StackExchange link is confused, and someone mistook the payload penalty (about 15% for a barge landing) for the amount of fuel required.
- TrevorJ 11y ago
  38 tons for RTLS is 76,000 pounds, so the figure isn't really all that exaggerated for the worst case. In any case, the fuel is cheap relative to everything else so as long as they can put the payload on orbit expending almost any amount of fuel to recover the rocket is perfectly reasonable. Not sure why the ULA guy thought spending 100,000 of fuel to recover a 15 million dollar core would be a bad thing.
  - mikeash 11y ago
    He was specifically referring to SES-9, which was the absolute minimum fuel usage. Probably a bit beyond minimum, given that it cratered. And yeah, even if it was 100,000 pounds, what's the point?

mikeash 11y ago

First, a quick overview on how the landing works. After the first stage separates, it executes a "boostback burn," using the engines to get it moving toward the landing site. Sometime later, it fires the engines again as part of the "reentry burn," which is used to soften the blow of hitting the atmosphere at Mach Whatever. Apparently the rocket exhaust acts a little bit like an ad hoc heat shield, plus it helps slow the thing down. Finally, right as it approaches the landing site, it fires the center engine a final time to, hopefully, bring the rocket to a stop at zero altitude. The amount of fuel used varies a lot, because they can land in different places. At one extreme, you have a landing at the launch site, which uses the most fuel in order to get the first stage turned around and coming back. Barge landings use less fuel, because they can be positioned under the flight path so the rocket doesn't have to maneuver as much. SES-9 was at the other extreme. It reserved the bare minimum of fuel for the landing attempt because of its heavy, high-performance payload. That meant no maneuvering boostback burn, no or minimal reentry burn, and the landing burn itself was done with three engines instead of the typical one. That saves fuel because the rocket decelerates faster and spends less time fighting gravity, but of course the control challenge is magnified, which is probably why it went all explodey. Putting some numbers on it, I found some estimates here: https://www.reddit.com/r/spacex/comments/2vuy1v/how_much_fuel_do_the_boost_back_reentry_and/ https://www.reddit.com/r/spacex/comments/2vuy1v/how_much_fue... According to that, landing at the launch site would require about 38 tons of fuel, and a (normal) barge landing would require about 20 tons. SES-9's barge landing would have been even less. I believe the 15% from your StackExchange link is confused, and someone mistook the payload penalty (about 15% for a barge landing) for the amount of fuel required.

TrevorJ 11y ago
38 tons for RTLS is 76,000 pounds, so the figure isn't really all that exaggerated for the worst case. In any case, the fuel is cheap relative to everything else so as long as they can put the payload on orbit expending almost any amount of fuel to recover the rocket is perfectly reasonable. Not sure why the ULA guy thought spending 100,000 of fuel to recover a 15 million dollar core would be a bad thing.
- mikeash 11y ago
  He was specifically referring to SES-9, which was the absolute minimum fuel usage. Probably a bit beyond minimum, given that it cratered. And yeah, even if it was 100,000 pounds, what's the point?