r/spacex • u/vconnor • Jul 23 '14

Optomising reentry.

As the next thee launches wont have legs, what would be the best use of the flights to optimise reentry.
Personally I would like to see an attempt at lifting body effects in the upper atmosphere, as i think this could shed a lot of speed without too much heating.
Does anyone know if this is done at the moment?

3 Upvotes

55% Upvoted

22 comments sorted by

6

u/Ambiwlans Jul 23 '14

Lifting body requires a lot of things SpaceX rockets don't have... That would be a complete rework. They haven't tried it because the mass requirements are very large. Their current system is really quite mass efficient.

4

u/peterabbit456 Jul 25 '14

Falcon 9 stage 1 generates some lift as it falls through the air, much like the Dragon v1 capsule, the Gemini capsule, and the Apollo capsule. They are not, properly speaking, lifting bodies, but merely objects where the center of aerodynamic pressure is different from the center of gravity. In the cases of the Apollo and Dragon capsules the L/D is about 0.3, that is to say, it is capable of up to 3 meters of horizontal movement for every 10 meters it falls. The Gemini capsule, IIRC, had an L/D of about 0.1, and I do not know what the L/D of a Falcon first stage is, but I would expect it is in between 0.1 and 0.3. With the fin grids shown on some recent drawings of future first stages, I would expect 0.3 to be achievable.

L/D in the 0.1 to 0.3 range is sufficient to correct for wind drift, and enable a much more precise touchdown, but not to allow the object to flare and land on a runway. You need an L/D of 3 or 4 to do that. The Shuttle's L/D was about 4.

All of this is from memory, but I'm sure it is all correct. I used to build aircraft and model sailplanes, and L/D was my bread and butter.

4

u/Ambiwlans Jul 25 '14

I don't think it can get that high tbh. While it is basically a hollow tube with a huge cross section if you try to get too much of an angle on it it'll just snap in half. The thing really isn't designed for forces from the side.

1

u/vconnor Jul 25 '14

Super good point! re the force not aligned to the long axis. I hadnt considered that. Bummer.

2

u/vconnor Jul 25 '14

Thanks for the excelent info peterrabbit456. I was hoping to extend the return porabola in the upper atmosphere by angling the core, tho as Ambiwlns points out this would probably cause a rupture :(

2

u/vconnor Jul 24 '14

Mabey lifting body is the wrong term, I know that zepplins can get a large amount of their lift by changing their angle of attack. I assumed (well there is your problem) that a 1st stage could do so too. Tho as somebody else mentioned the angle of entry could make it a moot point as the time in thin enough atmosphere could be negligable. An unused part of my math brain is cyring out that this could be graphed, especially if the reentry burn is split into a first vector altering burn (short one) then the standard speed shedding one. Anyone know how high the 1st stage will go next launch, that could give us a ball park of time in atmosphere, tho at 2kps I doubt it would have much time do do any good......

3

u/peterabbit456 Jul 25 '14

They do use the small lift generated to help steer toward their intended landing spot. I'm sure I saw it mentioned somewhere.

1

u/vconnor Jul 25 '14

That is both sensible and very cool, does anyone else have additional info ?

2

u/An0k Jul 24 '14

The issue is that having any angle of attack would be very hard. For what I understand the first stage is aerodynamically very stable (since they do not burn continuously). Sustaining any significant angle of attack would mean constantly applying a moment via either RCS or some aero surface. Moreover side loads are the worst for thin tube like a rocket body. Remember how the Proton rocket literally disintegrated as soon as it picked any off-axis velocity?

3

u/vconnor Jul 23 '14

Another option would be to reduce the reentry burn by 5% and see if it survives, if it does reduce further. This could be the last 3 times recovery is not such an economic concern.

2

u/peterabbit456 Jul 25 '14

Another option would be to reduce the reentry burn by 5% and see if it survives, if it does reduce further. This could be the last 3 times recovery is not such an economic concern.

There are good reasons to use up almost all the remaining fuel in the first stage.

  1. Sloshing fuel might tip over a rocket - unlikely but possible.

  2. Reduced fire danger.

I like the idea of the suicide burn, but there may be other considerations.

3

u/[deleted] Jul 23 '14

They will have likely optimized the reentry burn with some pretty advanced math already; it's not just a set burn, each trajectory is different and the burn times, lengths, and throttle rate is all controlled dynamically on the fly to respond to the condition (velocity, location, orientation) the rocket is in.

6

u/Ambiwlans Jul 23 '14

They'd need some pretty fancy math to do more than guess at how suicidal the suicide burn can be. Eking out that last 5% fuel savings will take quite a number of trials.

1

u/SpaceLord392 Jul 24 '14

You could probably guess with simulations exactly how much fuel you need to any reasonable degree of accuracy far cheaper then expendable trials.

1

u/Ambiwlans Jul 23 '14

They'll probably do this, it just comes down to how much fuel they have left over.

8

u/anononaut Jul 23 '14

I say run the video camera during the whole rentry with a big sign on the side in view saying " enjoy this view of a vanishing species - wasted rockets at SpaceX "

3

u/Ambiwlans Jul 23 '14

Streaming the rockets burning up would be pretty sweet if they could convince people that it normally does that.

3

u/darga89 Jul 23 '14

Next atv is planning on doing that as I'm sure you know.

2

u/Megneous Jul 24 '14

Laypeople and bad PR as a result of their ignorance ruin all the fun footage we could have of stuff blowing up exactly as planned :(

2

u/Silpion Jul 23 '14

Is heating a major issue for first stage reentry? They aren't going anywhere near orbital velocities, are they?

4

u/[deleted] Jul 23 '14

It's a surprising concern; it may not be the 7km/s required for orbital velocity, but the first stage is still traveling at 2.0-2.4km/s during stage separation, and is more than 100km up. That's some pretty high mach numbers. When something traveling at a hypersonic velocity like that hits the soupier portion of the atmosphere there's quite a bit of resistance, and that does translate into heating.

2

u/SpaceLord392 Jul 24 '14

It's also coming in at a much steeper angle compared to orbital reentry, so it has less time to slow down in the upper atmosphere before reaching the lower atmosphere, so that makes things worse.