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The Tyranny of the Rocket Equation (2012)

nasa.gov

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Re: The Tyranny of the Rocket Equation (2012)

#131

Post by phlo »

wlievens wrote: post #108
keenerd wrote: post #69

Earlier quoted context omitted.

I would certainly dismiss elevators because there is a much more practical alternative that doesn't require materials that don't exist in sufficient quantities to reach a quarter of the way to the moon. The Lofstrom Loop ( http://en.wikipedia.org/wiki/Launch_loop ) could be built with materials we have today, although it requires sufficient amounts of money and land that only large countries or multibillionaires coul…

I liked Elon Musk's note on the space elevator; he basically said "we should look at it once we have a bridge from Los Angeles to Tokyo, because that's far easier to build".

I disagree with that argument. Great solutions require great problems, and moving things between LA and Tokyo just isn't anymore. Right now, shipping a person costs some $1k (return included) and takes less than 12 hours; a 40 ft container of cargo around $800 and 22 days.

Launching a satellite still costs millions. The closest thing to inexpensive space flight is probably Virgin Galactic's SpaceShipTwo, which is still in testing. Space remains interesting and unsolved. And as such, I'll bet you a beer we'll have a space elevator sooner than either a bridge or a tunnel directly connecting LA and Tokyo.

Re: The Tyranny of the Rocket Equation (2012)

#132

Post by mcguire »

mikeash wrote: post #102
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)…

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 Space Shuttle was reusable, but that reusability never actually turned into a win.[2] (I have a rocket-scientist friend who might convincingly argue that putting the Space Shuttle's budget into building a Saturn V assembly line would have been a net win. Building the same rocket over and over seems to have worked for the Russians.)

There are two downsides to reusability, particularly for man-rated rockets: How do you land, and what do you have to do to turn it around.

As was pointed out in the article, IIRC, the Shuttle could put 120 tons into orbit, but 100 tons of that was coming back down with the re-entry vehicle. Kind of reduces the effective payload. I don't know the details of the SpaceX reusability design, but I'm wondering where the landing fuel comes from; if it rides the rocket the whole time, it's coming out of the 10%.

The turn-around part is bad, too. Take a look at [1] for the Shuttle. The interesting parts are:

* "Transfer engines to the Main Engine Processing Facility and service for future flights," and "When required, the orbital maneuvering system (OMS)/reaction control system (RCS) pods and forward RCS may be removed and taken to the Hypergol Maintenance Facility in KSC’s industrial area for maintenance." Yes, SOP involves major disassembly every time.

* "Visual inspections are made of the orbiter’s thermal protection system, selected structural elements, landing gear, and other systems to determine if they sustained any damage during the mission. Any damage to the thermal protection system must be repaired before the next mission." If!? I don't know the numbers, but after every flight, every tile was checked and a goodly number needed replacement. (They're not cheap, either.)

Is SpaceX flying man-rated yet?

Anyway, turn-around costs and not caring about re-entry make for a bit of cheapitude, too.

[1] http://www.nasa-klass.com/Curriculum/Get_Oriented%202/Space%...

[2] http://en.wikipedia.org/wiki/Criticism_of_the_Space_Shuttle_...

Re: The Tyranny of the Rocket Equation (2012)

#135

Post by ChuckMcM »

I really enjoyed this. It lays out in some very accessible ways, the challenges of getting into space. The recoverability of the Falcon9 will cut its costs dramatically as it reduces the cost of the launch by several tens of millions of $. I'll mention on-orbit refueling as well since you don't need recoverability per-se if you can refuel in orbit. Then your Mars lander / Crew Module can launch with enough fuel to get to Low Earth orbit, refuel, and then head out to Mars.

Air launched (Skylon, Pegasus, Et alia) are also interesting, laser boosting (using lasers to add energy during the initial launch) would also help. As Elon points out though, a multi-gigawatt laser for boost to orbit is impractical both from a construction standpoint and a diplomatic stability standpoint.

If the quantity of water on the moon is accurate, then it should be possible to create a 'refinery' on the Moon which could more easily get material into Earth orbit. We'll see though if we can get a group established there.

I had hoped to visit the Moon at some point (I was assured by NASA in my youth that would be able to :-)) but I don't expect that to come to pass unless something amazing changes.

Re: The Tyranny of the Rocket Equation (2012)

#136

Post by Florin_Andrei »

EliRivers wrote: post #82
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)…

"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, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.

Anthropic principle my a$$. This Universe can barely support an expanding civilization.

We barely became self-aware, and the useful lifespan of the Sun is almost over - it will only make things more difficult for the Earth from now on.

If the planet is too small, it's not stable enough to support life. If it's too big, you're trapped there forever. And the range in between is narrow.

Re: The Tyranny of the Rocket Equation (2012)

#138

Post by ubernostrum »

baq wrote: post #81
lmm wrote: post #68

Earlier quoted context omitted.

It's escape velocity, not escape speed - and it's orbital velocity that matters here, we're not escaping entirely. If you're in the same place, travelling at the same speed, but pointed down, you're not in orbit, right? It's the same if you're pointed straight up. You need to be at an orbital altitude and travelling at the right speed in the right direction . (well, any combination of speed, direction, and position w…

escape speed is technically correct. you need to go fast enough, direction doesn't really matter.

Some directions may require more speed than others.

Or perhaps you are having a bad problem and will not go to space today.

Re: The Tyranny of the Rocket Equation (2012)

#139

Post by brent_noorda »

mikeash wrote: post #102
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)…

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’s not necessarily gasoline, but other fuels that are also either fossil fuels or create from burning fossil fuels). So a person would have to drive a Tesla for 120 years to save enough fuel for one spot on a SpaceX launch into orbit. Do environmental fuel costs only matter when you can save a few gallons on a two hour car trip, but not when you use 25,000 gallons for a two our jaunt into space and back?

Re: The Tyranny of the Rocket Equation (2012)

#140

Post by scentoni »

snowwrestler wrote: post #87
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)…

I wonder about a fusion rocket drive, like Larry Niven wrote about (not the Bussard, the reaction drives). There are two big problem in fusion energy research: plasma leaks, and high-energy neutrons. It seems to me that a fusion rocket answers both questions: just throw it all out the back. And hydrogen is abundant and cheap.

Because they're neutral, neutrons can't easily be deflected to throw them in some particular direction.
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