Earlier quoted context omitted.
Another perspective is that the Falcon9 carries about as much kerosene as a 747.
That's a good one, although a 747 gets its oxygen for free.
Yes, but oxygen's pretty easy to come by it turns out.
SpaceX Grasshopper Flies High
151–160 of 207 posts
Re: SpaceX Grasshopper Flies High
#152Re: SpaceX Grasshopper Flies High
#153Earlier 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.
Well, one reason is that parachutes at this scale aren't that simple. For instance, check out the ones on the Shuttle SRBs: http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Boos...
Re: SpaceX Grasshopper Flies High
#154Earlier 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.
Not "massive". The empty stage is very light, so a little propellant goes a long way. It's less than 10% of the fuel for the entire descent phase (and the optimal upper stage length goes up to compensate somewhat).
Re: SpaceX Grasshopper Flies High
#155Earlier 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…
> The rocket equation is a harsh mistress Love this, totally making it mine ;) Also, there is the problem of efficiency loss due to over/underexpansion as you switch from atmospheric to vacuum conditions, and I have never heard of a rocket engine with variable nozzle geometry to compensate for it.
That was the promise of aerospike engines, but they have their own problems.
Re: SpaceX Grasshopper Flies High
#156Earlier quoted context omitted.
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.
…or not deploying when it should.
Re: SpaceX Grasshopper Flies High
#157Earlier quoted context omitted.
"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 rock…
Right, excellent. Except the first stage doesn't run out of propellant does it, because it has to LAND again. Hence my puzzlement. I know why multi-stage rockets are used... I'm asking why this rocket needs them, since they solve a problem it doesn't have by design.
>they solve a problem it doesn't have by design.
This is where you're incorrect. Not having to accelerate the empty first stage to orbital speed (and back!) is a huge fuel savings.
Re: SpaceX Grasshopper Flies High
#158Earlier 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.
A single stage rocket is desirable from a technological point of view since it sheds a lot of complexity: you don't have to deal with multiple engines, separation mechanisms, structural considerations, etc. However, multi-stage rockets can deliver more real payload to orbit, since they maximize the amount of fuel that is spent on taking the actual payload up there. Single-stage rockets need to carry a lot of dead mas…
>once it's cleared out most of the Earth's gravitational pull
There's about 90% as much gravity at the altitude of the ISS.
It's the atmosphere (and associated gravity drag), not the 1/r^2 scaling, that does it.
Re: SpaceX Grasshopper Flies High
#159For 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?
Fuel cost is only 2-3% of the mission's cost, according to Elon Musk.
Re: SpaceX Grasshopper Flies High
#160I 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?
Yep, Elon Musk has publicly mentioned that it will light up the engine to boost back to the launch site.
>So, I think, there's a number of improvements across the board, in structures, avionics, engines and then, as I said, this version [the Falcon 9 1.1] is really designed to be able to have the first stage come back - boost back – to launch site, deploy landing gear and actually land propulsively.[1]
With a flyback stage the optimal separation altitude and velocity drops, and the second stage gets even larger to compensate. But yes, the engine will cancel the downrange component of velocity and reverse it. Since the stage is empty the fuel requirements are actually quite modest.
[1] http://shitelonsays.com/transcript/crs-2-post-landing-teleco...