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

Re: The Tyranny of the Rocket Equation (2012)

#121

Post by mikeash » Fri, Oct 31, 2014, 4:15 PM UTC

marcosdumay wrote: post #118

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…

People have been trying to make reusable rockets since the beginning. The problem is that a reusable rocket is more complex, what means it's heavier, and that it needs much more fuel to launch, thus a bigger rocket, and the rocket equation makes everything astronomical. Like everybody else, I cheering for SpaceX to solve this problem, but the challenge is not making a reusable rocket - it's making a light enough reus…

I'm not so sure. The Space Shuttle had a pretty big payload capacity. It had a lot of problems as well, but those were mostly due to being underfunded and hit with weird requirements beyond simple reusability. And that's really the only serious attempt at reusability that got beyond the early stages. It's still a pretty unknown area at this point, but I don't think building stuff sufficiently lightweight is necessarily the challenge. SpaceX doesn't seem to think it is, anyway. Their engines are not particularly high performance (meaning they need more fuel for the same job) and their rockets are engineered more for cost effectiveness than light weight. They seem to be making great strides precisely because they're not concentrating on weight. For example, their first stage reuse system involves carrying a bunch of extra fuel, where virtually every other attempt at reusability involved some sort of unpowered or nearly-unpowered landing after expending all fuel on the launch phase.

Re: The Tyranny of the Rocket Equation (2012)

#122

Post by nitrogen » Fri, Oct 31, 2014, 4:15 PM UTC

claudius wrote: post #41

has2k1 wrote: post #22

Earlier quoted context omitted.

You would have to build up a lot more speed if all you are going to do is decelerate once you "take-off". Also noteworthy, once you get to the escape velocity you would require an additional force just to keep you circular.

You can still have a rocket on your vehicle to accelerate further after take-off, but you certainly need to accelerate less if you already at a certain velocity than if you are stationary. Since the “certain velocity” is free by assumption (i.e. supplied by ground-based electricty generators or something similar), I don’t see how you could be off worse. But I still didn’t do the maths fully.

Air friction, perhaps? There is more air in a tangent line.

Re: The Tyranny of the Rocket Equation (2012)

#123

Post by edraferi » Fri, Oct 31, 2014, 4:16 PM UTC

The common soda can, a marvel of mass production, is 94% soda and 6% can by mass. Compare that to the external tank for the Space Shuttle at 96% propellant and thus, 4% structure. The external tank, big enough inside to hold a barn dance, contains cryogenic fluids at 20 degrees above absolute zero (0 Kelvin), pressurized to 60 pounds per square inch, (for a tank this size, such pressure represents a huge amount of stored energy) and can withstand 3gs while pumping out propellant at 1.5 metric tons per second. The level of engineering knowledge behind such a device in our time is every bit as amazing and cutting-edge as the construction of the pyramids was for their time.

That's awesome, I had no idea.

Re: The Tyranny of the Rocket Equation (2012)

#124

Post by danielweber » Fri, Oct 31, 2014, 4:18 PM UTC

fpgaminer wrote: post #12

ars wrote: post #6

Lest you despair of ever making space flight routine, a rocket is not the only way to get into orbit. Virtually all of the needed velocity is tangent to the surface, not away from it. So you can accelerate the vehicle along the ground at least part of the way, and only then turn heavenward and burn fuel to get into orbit. With this boost you significantly reduce the amount of fuel needed. There are many way of doing…

I wonder if you could build a particle accelerator of sorts. A toroidal tube in which the vehicle is placed, and magnetically accelerated to launch velocity. Would just need some way to let the vehicle out; some kind of track switch. Such a setup would not require a large area to bring the spacecraft up to speed, and could be vacuum sealed to negate air resistance.

A long time ago I did the math for putting one of these on Mt Kilimanjaro, with another few thousand or so feet of height magically added. If you build up any significant speed in an evacuated tunnel, even at those very high heights you still face an incredible pressure wave as you exit.

Re: The Tyranny of the Rocket Equation (2012)

#125

Post by nraynaud » Fri, Oct 31, 2014, 4:19 PM UTC

SideburnsOfDoom wrote: post #78

nraynaud wrote: post #61

Earlier quoted context omitted.

what about vacuuming the cannon? like those ping pong ball cannons?

What happens when you leave the cannon? Either the cannon exit is near ground level, in which case you still have the air resistance problems; or it's not, in which case you have the problems of building very a tall cannon.

you don't have to exit the cannon at high speed, the v^2 drag will kill you. You're just trying to put some kinetic energy in the rocket in a way that doesn't make you carry it. you don't have to put all the energy in it that way, the current system works, it's just a tentative improvement.

But your answer makes me feel you've not seen the ping pong ball cannons, because they have very good results with a very low mass.

