The Tyranny of the Rocket Equation (2012) • Hacker News Board

Re: The Tyranny of the Rocket Equation (2012)

#81

Post by baq » Fri, Oct 31, 2014, 2:05 PM UTC

lmm wrote: post #68

has2k1 wrote: post #25

Earlier quoted context omitted.

I see it this way. You have to accelerate the mass to the escape velocity. You also have to achieve a net effect of being in orbit (some distance x above surface of earth). The most direct vector to that distance is perpendicular. The two combined should give you the minimal energy requirement. Any engineering (and aerodynamics) creativity cannot give you anything better.

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.

Re: The Tyranny of the Rocket Equation (2012)

#82

Post by EliRivers » Fri, Oct 31, 2014, 2:05 PM UTC

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)

Re: The Tyranny of the Rocket Equation (2012)

#83

Post by DennisP » Fri, Oct 31, 2014, 2:08 PM UTC

pjc50 wrote: post #30

ytturbed wrote: post #2

>Currently, all our human rated rocket engines use chemical reactions (combustion of a fuel and oxidizer) to produce the energy. Yes, however, for completeness: an explanation of why we must limit designs to chemical rockets ought to include an explanation of why the dozen or so fusion projects underway around the world will all fail, i.e. let's inject some rational optimism. Note that the Apollo programme began befo…

The fusion projects have been running for a long time with little success. Apollo was built on scaling tech that already worked (1940s rocketry could reach space although not achieve orbit)

If you consider exponential progress since 1970 or so to be "little success," you're right. Fusion has a very high threshold before it becomes useful, but we've come a long way, and we're not that far from the breakeven point now.

NASA is currently working with John Slough's company on a fusion rocket for interplanetary travel.

http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII...

http://www.washington.edu/news/2013/04/04/rocket-powered-by-...

Re: The Tyranny of the Rocket Equation (2012)

#84

Post by bcoates » Fri, Oct 31, 2014, 2:11 PM UTC

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)…

Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.

Re: The Tyranny of the Rocket Equation (2012)

#85

Post by DennisP » Fri, Oct 31, 2014, 2:13 PM UTC

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's an interesting read.

http://www.niac.usra.edu/files/studies/final_report/521Edwar...

Re: The Tyranny of the Rocket Equation (2012)

#86

Post by rquantz » Fri, Oct 31, 2014, 2:15 PM UTC

keenerd wrote: post #69

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…

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 never have understood why the space elevator gets so much attention, and the launch loop is almost unknown. The elevator obviously had some high-profile proponents (like Arthur C. Clarke), but given how realistic the launch loop seems (only 8 billion dollars??), I would think it would be the thing everyone talks about.

The fact that it is not makes me think that it is less realistic or more constrained than it is made out to be. I certainly don't have the skills necessary to evaluate that, but I bet someone here does...

Re: The Tyranny of the Rocket Equation (2012)

#87

Post by snowwrestler » Fri, Oct 31, 2014, 2:25 PM UTC

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.

Re: The Tyranny of the Rocket Equation (2012)

#88

Post by rtkwe » Fri, Oct 31, 2014, 2:27 PM UTC

keenerd wrote: post #69

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…

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…

The failure mode of a launch loop is really bad though. The constant power requirement is a pretty big problem and the solutions to loss of power aren't that promising. Compared to the elevator where there's a larger safety margin.

Re: The Tyranny of the Rocket Equation (2012)

#89

Post by derekp7 » Fri, Oct 31, 2014, 2:29 PM UTC

bcoates wrote: post #84

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)…

Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.

I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam over the course of a few minutes. And be able to hold that laser on the target.

Edit: The Saturn V held 3.2 million liters of fuel. And I think it takes about 2600 joules of energy to boil off 1 liter of water. So that is 8.3 billion joules of energy. If the flight time to orbit is 5 minutes, then a 27 megawatt laser should should do the trick.

Second edit -- I didn't see the "k" in front of joules on my random web searches for number of joules to boil off 1 liter of water. So that would be a 27 gigawatt laser (10 times the energy need for time travel). And, according to Retric below, I'm off even further. Point I was originally trying to get at is using a laser is more than a shade past impractical.

Re: The Tyranny of the Rocket Equation (2012)

#90

Post by snowwrestler » Fri, Oct 31, 2014, 2:35 PM UTC

rquantz wrote: post #86

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 never have understood why the space elevator gets so much attention, and the launch loop is almost unknown. The elevator obviously had some high-profile proponents (like Arthur C. Clarke), but given how realistic the launch loop seems (only 8 billion dollars??), I would think it would be the thing everyone talks about. The fact that it is not makes me think that it is less realistic or more constrained than it is m…

The launch loop must be perpetually active once established. That's a lot of continuous power needed for something we do pretty rarely--launch stuff into space.

And if the power turns off, it falls back to Earth. A heavy, high-speed belt falling 50 miles down along a length of 1,200 miles...not an easy problem to solve. Even if it drops into uninhabited territory, it's not going to be in great shape.

A space elevator is a passive system. Once established, it stays up. If you lose power you can't go up it, but it doesn't fall down. In that respect it is more like the passive infrastructure we're comfortable with, like highways, bridges, buildings, etc.