Science & Technology

Rockets: Fuels and Propulsion

Chris Hadfield

Lesson time 20:59 min

Chris explains the pros and cons of different types of rocket fuels including liquid fuel, solid fuel, and ionized gas.

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Like all spaceships, rockets are a compromise design. You're trying to take something to space, and you have a finite amount of fuel inside your fuel tanks, you only have so much ability to steer through the atmosphere, and so you really want this thing we're taking up to be as light and as small as possible, because then you don't need so much fuel. The problem gets harder and harder if you're going somewhere, if, say, you're going all the way to Mars, because when you get to Mars, somehow you have to be able to slow down and land. So you're not just bringing the stuff that you need, but you have to bring another rocket ship up here in the end that will be able to take you down to land on the surface of Mars. And you need fuel for that rocket. And every ounce that is carried on the top is going to take pounds and pounds of fuel to get it away from the world and to slow down when you get to Mars. And it gets even worse, because you got to come back from Mars probably. And where does that fuel come from? Unless you can manufacture fuel on the surface of Mars using the raw materials that are there, that means not only are you bringing enough fuel to leave Earth, but you're bringing enough fuel to leave Earth and slow down and stop at Mars, land on Mars, blast off of Mars, accelerate, come all the way back to Earth again, and then land back on Earth. And it just magnifies on the amount of fuel that you need. So what does it really take, say, to put one thing on the surface of Mars? For every pound that we put on the surface of Mars, it takes about 200 pounds of rocket here on the surface of the world. Most of that is fuel. But for every pound that gets to the surface of Mars, it took 200 pounds to get it off the surface of the world, accelerate it out to the speed, and safely there. So every little fraction of a pound, every ounce that you can save on what it is you're putting on Mars will decrease the size of the rocket that you need to leave the Earth. So let's think. I want my T-shirt to be on Mars. Your T-shirt weighs some fraction of a pound. If your T-shirt weighs, I don't know, a third of a pound, then you need a third of 200. You're going to need, like, 70 pounds of rocket just to get your T-shirt onto Mars. So you really want to be efficient in packing, and thinking about what's the minimum amount of stuff that we can bring to Mars so that we can keep the size and the scale and the complexity and, therefore, the cost of the rocket to be as small as possible. What type of fuel do rockets use? It's varied over time. But essentially, it's fairly similar. We have some sort of fuel in a tank or a solid fuel inside a rocket like this, and then we have the oxidizer or oxygen that's stored in a separate tank. On the Saturn V, the rocket that took us to the moon, these first stages couldn't have been simpler. It was just kerosene. That was the fuel. Kerosene and oxygen. It was a pretty good first fuel, just using kerosene, oxygen....


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Comments

Ken C.

This was a good lesson on some of the details and problems a rocket designer faces. What trade offs must be made to accomplish a mission.

A fellow student

This is not my area of expertise, but I thought there was some discussion of leaving from orbit. Maybe send the Mars ship up in pieces and put it together at the ISS or on the moon and launch from there. You wouldn't need as much fuel to escape from those places (I think) as from the earth. Just wondering.

Alexis P.

Isn't there a way to turn Drag to our advantage, rocket wise? Use all that drag to basically store and pressurize oxygen at the same time and keep on using that until we're out? Or is the oxidizer not an issue during ascend? I also get the fact that pressurized air isn't pressurized oxygen ...

A fellow student

The point on Ion engines is a bit misleading. You don't discuss the mass of the mechanism or the source of energy needed to accelerate the expelled Ion gas 'fuel very much'. On a solid rocket, the mechanism is a tiny fraction of the total vehicle mass - essentially the containment shell. It's not much larger for a liquid fueled vehicle - some mixing chambers and nozzles. But I expect the mass of the Ion engine to take a non-trivial share of the remaining 20%. You mentioned solar and nuclear but it would be nice to talk more about the impracticality of both (in today's tech) and the percent share of the remaining 80% payload space each would (reasonably) take.

Tobias

ok so if you can go to a top speed and escape earth gravity then why don't you just turn around?

H

The sound of the chalk is so sandy and grinding that gives me goosebumps and sore teeth.

Yusuf N.

What effect does the gravitational pull of the moon have on the rocket? Will it cause the orbit to deform?

Ronald A.

That rocket equation was discovered by Constantin Tsiolkovski (1857- 1935). He builded that equation with the scenario of a fisherman getting to the opposite shore while trowing big rocks, pilled in the middle of the boat, in the opposite direction of the heading. I tough this should have being mentioned. Newton was not the only one interested in space.

Richard G.

I feel like liquid would be the most efficient way to get to Mars, considering the tech that is abtainable, because Ion rockets can't be used efficiently to get to even the moon, let alone Mars. Solid rockets would get to the place quicker, but would burn up more of our fuel supply, it's like the idea of getting the job done quickly gives you 85% hope, added to the fact that the storage spaces gives you about 10% hope. Giving you 95/100. If we used liquid rockets, we would be able to transport things to Mars at an acceptable rate, with 23% storage which gives roughly 50% hope, and the speed is only half of the incredibly powerful solid rocket, which is still monster force, which again gives approximately 50% hope, and 50 + 50 is 100/100, rather than 95/100. (:

Denny C.

Professor Hadfield; On the friction of Mars to slow you down, isn't that aerobraking?