Re: The Tyranny of the Rocket Equation (2012)

#126

Post by Mvandenbergh » Fri, Oct 31, 2014, 4:23 PM UTC

mcguire wrote: post #120

" If a vehicle is less than 10% propellant, [c]hanges to its structure are readily done without engineering analysis; you simple weld on another hunk of steel to reinforce the frame according to what your intuition might say. " This is why you don't let cabinet makers build ships.

The point is that they can actually. A cabinet maker could build a fairly large boat at least (maybe not a ship...) entirely by hand and mostly using very simple rules of thumb. It probably wouldn't perform very well but it would float and be able to sail around.

Amateurs build small boats that cross the Atlantic and even the Pacific all the time, precisely because ship building is easier than building rockets.

When I say it's easier, that doesn't mean that naval architects aren't as smart as rocket engineers, but the ratio of engineering effort to performance is much more favourable.

Re: The Tyranny of the Rocket Equation (2012)

#127

Post by danielweber » Fri, Oct 31, 2014, 4:23 PM UTC

im2w1l wrote: post #93

What about firing the rocket from Jules Verne's space gun[0]? Now, I don't mean an actual gun what with the high g-forces and so, but only some machinery that gives the rocket high initial speed. Maybe a super sonic evacuated tube maglev that ends with a ramp? Put it on Mt. Everest for less air resistance. [0] http://en.wikipedia.org/wiki/Space_gun

You still get a massive hit from the atmosphere when you exit the tunnel if you've built up any significant speed. Air pressure up there is still a third of air pressure at sea level.

Re: The Tyranny of the Rocket Equation (2012)

#128

Post by danielweber » Fri, Oct 31, 2014, 4:24 PM UTC

AndrewDucker wrote: post #52

Why not assemble some rockets in space? Get the pieces up there as efficiently as possible, and then assemble a more efficient rocket that's not designed to leave the gravity well.

If you skip the middle step, that's called staging.

It's really expensive to try and keep a permanent base in space.

Re: The Tyranny of the Rocket Equation (2012)

#129

Post by gizmo686 » Fri, Oct 31, 2014, 4:25 PM UTC

mrfusion wrote: post #79

So this is something I've always wondered about. Can anyone explain the equation for getting to an earth sun Lagrange point? Since were already on earth orbiting the sun it would seem we already have the correct orbital velocity to hang out at a Lagrange point. So we could really approach these points at any speed no? Is there potential to use less fuel than you'd need to reach an earth orbit?

By "orbital velocity", I assume you mean orbital velocity relative to the sun. The need for this clarification should be the first indication that we do not already have the necessary orbital velocity (as there is no particular reason to use the sun as the reference other than the fact that it is the more massive body). This diagram [0] shows the Earth-Sun Lagrange points. As you can see, they all have a slightly different orbital radius around the sun, and therefore will have different orbital velocities (larger radius=slow velocity). Additionally, (most) of these points have a different direction, which would means a different velocity. (However, we can change direction for free simply by orbiting, and as long as our orbit is different from Earth's we will change our direction relative to Earth as well).

The bigger issue is 'escaping' Earth's gravity well[1]. The most efficient way to do this is through an orbit. To see this, consider the effect of gravitational drag. That is to say, the speed lost due to the force of gravity. If you are in orbit, the net gravitational drag is 0 [2]. Suppose you are at point P of an orbit, travelling at speed V. If you accelerate with X delta-V at this point in the orbit, your new orbit will still contain point P, and you will pass through P with velocity (V+X), indicating that there was 0 gravitational drag. Intuitively, this is because you are accelerating perpendicular to the force of gravity.

Another way to look at this is to remember that there is no particular reason to prefer using the Sun as a reference point, in which case we can see that Lagrange points are also in Earth orbit.

However, your idea does point to a (theoretical) way to reduce fuel usage. When calculating the fuel needed to establish an orbit, we assume that Earth is the only massive body. However, we can (in theory) use the gravitational force of the sun to reduce the needed fuel to reach Earth orbit (in a way that has nothing to do with Lagrange points). However, the effect is not significant enough to be worth talking about.

[0] http://upload.wikimedia.org/wikipedia/commons/e/ee/Lagrange_... [1] Of course, by definition of Lagrange points, we are not actually escaping [2] Unless the orbit is circular, gravity will change your speed, but the acceleration/deceleration will balance out.

Re: The Tyranny of the Rocket Equation (2012)

#130

Post by danielweber » Fri, Oct 31, 2014, 4:29 PM UTC

quarterto wrote: post #105

derekp7 wrote: post #89

Earlier quoted context omitted.

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 ove…

One method of laser propulsion is using the laser to ablate a metal reaction mass. Since metal is much denser than water, and is converted into plasma, a much smaller reaction mass can be used. This method has a specific impulse of about 5000s, an order of magnitude higher than chemical rockets.

That would be useful for station-keeping for the ISS. I don't think NASA would let you point a laser strong enough to ablate metal at their station, though.