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Return of a falcon9 stage fails
12-06-2018, 04:36 PM
Post: #1
Return of a falcon9 stage fails
Video

https://www.twitch.tv/dasvaldez/clip/Cle...EggPrimeMe

More info
https://danielmarin.naukas.com/2018/12/0...rcera-vez/

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12-07-2018, 10:17 PM
Post: #2
RE: Return of a falcon9 stage fails
As neat as it is to watch these SpaceX fired landings (regardless of whether they work or not!), the engineer in me always wonders we they don't employ at least some deployable aerodynamic means of creating drag to help slow the descent to offload some of the work from the engines. That could be a parachute or some other sort of other drag element that pops out.

Retaining enough fuel on board to do ALL the deceleration via fired burn adds a WHOLE lot of weight, which is so precious in rockets / lift vehicles. All that "landing fuel weight" reduces the lifting capacity.

I trust they know what they are doing, but it's still really curious to me. They are in the Earth's atmosphere... Why not take advantage of that to at least reduce the landing fuel that needs to be used? The cynic in me thinks the engineers probably wanted to, but Elon Musk told them they couldn't because it wouldn't look as cool on YouTube. :-)
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12-08-2018, 12:33 AM
Post: #3
RE: Return of a falcon9 stage fails
I suspect that reentry from near space at high speed would damage the engines which aren't protected by a heat shield, so a significant burn is needed anyway. Landing at a given spot under a parachute is unlikely with any significant surface winds. The structure is also designed for compressive loads while a parachute would apply large tensile loads. Finally, since the vast majority of the fuel and oxygen are already used, the lower stage is very light compared to at liftoff, minimizing the amount of fuel needed to propel it at high accelerations to bring it back. All told, the weight difference may be less than you'd expect with the tradeoff being high control of the return.
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12-08-2018, 12:39 AM
Post: #4
RE: Return of a falcon9 stage fails
(12-07-2018 10:17 PM)burkhard Wrote:  That could be a parachute or some other sort of other drag element that pops out.

Perhaps doing that would affect the accuracy on trying to land on a very specific, relatively small place, such as a platform in the sea or some delimited zone.

An unexpected stroke of wind and it would strike the sea water or land over some buildings nearby.
Just saying.

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12-10-2018, 06:59 PM (This post was last modified: 12-10-2018 07:04 PM by burkhard.)
Post: #5
RE: Return of a falcon9 stage fails
(12-08-2018 12:33 AM)Jim Horn Wrote:  I suspect that reentry from near space at high speed would damage the engines which aren't protected by a heat shield, so a significant burn is needed anyway. Landing at a given spot under a parachute is unlikely with any significant surface winds. The structure is also designed for compressive loads while a parachute would apply large tensile loads. Finally, since the vast majority of the fuel and oxygen are already used, the lower stage is very light compared to at liftoff, minimizing the amount of fuel needed to propel it at high accelerations to bring it back. All told, the weight difference may be less than you'd expect with the tradeoff being high control of the return.

You raise some good points Jim, although the issue of tensile v. compressive loading depends on where the parachute cables are ultimately anchored and applying force when they are under load. If they mount to the top of the structure, yes, it is tensile. If they actually mount near the bottom and are merely guided up through the center, it's a compressive load, just like the engine firing.

In any event, I wouldn't box oneself into thinking only parachutes either, nor would I expect it to be wholly without *any* engine firing. I'm talking about reducing the work the engines do, not eliminating it.

There are all sorts of extendable panels which could add some free drag, and yet still allow the engines to do 50% (or more) of the work and handle guidance on descent. On Earth, atmospheric drag is there for the taking. They let the thing fall like a stone (picking up kinetic energy which they later have to thrust like mad to reduce) for a while before even firing the engines at any significant level, don't they?

