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SpaceX Grasshopper Flies High

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Re: SpaceX Grasshopper Flies High

#141

Post by InclinedPlane »

jlgreco wrote: post #122
InclinedPlane wrote: post #62

Earlier quoted context omitted.

Perhaps on Mars, unlikely on Earth. With our dense atmosphere a vehicle like a mostly empty Falcon 9 first stage is going to have a fairly low terminal velocity, in the low hundreds of km/s range. Slowing down from that speed to a controlled hover/landing is pretty easy. The cost/benefit on Mars might be different though, since the atmosphere is thinner.

> in the low hundreds of km/s range I think you might mean m/s? low hundreds of km/s is very supersonic. ;)

What am I, a physicist? Jeez, boffing a few orders of magnitude like that is pretty bad, and it's too late to edit.

I meant kph, which was HOPEFULLY obvious from context.

I'll just leave this here: http://www.smbc-comics.com/?id=2679

Re: SpaceX Grasshopper Flies High

#142

Post by numbsafari »

InclinedPlane wrote: post #118
lutze wrote: post #66

Earlier quoted context omitted.

I don't understand the need for the intermediary stages, couldn't the main rocket just deliver the payload and then land? As it is in that video, it's having to carry engines and fuel for 3 separate stages, for no apparent reason.

It would be more convenient and cost effective to do so, if we could. But we lack the technology. The rocket equation is a harsh mistress, it demands exponential amounts of fuel the faster you want to accelerate a given stage (specifically, as a ratio to the exhaust velocity of the rocket). Given that orbital velocity is quite high (about 8.5 km/s) relative to the exhaust velocity of the best chemical propellants (ab…

What you need is a transporter that can teleport you from a space-bound craft to a surface location and back without all of this rocket launching business to deal with.

Re: SpaceX Grasshopper Flies High

#143

Post by numbsafari »

ParahSailin wrote: post #138
lutusp wrote: post #134

Earlier quoted context omitted.

On the bright side, SpaceX is so economical compared to NASA's past methods, that if it succeeds, we will all benefit. So what might be characterized as theft at one point in time, might eventually be seen as a great investment. Only time will tell.

For all Elon Musk's libertarian rhetoric, he sure disregards it whenever expedient.

Most libertarians do. It's called making money and all's fair in love and business.

Re: SpaceX Grasshopper Flies High

#144

Post by TheSOB88 »

numbsafari wrote: post #142
InclinedPlane wrote: post #118

Earlier quoted context omitted.

It would be more convenient and cost effective to do so, if we could. But we lack the technology. The rocket equation is a harsh mistress, it demands exponential amounts of fuel the faster you want to accelerate a given stage (specifically, as a ratio to the exhaust velocity of the rocket). Given that orbital velocity is quite high (about 8.5 km/s) relative to the exhaust velocity of the best chemical propellants (ab…

What you need is a transporter that can teleport you from a space-bound craft to a surface location and back without all of this rocket launching business to deal with.

That would be quite convenient... A little too convenient?

Re: SpaceX Grasshopper Flies High

#145

Post by russell »

I watched the videos (no sound - at work) and it seems like from the discussion that they are thinking of returning to the launch site. I looked at the Saturn V data and the first stage came down 350 miles down range and the second stage 2300 or so miles down range. Are the SpaceX launches going to have little downrange component for the first stage?

Re: SpaceX Grasshopper Flies High

#146

Post by HodCarrier »

petenixey wrote: post #35
EthanHeilman wrote: post #30

Earlier quoted context omitted.

Fuel is very inexpensive (~2% the cost of a rocket launch), rockets are very expensive. If you can trade off fuel for not having to build a new rocket you drop the cost to orbit by orders of magnitude. Additionally you can build more expensive rockets that are more efficient since rocket creation becomes a capital rather than a reoccurring cost.

But why not just use a parachute and airbags? They're going to be a vast amount lighter than the fuel you'd need for a rocket landing.

