Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The Tyranny of the Rocket Equation (2012)

nasa.gov

Post reply on HN
Print view View original

151–160 of 163 posts

Re: The Tyranny of the Rocket Equation (2012)

#151

Post by mikeash »

brent_noorda wrote: post #139
mikeash wrote: post #102

Earlier quoted context omitted.

Everyone Knows™ that putting stuff into space is expensive. Then Everyone Assumes™ that it's because of all the fuel. But no, fuel is cheap, hardware is what's expensive. If you look at the costs involved in putting something into orbit, the cost of fuel is a trivial detail, on the order of 1% of the total costs. Compare that to an airliner, where fuel is around 1/3rd of the total costs, or a car, where fuel can easi…

It blows my mind that SpaceX’s would think that “fuel costs don’t matter” given that the company is run by Elon Musk, who when he’s not doing SpaceX is running Tesla and being Chairman of SolarCity where fuel costs, in particular the environmental fuel costs, are basically the only thing that matter. It takes the equivalent of 25,000 gallons of gas to put a 200 lb person into orbit on a SpaceX rocket (yes, I know it’…

Constructing machinery has huge environmental costs as well. The CO2 emitted in the construction of a car rivals that emitted by driving it afterwards, for example.

Getting into orbit requires a huge amount of energy. That's just physics, and there's no way around it. Spending lots of money on extremely complicated and efficient machinery to use less fuel getting to orbit does not mean you're more environmentally friendly.

Also, numbers matter. The environmental costs of space travel are completely insignificant, while cars are choking the planet, simply because there are billions of them. A tiny efficiency improvement applied to billions of cars will dwarf a gigantic efficiency improvement applied to a few rocket launches per year.

Re: The Tyranny of the Rocket Equation (2012)

#152

Post by mikeash »

Florin_Andrei wrote: post #136
EliRivers wrote: post #82

Earlier quoted context omitted.

"A space vehicle that's an air-breather while it's in the atmosphere and transitions to rocket mode once out has been proposed many times, but making something that's both a rocket and an airplane is hard and adds a lot of weight." For those interested, these people are working on it. http://www.reactionengines.co.uk/ (They're the Skylon makers mentioned in another comment here)

Now imagine if this planet was a bit more dense or bigger. Let's say 2g instead of 1. I wonder if we had any space vehicles at all right now, in that case. EDIT: Nevermind, FTA: > If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank…

That paragraph doesn't make much sense. It's assuming a single-stage rocket. Stages let you do better, and that's why every orbital rocket is staged. There will be a point where that becomes impractical (e.g. your rocket has to be the size of a mountain) but there isn't a point where it's physically impossible.

Re: The Tyranny of the Rocket Equation (2012)

#153

Post by mikeash »

mcguire wrote: post #132
mikeash wrote: post #102

Earlier quoted context omitted.

Everyone Knows™ that putting stuff into space is expensive. Then Everyone Assumes™ that it's because of all the fuel. But no, fuel is cheap, hardware is what's expensive. If you look at the costs involved in putting something into orbit, the cost of fuel is a trivial detail, on the order of 1% of the total costs. Compare that to an airliner, where fuel is around 1/3rd of the total costs, or a car, where fuel can easi…

Fuel is indeed cheap (The Russkies are using kerosene and liquid oxygen---you can source those locally yourself.), but hardware isn't expensive, either. The expensive part is designing hardware that works correctly, given that it is 90% fuel. If a car was a one-time-use vehicle, it would be much, much cheaper than it is now, when it's designed to last at least through the end of the warranty period. Likewise, the Spa…

The Russians put stuff into orbit relatively cheaply, but not by a huge margin. Building the same rocket over and over again has worked out OK for them, but it hasn't dramatically reduced the cost of access to space.

The landing fuel for SpaceX's design is the same fuel as used to launch. A reusable Falcon 9 launch will have 30% less payload capacity than an expendable one, because of the need to save fuel for the landing. But the cost savings will be vastly more than 30%. So overall it's a big net win.

SpaceX isn't man-rated yet. They don't yet have a spacecraft that can carry people, so there's no point. That's being worked on, of course. The Dragon 2 spacecraft that will carry people includes a launch escape system, so it will be much more tolerant of launch mishaps than the Shuttle was, where the options for surviving a serious failure on launch were pretty much just "pray."

Turnaround costs and reentry shielding actually illustrate just how different SpaceX's approach is from the Shuttle's. They're only reusing the first stage for now. That means that there's no real need for a thermal protection system, so no worries with tiles. The engines are stressed a lot less, so they break less. The engines are also much less efficient (which is to say much less fragile) than the Shuttle's, and should need no refurbishment for subsequent launches. They're looking at reusing the capsules as well, but the heat shielding on those is tiny compared to what a Shuttle needed. They're not currently looking to reuse the second stage at all, but of course the rocket equation tells us that reusing the first stage is a much bigger deal. Just reusing the first stage gets you 90% of the cost savings of reusing everything.

Re: The Tyranny of the Rocket Equation (2012)

#154

Post by ericd »

Leszek wrote: post #66
ericd wrote: post #42

Earlier quoted context omitted.

I wouldn't dismiss the space elevator out of hand. It requires carbon nanotubes of a few meters length to achieve the required strength, and you wouldn't need to lift it pre-built - you could build it with a guideline and cable laying cars traveling up and down, adding to the cable, much like they do with suspension bridges. Long term, it seems like far and away the best approach if we can solve the materials science…

What would the guideline be made of?

The same stuff as the main cable, just thinner and less redundant. The main issue with the space elevator is the cable weight vs. cable strength.

