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Comment by ilaksh | Hacker News Reader

The hardest part is not running out of fuel on the way down. Old-school science fiction often assumed something like nuclear power where that wouldn't be a concern. Seems like for many years most people haven't even attempted this full recovery/landing thing because of fuel and weight issues.

Has SpaceX really cracked that problem? What portion of flights on this smaller rocket can do it? It says some missions won't have enough fuel left over.

Will the Falcon 9 Heavy be able to do all or almost all missions with a landing and full recovery at the end? How much fuel will Heavy have left on average after a return? Theoretically it would not wait until the last possible second to start slowing, and therefore use up almost all of the fuel in order to give the descent a larger margin of error and also reduce the possibility of damage, I assume.

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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 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.

Is SpaceX's long term plan to also have 2nd stage retrieval?

(I didn't know that)

Yes; 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.

Cool. Where might one find those videos? I can't seem to locate them.

Their 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

Edit: Found the original copy on the Web Archive http://web.archive.org/web/20110615000000*/http://www.spacex...

30% 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.

There'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

An enormous amount of inertia to overcome so 30% doesn't sound so unlikely.

There 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.

Indeed. 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.

Not 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.

From 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.

Ahh good catch, thanks for the additional insight.

I'm sure that all this information is publicly available. There's nothing wrong with asking these question in a forum, but you probably shouldn't suggest that it's some big unknown and you should probably acknowledge that you're asking people to do research for you.

Far as I know, the answer to this question is by far the most relevant in the entire thread.

Even if I were an expert, the starting point is asking the question, and if I made claims about the answer, someone would come along and say it is still unknown as to whether they can do that.

It should be framed as a question because it is an open question. That's the way technology works. You don't really know you can do something until you do it.

So this question is an attempt to inform you and others of the correct context for discussion. Of course it failed.

Right now there is a bit of a slight impedance mismatch between attempted reusability and the commercial launch offering of the full expendable Falcon 9 capability. On the plus side the dual role nature of the Falcon 9 v1.1 means they can take advantage of expendable launches with sufficient margin to test the reusability features, on the minus side it means that when there isn't that margin they don't get free test flights.

However, once the F9 v1.1 has been thoroughly tested in the reusable configuration they will stop offering the expendable option. Every flight will have the first stage flyback for reuse, and only payloads which allow that flight profile will be flown, leaving more than enough fuel margin to allow return and some contingency.

Larger payloads will simply be flown on the Falcon Heavy, which will reuse the lower stage cores from day one. Even if that means no cross-feed and a massive hit to the payload capacity the Falcon Heavy should still be able to loft payloads larger than the Delta IV Heavy and with reusability it should be able to do so at less than the current cost of the F9.

Don't forget you can do an expendable configuration on the final launch before 'retirement'.

That's expendable launch (at reusable prices!) every 5-10+ launches per vehicle.

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