Elon wants rapid (think aircraft), turn around times. That's what's driving the decisions.

Re: SpaceX Grasshopper Flies High

#147

Post by JshWright »

InclinedPlane wrote: post #105
JshWright wrote: post #89

Earlier quoted context omitted.

>very large contracts which result in absolutely nothing That's a moderately ironic statement, given the fact that SpaceX wouldn't be doing anything remotely close to what they're doing today without the very large contract they got from NASA.

Without NASA contracts (to be noted: fixed price contracts predicated on delivery of goods, for the most part) SpaceX would have much less cash on hand and their pace of R&D would be much slowed. But they would still exist and still be pushing the state of the art, just at a slightly slower pace. They have one of the most competitive orbital launchers on the market, they have a ton of commercial business already on t…

That's essentially what I meant... They'd still be making progress, they just got to make progress much faster thanks to some big contracts from NASA.

Re: SpaceX Grasshopper Flies High

#148

Post by JshWright »

slacka wrote: post #136
mikeash wrote: post #84

Earlier quoted context omitted.

To put it in perspective, the fuel used to launch a Space Shuttle cost in the neighborhood of $1 million, while the cost of the entire launch was somewhere around $500 million to $1 billion.

Could and real or kerbal rocket scientists here explain why SpaceX isn't doing something like the curiosity rover landing? Sure there's 3 extra steps, deploy parachute, cut parachute, and evasive maneuver, but the fuel saving would massive.

I'm not sure the savings would be as massive as you think (and would be offset by other costs).

Parachutes (and associated equipment) are heavy, so you'll burn fuel lifting that extra weight.

Parachutes are complicated, and would be an extra system to develop, test, and validate.

Parachutes are annoying to repack/replace (increasing turnaround time).

Parachutes put odd stresses on large objects when they deploy (increasing the amount of inspection you would have to do after each flight).

All that hassle to reduce the terminal velocity by a couple hundred miles an hour. That's not that big a win for a pretty high cost.

Re: SpaceX Grasshopper Flies High

#149

Post by cbhl »

deepblueocean wrote: post #25
jessriedel wrote: post #21

For those of you who haven't seen it, here is SpaceX's fantastic animation of their ultimate goal for Falcon 9 (complete with soundtrack by Muse): http://www.youtube.com/watch?v=sWFFiubtC3c This will give you an idea of how grasshopper fits into the full flight plan.

I find this confusing. It's awesome, that's for sure. But it seems like lifting all the fuel needed to do a soft landing is inherently a tradeoff for less payload capacity. What is the benefit, then? Is it just so much cheaper to be able to re-use the first two stages?

"With an advanced rocket you can do maybe two to three percent of your lift-off mass to orbit, typically. And then reusability subtracts two to three percent. So then you've got nothing toward or negative and that's also not helpful. So, the trick is to try to shift that from two to three percent in the expendable configuration, to make the rocket mass efficiency, engines efficiency, and so forth so much better so that it moves to around three and half to four percent in the expendable configuration and then try to get clever about the reusablility elements and try to drop that to around the the one and a half to two percent level so that you have a net payload of about two percent."

http://youtu.be/vDwzmJpI4io?t=27m

(Watch from 27m for about 50 seconds.)

Re: SpaceX Grasshopper Flies High

#150

Post by JshWright »

dspeyer wrote: post #120
jessriedel wrote: post #21

For those of you who haven't seen it, here is SpaceX's fantastic animation of their ultimate goal for Falcon 9 (complete with soundtrack by Muse): http://www.youtube.com/watch?v=sWFFiubtC3c This will give you an idea of how grasshopper fits into the full flight plan.

How does the first stage not get damaged by the backblast from the second stage seperating?

SpaceX uses pneumatic 'pushers' to separate the stage. After a few seconds the two stages are far enough apart that igniting the second stage won't cause any real damage to the first stage.

Check out the video on this page for a good view of what that process looks like: http://www.extremetech.com/extreme/149741-spacex-falcon-9-la...

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