Author Topic: First stage recovery at down-range locations  (Read 146614 times)

Offline joek

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Re: First stage recovery at down-range locations
« Reply #40 on: 03/28/2013 01:47 am »
The main issue with down range recovery, at least as far as SpaceX is concerned, is that it doesn't work for the "rapidly" part of Elon's often stated
 "Fully and rapidly reusable" mantra.

Consider it a goal.  A logical progression might be: (a) reusable; then (b) cost-effective reusable; then (c) rapidly reusable; then (d) cost-effective rapid reusable.  No reason they have to attempt or accomplish all at the same time or within a fixed time frame.  Say again: (a) make it work; (b) make it better; (c) lather-rinse-repeat.

Online llanitedave

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Re: First stage recovery at down-range locations
« Reply #41 on: 03/28/2013 02:04 am »
How much risk NASA and ULA do/should take is off-topic for this thread.

My apologies, you're correct.
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Offline deltaV

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Re: First stage recovery at down-range locations
« Reply #42 on: 03/28/2013 02:28 am »
A really big catamaran would be easy and cheap to do, say 100mx100m would only cost a few millions (100m barges cost as little as 2-3million)

A SWATH boat is basically a type of catamaran designed for wave resistance.

SWATH boats are smoother than traditional displacement hulls, but they're still moving plenty in multiple axis. Their stability is most evident upon being under way, which makes for a moving target.

Why do you think "moving target" is a problem? As long as the boat is moving at a steady speed in a steady direction one can use the time-dependent coordinate system where the boat is stationary. The only net effect of steady movement is changing the effective wind, which is actually helpful if you're moving downwind.

Quote from: Halidon
And if "downwind" involves taking wave action at a bad angle your SWATH landing pad will be bouncing around quite dramatically.

From the Silver Cloud link I posted:
Quote
The building contract for Silver Cloud was most unusual in that it
specified the maximum vertical acceleration acceptable. For comparison,
NATO naval vessels require a maximum of 0.2g rms (1g = force of
gravity, rms = statistical average), a standard hardly ever achieved in
practical operation by ships less than 100 metres in length. On trials in a
two metre head sea, Silver Cloud recorded a dramatically smaller vertical
acceleration of 0.035g rms. The roll angles were also very low at 1.2
degrees rms in beam seas.

The video I quoted shows much larger seas (5m maybe?). It's hard to see from the video what the SWATH boat's acceleration is but I'd guess its at most a few tenths of a gee.

Can you give a citation for the poor performance of SWATH boats you're concerned about?

Make note here. Landing on a hard vessel deck is _worse_ than landing on land. If you catch a boat on an upswing your impact speeds are even worse than landing on land. If you're going to land on a hard surface anyway, just land on land.

I quoted above that Silver Cloud has 0.03 g vertical acceleration in a 2m sea. Even if this increases by an order of magnitude in rougher seas I don't see this being a serious problem. All the stage needs to do to compensate is add the few tenths of a gee vertical acceleration of the boat to its ~2 gees of thrust. If the rate of throttle change is limiting then small ~0.1 gee thrusters can solve that problem.

Just because it's feasible doesn't mean it's a good idea. Using landing legs its better. It gives a better margin for a safe landing if there is an error, or a gust of wind or something.

I'm not sure your statement that landing on legs have more margin is necessarily true. A gust of wind may actually be a bigger threat to landing on landing legs (even on land) then it would be landing on a hook/cable system at sea. To land on legs the stage needs to end up touching the ground, not moving much in any of the three dimensions, and not rotating much in either pitch or yaw. That's 7 parameters all simultaneously near-zero. A hook/cable system only needs the hook to be engaged with the wire (two degrees of freedom) and that hook should be moving slowly in those two directions, a total of 4 state parameters that need to be accurately near-zero. The other parameters can't be too huge but the required accuracy for the remaining state parameters should be a lot less than required for landing due to the inherent stability of hanging. The reduced number of state parameters that need to be controlled accurately should make the control problem easier. The cable/hook approach also seems more amenable to reducing accuracy requirements using shock absorption than a landing pad is.

Offline Lar

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Re: First stage recovery at down-range locations
« Reply #43 on: 03/28/2013 02:35 am »
Except during earthquakes the typical rms acceleration (vertically AND horizontally) of land is 0.000 g
« Last Edit: 03/28/2013 02:39 am by Lar »
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"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline deltaV

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Re: First stage recovery at down-range locations
« Reply #44 on: 03/28/2013 03:56 am »
Except during earthquakes the typical rms acceleration (vertically AND horizontally) of land is 0.000 g

Technically speaking the rotation of the Earth makes land accelerate downwards by up to about 0.03 m/s/s (depending on latitude). We don't notice because its effect on the surface of earth is (locally) equivalent to negative gravity.

More seriously I agree that land is more stable than a boat, but this seems like an easily solvable issue. The advantage of landing on a boat is increased payload to orbit. However landing at sea does need to be traded against just making the rocket a little bigger. Making the rocket bigger isn't always an easy option; e.g. a bigger Falcon Heavy would require non-roadable stages (or more cores, which presumably makes horizontal integration harder) and either oodles of engines or a new bigger engine. I guess landing at sea would only make sense economically if either you have lots of payloads a bit bigger than your rocket can handle (in RTLS mode) or launch prices drop so dramatically that the extra fuel burned by the bigger rocket is a big deal.

