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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)

#141

Post by nickff »

ChuckMcM wrote: post #135

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 ge…

If you do the math, air-launch does not save much in fuel costs, while it adds a great deal of complexity to the launch system, and you do not save much in terms of reduced energy requirements. Most of the rocket fuel and oxidizer are used to increase velocity; comparatively little is used to gain (the first 30k') altitude or lost to atmospheric drag. The main benefit to air-launch is reduced range safety costs, and ease of scheduling, as the launch vehicle can be taken out over an unoccupied stretch of ocean, and range rental/lease costs are basically eliminated.

Re: The Tyranny of the Rocket Equation (2012)

#142

Post by dyeje »

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…

The useful lifespan of the Sun thing caught my attention, could you elaborate on that?

Re: The Tyranny of the Rocket Equation (2012)

#143

Post by Lambdanaut »

If you've got a poor mind for Math but want to understand rocket science on a more intuitive level, check out Kerbal Space Program. It's the most educational game I've ever played that still retains being fun.

I usually avoid video games because I feel like I'm wasting time, but I make an exception for KSP.

Re: The Tyranny of the Rocket Equation (2012)

#144

Post by Florin_Andrei »

dyeje wrote: post #142
Florin_Andrei wrote: post #136

Earlier quoted context omitted.

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…

The useful lifespan of the Sun thing caught my attention, could you elaborate on that?

It's not going to happen tomorrow, but "soon" compared to all its previous history. It is estimated that in 1 billion years the Earth will be too hot for any meaningful ecosystems to exist. Things would start to degrade before that limit, of course.

Overall, it looks like life has basically one single shot at producing intelligence on an Earth-like planet. We popped out kinda towards the end.

Re: The Tyranny of the Rocket Equation (2012)

#145

Post by Retric »

derekp7 wrote: post #119
Retric wrote: post #112

Earlier quoted context omitted.

Your off by ~61,545 times. As, you don't want the energy to boil water, you want the energy you get from burning hydrogen. 1 liter of water is 11.19% hydrogen or ~0.1119kg. Hydrogen is 143 MJ/kg so 1 liter of water ~= 143,000,000J * 0.1119 = 16,001,700J. So, you want a 1,661,715 megawatt laser. Good luck with that. PS: You might be able to do laser assisted rocketry where you hit the combustion chamber with energy to…

Ah, I didn't do the calculations myself, just did a couple random Google searches for how many joules it takes to boil off one liter of water (not how much it takes to bring a gallon to boiling point, which is of course much much less). Here's one site I used: https://www.physicsforums.com/threads/energy-required-to-boi... , but I didn't see the "k" in front of joules. Thanks.

Your still confusing a state change (boil) with a chemical change (burn). The reason this is important is how fast stuff comes out the bottom of your rocket is really important and pushing it faster takes more energy. To beat chemical rockets you need to use more energy to push harder.

Re: The Tyranny of the Rocket Equation (2012)

#146

Post by Retric »

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.

Rocket exhaust is normally plasma. Also, density is not really much of an issue unless it's really low. Rockets use liquid hydrogen which is normally ~1/14th the density of water.

The important bit is mass not density.

Re: The Tyranny of the Rocket Equation (2012)

#147

Post by Animats »

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.

It takes a kilowatt laser to lift a gram. A megawatt laser to lift a kilogram. A gigawatt laser to lift a metric ton.

Biggest chemical laser so far is about a megawatt. The laser diode people are making real progress, into the kilowatt range (http://teradiode.com/technology/) but gigawatt laser array are still a ways off.

A Saturn V at launch was 190 gigawatts.

Re: The Tyranny of the Rocket Equation (2012)

#148

Post by mcguire »

Mvandenbergh wrote: post #126
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 read this story[1] once, although I don't remember where:

Someone somewhere needed a large number of cargo vessels built quickly (think WWII liberty ships, but those weren't made of wood), so they brought in a bunch of cabinet makers to bolster their shipwrights. It worked great, until the ships built by normal woodworkers saw significant wave action, at which time they broke up and sank. The punchline being that, on land, rigidity is the primary constraint; if you build it not to be floppy, it will be plenty strong. At sea (and especially in aerospace), strength is primary; if you build it to be rigid, it will be too heavy and if you build it to be not-heavy, it will probably fall apart.

An amateur can build a fairly large boat (although usually to plans by an actual naval architect), and a small boat can make it across an ocean, but if you're serious about schlepping things around, the design constraints for ships aren't much looser than those in aerospace.

[1] Maybe this, although that's not the cover I remember: http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/03...

Re: The Tyranny of the Rocket Equation (2012)

#149

Post by mcguire »

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…

How much does your fusion plant weigh?

Re: The Tyranny of the Rocket Equation (2012)

#150

Post by ChuckMcM »

nickff wrote: post #141
ChuckMcM wrote: post #135

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 ge…

If you do the math, air-launch does not save much in fuel costs, while it adds a great deal of complexity to the launch system, and you do not save much in terms of reduced energy requirements. Most of the rocket fuel and oxidizer are used to increase velocity; comparatively little is used to gain (the first 30k') altitude or lost to atmospheric drag. The main benefit to air-launch is reduced range safety costs, and…

An interesting paper that is doing that math : http://mae.engr.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pd...
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