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SpaceX Grasshopper Flies High

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Re: SpaceX Grasshopper Flies High

#71

Post by rplnt »

jerf wrote: post #44
rplnt wrote: post #12

Earlier quoted context omitted.

I believe same thing was done by NASA in the 70s. So not really big step forward in this particular regard. edit: Can't find it at the moment. So far I've found http://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X which did what grasshopper and more almost 20 years back. I still believe I've read something earlier though. Will edit when/if I find it.

No. It was a stated goal of the Space Shuttle project, but it failed. Had the Space Shuttle been a commercial venture it would have been abandoned decades ago; other than the fact that it generally worked (which, to be clear, is saying something, that's not guaranteed), it was an awful design.

Can't edit the parent post anymore, indeed I was wrong, but only about the year. The project I've added however is around 20 years old, so I still don't find Grasshopper revolutionary in this one aspect - reusable VTVL rocket.

Re: SpaceX Grasshopper Flies High

#72

Post by rst »

lutze wrote: post #66
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 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.

The video actually shows two stages and the Dragon capsule, which is payload. As to the use of staging, there are several reasons:

1) During the late stages of the boost, the one second stage engine is pushing only its single engine, fuel and tankage. You're no longer dragging around the nine engines of the first stage and their tanks. That weight saving means you get a lot more delta-V for each unit of expended fuel.

2) The engines themselves are also different. Rocket nozzles designed for optimal performance at sea level aren't optimal for high-altitude or vacuum conditions; those optimal for vacuum won't work at sea level (their exit pressure is so low that the exhaust has trouble pushing air out of the way). And compromise designs aren't optimal in either environment.

3) In the proposed SpaceX reuse architecture, they don't need to protect that long, thin first-stage tank from re-entry at orbital velocity. It's not clear how they could.

Re: SpaceX Grasshopper Flies High

#73

Post by rst »

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.

Parachutes were the first thing they tried, on the first stages of the first Falcon 9 launches. It didn't work; the stages broke up on re-entry before they slowed down enough to deploy the parachutes. The response was to try active control to keep them intact --- and I guess they figure they might as well keep it all the way down.

Re: SpaceX Grasshopper Flies High

#74

Post by gamegoblin »

lutze wrote: post #66
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 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.

"A multistage (or multi-stage) rocket is a rocket that uses two or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Taken together these are sometimes called a launch vehicle. Two stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched. By jettisoning stages when they run out of propellant, the mass of the remaining rocket is decreased. This staging allows the thrust of the remaining stages to more easily accelerate the rocket to its final speed and height."

https://en.wikipedia.org/wiki/Multistage_rocket

Re: SpaceX Grasshopper Flies High

#75

Post by jordan0day »

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?

I imagine there's a lot less fuel required to land than is required to take off. That is, on launch, the rocket is probably (eventually) reaching speeds of several thousand miles per hour.

Upon landing, the atmosphere does most of the slowing-down for you, so you only need enough fuel to reduce the speed from terminal velocity (not sure what this would be for a rocket, probably several hundred miles an hour, at least) to 0. So, I'm sure it's not an insubstantial amount of fuel, but maybe less than you'd think?

Re: SpaceX Grasshopper Flies High

#76

Post by InclinedPlane »

skorgu wrote: post #57
InclinedPlane wrote: post #51

Earlier quoted context omitted.

I'm pretty optimistic about the future. The information age is young yet and only recently starting to come into its first full stage of maturity. And there are a ton of very hopeful signs out there, especially in terms of long term trends. I think the chances that things will turn out well are actually higher than not doing so.

Don't get me wrong, I'm optimistic myself. I'm just a bit concerned about how some poorly defined cohort of 'people who don't read HN' are going to fare; not all of those trends are looking super rosy.

Oh, certainly. There are a lot of scary trends right now, but there are also a lot of very positive trends. Personally I think the forces behind the latter are more powerful, but it's not exactly a sure thing necessarily. If someone told me that in 2070 the developed world was basically neo-feudalist with strong class divisions amidst an effectively world-wide police state I could easily believe it.

I think there are a few major, mostly technological, developments which haven't happened yet but are essentially inevitable which will tend to tip things even stronger toward the "good" side. I should probably write about that at some point.

Re: SpaceX Grasshopper Flies High

#79

Post by JshWright »

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.

Weird bit of trivia... The pad the the rockets are landing on is a helipad (green perimeter lights). I suppose that makes sense for the video, since there isn't, AFAIK, an FAA spec for rocketpad lighting...

Re: SpaceX Grasshopper Flies High

#80

Post by danielweber »

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.

Parachutes are really hard. Ask Carmack at Armadillo, but make sure he's in a good mood first. It's not because he's an idiot or anything.

Plus, parachutes create a new failure mode: parachute deploying when it shouldn't.

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