(Message substantially edited.)
« Last Edit: 03/28/2013 04:13 am by deltaV »

Offline meekGee

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Re: First stage recovery at down-range locations
« Reply #45 on: 03/28/2013 05:12 am »
How much risk NASA and ULA do/should take is off-topic for this thread.

My apologies, you're correct.

Mine too, this one strayed OT even more than usual.
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Offline meekGee

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Re: First stage recovery at down-range locations
« Reply #46 on: 03/28/2013 05:20 am »
If SpaceX's eventual main business will be Mars transport, then the launch direction will be fixed, and so a fixed asset can be used.

If the first stage lands, refuels (very partially), and flies back, then this solves all of the problems - no extra dV for boost back, no shipping infrastructure, etc.  We already know what a GH landing pad looks like, and there's no payload or US integration, etc.   The fly-back doesn't incur major stresses, and doesn't risk a payload, so in principle there's exactly zero processing at the pad.

It's a bit more difficult if the rocket can launch on a different azimuth every time, but Boca Chica kind of makes this problem go away.

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Offline mlindner

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Re: First stage recovery at down-range locations
« Reply #47 on: 03/28/2013 04:49 pm »
Looks like this thread has become semi-futile. Elon just re-iterated flight back to launch site in the most recent press conference and they're planning on attempting full flight back to launch site mid-2014.

Q: How will v1.1 booster recovery be attempted?
Musk: Initial attempt with water landing after ballistic arc, burn before atmosphere to reduce speed, then 2nd burn before splashdown. We don't expect success in the first several attempts. Later we will try to return booster to launch site with landing legs.

Q: First attempt to bring 1st stage back to launch site?
Musk: Not a specific flight, perhaps middle of 2014
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Offline meekGee

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Re: First stage recovery at down-range locations
« Reply #48 on: 03/28/2013 04:51 pm »
yup.
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Offline Avron

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Re: First stage recovery at down-range locations
« Reply #49 on: 03/28/2013 05:11 pm »
Clearly looks like downrange locations will not be needed..

Online llanitedave

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Re: First stage recovery at down-range locations
« Reply #50 on: 03/28/2013 05:36 pm »
For v. 1.1 and Falcon Heavy boosters, sure.  But I'm still skeptical they can do it with the Falcon Heavy core.
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Offline ArbitraryConstant

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Re: First stage recovery at down-range locations
« Reply #51 on: 03/28/2013 05:44 pm »
For v. 1.1 and Falcon Heavy boosters, sure.  But I'm still skeptical they can do it with the Falcon Heavy core.
I'm reserving judgement on the FH core. Seems very difficult no matter how you slice it since it won't have a full heat shield.

Offline guckyfan

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Re: First stage recovery at down-range locations
« Reply #52 on: 03/28/2013 05:54 pm »
For v. 1.1 and Falcon Heavy boosters, sure.  But I'm still skeptical they can do it with the Falcon Heavy core.
I'm reserving judgement on the FH core. Seems very difficult no matter how you slice it since it won't have a full heat shield.

If it had a full heatshield it would need to enter head first. That would mean it would have to flip in dense atmosphere. No way this can be done with a first stage IMO.

Offline Lar

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Re: First stage recovery at down-range locations
« Reply #53 on: 03/28/2013 06:10 pm »
I remain as convinced as ever (based on nothing at all other than my readings) that SpaceX won't do down-range recovery operationally. It just doesn't make economic sense.

That said I could see them doing some tests in various places such as barge landings. But I just do not see it as very likely, with Elon saying they want to be doing RTLS by mid 2014... The window for non launch site landings would be small.

I agree that the center core of FH might be the most challenging... harder to boost back than the side cores due to higher velocities, and maybe not high enough to do a once around without some deltaV in the forward direction...
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"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline oiorionsbelt

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Re: First stage recovery at down-range locations
« Reply #54 on: 03/28/2013 07:06 pm »
Perhaps boost forward to Vandenberg landing site may be more likely for FH core

Offline ArbitraryConstant

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Re: First stage recovery at down-range locations
« Reply #55 on: 03/28/2013 07:18 pm »
Perhaps boost forward to Vandenberg landing site may be more likely for FH core
Reentry would be brutal. That would be more than half way around the world - very close to orbital velocity.

Offline meekGee

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Re: First stage recovery at down-range locations
« Reply #56 on: 03/28/2013 07:23 pm »
Maybe come done with a real high horizontal velocity and skip the water like a stone around the globe.

Oh, Asia.

Nevermind.
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Offline oiorionsbelt

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Re: First stage recovery at down-range locations
« Reply #57 on: 03/28/2013 07:42 pm »
Perhaps boost forward to Vandenberg landing site may be more likely for FH core
Reentry would be brutal. That would be more than half way around the world - very close to orbital velocity.
yes, similar to second stage re-entry, which they are also working on.

Offline ArbitraryConstant

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Re: First stage recovery at down-range locations
« Reply #58 on: 03/28/2013 08:15 pm »
yes, similar to second stage re-entry, which they are also working on.
The second stage gets a full heat shield and is much shorter. The penalty for doing this on a full first stage seems prohibitive.

Offline Mongo62

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Re: First stage recovery at down-range locations
« Reply #59 on: 03/29/2013 01:24 am »
yes, similar to second stage re-entry, which they are also working on.
The second stage gets a full heat shield and is much shorter. The penalty for doing this on a full first stage seems prohibitive.

Although the reward (nine engines instead of just one) is much higher as well.

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