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Background on the Falcon 9 Launch

spacex.com

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Re: Background on the Falcon 9 Launch

#21

Post by msandford »

msandford wrote: post #4
mrec wrote: post #3

One thing I've not seen mentioned in the coverage so far: how much payload is sacrificed by the need to keep fuel in reserve for the return to base? I'm guessing the sacrifice is roughly equal to the mass of unburnt fuel in the booster at the point of booster separation, but don't much trust my intuition on these things.

You're right not to trust your intuition! Good call. It's actually going to be much, much less than that due to the nature of the rocket equation. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Given that I'm not wrong as the above link shows (http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge...) would anyone care to explain to me for a second why I'm getting downvoted?

I'm happy to be less stupid if that's the problem, but I don't know what thing it was that I did that was stupid.

Re: Background on the Falcon 9 Launch

#22

Post by panic »

Nice article, but I couldn't pass up a chance to correct Elon Musk's math:

It is important to note that the amount of energy needed to achieve a given velocity increases with the square, so going from 1000 km/h to 2000 km/h takes four times as much energy as going from 0 km/h to 1000 km/h, not twice as much.

Three times, not four--you already spent a quarter of the energy getting to 1000 km/h. Getting the rest of the way to 2000 km/h takes the remaining three quarters.

Re: Background on the Falcon 9 Launch

#23

Post by w_t_payne »

If we can land the rocket accurately enough to put it down on a tiny barge only slightly larger than the rocket itself, then why do we need to tolerate the weight of the landing legs?

We already have industrial robots that can move and grasp heavy weights relatively quickly over distances of several metres -- it doesn't take much imagination to conceive of a similar contraption being used to arrest the descent of the rocket over the final few tens of metres of its' descent - a sort of brobdingnagian robotic catcher's mitt.

Granted, this might be a bit on the expensive / elaborate / bizarrely over-engineered side -- but it would look utterly awesome.

Re: Background on the Falcon 9 Launch

#24

Post by kibwen »

The article mentions that the water landing requires less fuel to return the rocket because it doesn't have to spend fuel overcoming its initial ballistic trajectory. In the water landing scenario, how far away from the launchpad is the landing barge? I'm wondering about the economics of launching from a site where your first stage trajectory is entirely overland, to avoid the complications of landing on a barge that's being tossed in the sea. Though it might be hard to find such a site in U.S. territory that's both near the equator and sparsely populated.

Re: Background on the Falcon 9 Launch

#25

Post by tunesmith »

blizkreeg wrote: post #17

Clearly this man's intellect is through the roof. Reading this post and him explaining these concepts in such first-principle terms (despite not being a physicist/rocket engineer) indicates his in-depth understanding (nothing new there). I know he has a Bachelors in Physics but bear with me. But, I'm just in awe and I keep thinking 'how does he do it?'. He's running two intensely technical and risky companies. Yet he…

He's commented on this type of thing before and it seems like he thinks it isn't any great trick. The main thing is that if you're going to reason well from first principles, you have to learn from first principles. If you build your "tree of knowledge" efficiently, then adding on to it isn't so difficult because you have a good core. Contrast this with the hobbyist that tries to learn rocket science when they don't have a good handle on first-year physics.

Re: Background on the Falcon 9 Launch

#26

Post by idlewords »

I'm having some trouble with the opening salvo here:

"Now imagine placing a marble somewhere on that slippery sheet -- it is guaranteed to fall into one of the funnels. "

This holds for the case where there are two objects initially at rest, but I don't see it as obviously true if there are more than two objects in the universe.

Re: Background on the Falcon 9 Launch

#27

Post by w_t_payne »

w_t_payne wrote: post #23

If we can land the rocket accurately enough to put it down on a tiny barge only slightly larger than the rocket itself, then why do we need to tolerate the weight of the landing legs? We already have industrial robots that can move and grasp heavy weights relatively quickly over distances of several metres -- it doesn't take much imagination to conceive of a similar contraption being used to arrest the descent of the…

... or (as a less mechanically complex solution ...) use the same concept as the arrester cables on an aircraft carrier flight deck -- but upended to catch a vertically descending vehicle.

(No flight deck required -- just cables suspended between two towers and an arrester hook at the top of the rocket -- which just has to be lighter than folding legs at the bottom).

Re: Background on the Falcon 9 Launch

#28

Post by tcheard »

mikeash wrote: post #8
mannykannot wrote: post #6

Earlier quoted context omitted.

I don't think you could allocate all of the landing fuel load to payload as any addition to the payload also requires additional 2nd. stage fuel.

You can run the first stage longer without any extra second stage fuel. Adding performance to either one (within limits) increases the performance of the system. Using the landing fuel for a launch would allow for a heavier payload without changing the second stage. The first stage would do more of the total work.

Except that the longer you run the first stage, the longer you have to carry the weight of the first stage. It eventually becomes much more efficient to jettison the first stage and use a slightly less powerful but much lighter weight second stage, because you no longer need as much power to get through the atmosphere. Which is the whole point behind staging (https://en.wikipedia.org/wiki/Multistage_rocket#Optimal_stag...).

It turns out when you add 10 lbs of weight to the first stage for reuse you lose approximately 1 lbs of weight from the max payload. (http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenge...)

On the second stage every 1 lb of reuse you lose 1 lb of payload, which is one of the big reasons why second stage reuse isn't really feasible on the F9.

Re: Background on the Falcon 9 Launch

#29

Post by jholman »

idlewords wrote: post #26

I'm having some trouble with the opening salvo here: "Now imagine placing a marble somewhere on that slippery sheet -- it is guaranteed to fall into one of the funnels. " This holds for the case where there are two objects initially at rest, but I don't see it as obviously true if there are more than two objects in the universe.

I see two aspects to your question/comment: at-rest, and two-objects.

The whole point of that comment was to apply only to at-rest things (he goes on to contrast it with objects with velocity), though I think maybe you got that.

If you have a marble and two other objects, the other objects make two funnels, and there is a saddle (itself curved) where the two objects's funnels are equipotent. That's the three-body case. If it were possible to balance perfectly on this line of equipotence, you'd just slide to the lowest point on that line, and stay there, out of the funnels. This, IIUC, is one of the Lagrange points... L1, I think. In theory it's possibly-stable, but its stability exhibits negative feedback, so in practice it's impossible (although with station-keeping rockets, it's cheaper to hover there than most places).

As you add more funnels, you just get more of these saddle lines intersecting. There are many infinitely-fussy places in the universe where you could in-theory-but-not-in-practice hover without falling into a funnel (if it weren't for brownian motion and maybe some other quantum effects that disturb your infinitely-difficult equilibrium).

Of course, this whole analogy doesn't account for the fact that all of the bodies are acting on each other, not just the funnels acting on the marble.

Re: Background on the Falcon 9 Launch

#30

Post by mrfusion »

w_t_payne wrote: post #23

If we can land the rocket accurately enough to put it down on a tiny barge only slightly larger than the rocket itself, then why do we need to tolerate the weight of the landing legs? We already have industrial robots that can move and grasp heavy weights relatively quickly over distances of several metres -- it doesn't take much imagination to conceive of a similar contraption being used to arrest the descent of the…

That's a great idea. But I think they want something really general purpose that can land anywhere and possibly even on Mars.

Also the empty rocket might be too weak to be grabbed by anything. They compare the thickness to a tin can.

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