I'd like to get a couple of SpaceX engineers out for some adult beverages and after a few hours get some straight unfiltered opinions out of them. Musk is a master showman and viral internet direct marketer. I can't help but think that the burn-only landing is strongly influenced by his hand and his flair for the dramatic rather than by pure engineering judgement.
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12-11-2018, 10:58 AM
Post: #6
RE: Return of a falcon9 stage fails
Agreed, it would be good to spend some quality time with rocket scientists. Fortunately, you don't need to, because there are lots of resources out there.

The question of parachutes for SpaceX is almost an FAQ... in fact I see it is in the FAQ.

If you play a few rounds of a lunar-lander type game you'll see that burning as late and as hard as you can is the way to use least fuel. The nature of the rocket equation is that using least fuel is very desirable indeed. One way to see this is to time-reverse a landing, using negative fuel consumption. If you took off really slowly, climbing up in the gravity field is just spending fuel unnecessarily. Your rocket is heavy, and you want to get it into free-fall. Imagine a rocket with only just enough thrust to hover: it spends all its fuel and gains no kinetic energy at all.

There are also relevant answers on the Space Exploration Stack Exchange:
AIUI, the problem with parachutes for re-entry is that by the time you've picked up enough atmosphere to get any useful drag, you're already moving so fast that you will have a serious heat problem - as well as a parachute problem.

It might be relevant for SpaceX especially, that they want to get the boosters back to land, or not too far out to sea. So they need to kill a lot of horizontal velocity, fast. See also

Having said all that, it seems SpaceX are thinking, or were thinking, of some kind of toroidal ballute for getting the second stage back down for reuse. The second stage is much smaller and will be going much faster, so the tradeoffs are different: no chance to get back to near shore, and a shallow trajectory.
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12-11-2018, 12:52 PM
Post: #7
RE: Return of a falcon9 stage fails
Adding another system would also add complexity. Think about your car's heater, the engine is already making heat so adding a heater is trivial. But adding in air conditioning adds a bunch more complexity. Same applies here, you already have a rocket engine and all its controls so adding a little more fuel is trivial. Adding a secondary system like a parachute adds complexity.
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12-11-2018, 04:23 PM (This post was last modified: 12-11-2018 04:39 PM by burkhard.)
Post: #8
RE: Return of a falcon9 stage fails
(12-11-2018 10:58 AM)EdS2 Wrote:  Agreed, it would be good to spend some quality time with rocket scientists. Fortunately, you don't need to, because there are lots of resources out there.

Harumph.... I'd prefer to spend time with the engineers as these are questions of engineering, not science, but I get your point.

Lots of good links you sent with some pretty detailed answers. Thanks for sending those. Among the more interesting points was that they retain only 10% of the fuel for landing, which perhaps isn't too bad. 48,000 kg seems like a big number, but it does need some perspective.

I like the commercialization of the launch vehicles as it enables varied ideas to compete for the best solution. Maybe SpaceX has the future paradigm here which everyone else will ultimately follow. Or maybe it will get abandoned. Remember in the early days of the "innovative reusable" shuttle when NASA tried to tell people they would launch every couple of weeks and chances of a catastrophic shuttle mission failure were on the order of 1:10,000? Time showed catastrophic failures were more like 1:75, but we were locked into that design for decades. Having multiple players in the mix enables the best ideas to shake out. We'll see what works and there will be lots of learning in the process.

In the interim, I enjoy watching them land (especially that pair in unison some months back... stunning). But even if they don't land successfully, it's good to see them fail spectacularly right under the cameras at a landing site, rather than far out at sea, wherever an unguided parachute might take them. That alone is a good "feature". :-)
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12-11-2018, 04:33 PM
Post: #9
RE: Return of a falcon9 stage fails
(12-11-2018 12:52 PM)EugeneNine Wrote:  Adding another system would also add complexity. Think about your car's heater, the engine is already making heat so adding a heater is trivial. But adding in air conditioning adds a bunch more complexity. Same applies here, you already have a rocket engine and all its controls so adding a little more fuel is trivial. Adding a secondary system like a parachute adds complexity.

That is a fair point, especially as additional systems reduce reliability. With recovery still rather dicey at present, putting the brakes on "complexity creep" is probably prudent.
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