Re: The Tyranny of the Rocket Equation (2012)

#155

Post by brent_noorda »

mikeash wrote: post #151
brent_noorda wrote: post #139

Earlier quoted context omitted.

It blows my mind that SpaceX’s would think that “fuel costs don’t matter” given that the company is run by Elon Musk, who when he’s not doing SpaceX is running Tesla and being Chairman of SolarCity where fuel costs, in particular the environmental fuel costs, are basically the only thing that matter. It takes the equivalent of 25,000 gallons of gas to put a 200 lb person into orbit on a SpaceX rocket (yes, I know it’…

Constructing machinery has huge environmental costs as well. The CO2 emitted in the construction of a car rivals that emitted by driving it afterwards, for example. Getting into orbit requires a huge amount of energy. That's just physics, and there's no way around it. Spending lots of money on extremely complicated and efficient machinery to use less fuel getting to orbit does not mean you're more environmentally fri…

Good point about billions of cars having more impact than a few fuel-hoggy rocket launches.

I just hope that space-tourism doesn't catch on, because while you're correct that overproducing rockets to make them slightly more efficient does not make you more environmentally friendly, burning 25,000 gallons for a couple of recreational hours off the planet just because you have way too much money does make you an environmental monster.

Re: The Tyranny of the Rocket Equation (2012)

#156

Post by Animats »

mikeash wrote: post #153
mcguire wrote: post #132

Earlier quoted context omitted.

Fuel is indeed cheap (The Russkies are using kerosene and liquid oxygen---you can source those locally yourself.), but hardware isn't expensive, either. The expensive part is designing hardware that works correctly, given that it is 90% fuel. If a car was a one-time-use vehicle, it would be much, much cheaper than it is now, when it's designed to last at least through the end of the warranty period. Likewise, the Spa…

The Russians put stuff into orbit relatively cheaply, but not by a huge margin. Building the same rocket over and over again has worked out OK for them, but it hasn't dramatically reduced the cost of access to space. The landing fuel for SpaceX's design is the same fuel as used to launch. A reusable Falcon 9 launch will have 30% less payload capacity than an expendable one, because of the need to save fuel for the la…

The Russians put stuff into orbit relatively cheaply, but not by a huge margin. Building the same rocket over and over again has worked out OK for them, but it hasn't dramatically reduced the cost of access to space.

Buran, the USSR's shuttle, was a reasonably good idea. Although it looked like the US shuttle, it was really a vehicle carried on a big booster; it had no main engines. It flew once, successfully, unmanned. The Boeing X-37 unmanned mini-shuttle is similar, and seems to work well. The USAF keeps sending one up, keeping it up for a year, and then bringing it down to land on a runway.

Re: The Tyranny of the Rocket Equation (2012)

#157

Post by ericd »

DennisP wrote: post #85
Animats wrote: post #32

Right. This has been known for a long time. It's why rockets aren't much better than they were 40 years ago. Chemical fuels are as good as they can get. Space travel with chemical fuels is just barely feasible. In the 1960s, it was assumed that nuclear power would be necessary for space flight. Everybody involved knew the rocket equation. The original plan for Apollo included a nuclear upper stage. The engine (NERVA)…

NASA did a study on space elevators about a decade ago, and found that it would require carbon nanotubes several centimeters long, bound together by a realistically strong epoxy. Launching their design would require seven space-shuttle flights to deploy a minimal elevator, which you use to pull up additional construction material. They addressed a lot of other practical issues too. Here's their final report (pdf), it…

Ah neat, that looks like an extension of the book he wrote.

I get a bit worried when he dismisses chemical rockets for lifting the initial spool into space, and starts going on about electric drive... it seems like he should limit the number of technical revolutions required to make this work, if he wants this to succeed on a reasonable time scale...

Re: The Tyranny of the Rocket Equation (2012)

#158

Post by neurobro »

clumsysmurf wrote: post #134

If anyone is interested, these researchers from Russia have proposed a way to accelerate a spaceship while in flight – firing a ground-based laser up its backside http://www.osa.org/en-us/about_osa/newsroom/news_releases/20...

Well that seems less preposterous than my own idea of using lasers to accelerate a ring of air, generating a plasma circuit that would pull the craft up magnetically. (Version 2 would have a series of plasma rings that the craft would fly through like a coil gun.)

Re: The Tyranny of the Rocket Equation (2012)

#159

Post by mikeash »

Animats wrote: post #156
mikeash wrote: post #153

Earlier quoted context omitted.

The Russians put stuff into orbit relatively cheaply, but not by a huge margin. Building the same rocket over and over again has worked out OK for them, but it hasn't dramatically reduced the cost of access to space. The landing fuel for SpaceX's design is the same fuel as used to launch. A reusable Falcon 9 launch will have 30% less payload capacity than an expendable one, because of the need to save fuel for the la…

The Russians put stuff into orbit relatively cheaply, but not by a huge margin. Building the same rocket over and over again has worked out OK for them, but it hasn't dramatically reduced the cost of access to space. Buran, the USSR's shuttle, was a reasonably good idea. Although it looked like the US shuttle, it was really a vehicle carried on a big booster; it had no main engines. It flew once, successfully, unmann…

The Buran is really interesting. It's a pity it never got enough love. I'm not hugely familiar with it, but it does seem superficially like a better approach than the Shuttle.

Re: The Tyranny of the Rocket Equation (2012)

#160

Post by [deleted] »

chii wrote: post #95
dalke wrote: post #53

Earlier quoted context omitted.

The rocket equation applies to all momentum machines which carry its own "fuel", that is, the mass to eject. Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.

is there a form of transport that is _not_ limited by the rocket equation?

[deleted]
Post reply on HN