Will someone please explain space propulsion?

Factual Questions
1

Forgive me if I am redundant, here. I am new to the boards, and I did do a search, but what I found seems to be more complex than my question.

I am having trouble rationalising the propulsion of spacecraft. I want to know what king of force propels them.

This started as a discussion about what mach level the space shuttle could reach. I will try to recap, so you can understand my lines of thought.

him: “So, what mach level do you think the space shuttle can reach?” (he likes planes.)
me: “Depends. Do you mean as it’s lifting off, with the boosters and the mass quantities of rocket fuel, by itself in space, or if it was traveling along the earth’s surface with its boosters?”
him: “Lifting off.”
me: “I doubt it reaches mach one. It’s fighting the pull of the Earth’s gravity all the way up through the atmosphere. That’s the only reason it needs that much fuel, and why it drops the boosters once its momentum is enough to propel it into orbit. Then it’s just floating along, just high enough not to get pulled back down.”

Now, I could be completely wrong about some things. Hence my questions. I have a basic working knowledge of physics, and it’s my understanding that for an object to move of its own accord, it has to push off somehow and create resistance to move away from. For instance, for me to walk, my legs push off from the floor. I understand that there is a system on the space shuttle that allows it to be directed, but from orbit, to reenter the atmosphere, my mind says it has to push off from something. If space is a vaccuum, where does the resistance come from? How is the force created to push the shuttle around in space, and to direct it back toward earth?

I am sometimes amazed at the higher concepts I can grasp while the ones that make them possible elude me.

2

Momentum is conserved. The burned fuel has mass and is expelled backwards – This is your momentum backwards. Therefore there must be forward momentum to compensate. This is what pushes the rocket.

3

Nope, you don’t need anything to push against. Thanks to conservation of momentum, so long as you chuck mass fast enough out the back of the rocket fast enough, the rocket will move forward. Imagine you’re, oh, sitting in a playground swing that’s not moving, and you’re holding a big-ass rock in your lap. Now, if you fling the rock forward as fast as you can, the reaction will push you backward a little.

This is conservation of momentum–the total momentum (which is mass x velocity) of the system wants to remain the same (because that’s how the universe works). In this case, the system is you plus the rock (I’m simplifying here, of course). This means that when you fling the rock away, you give it momentum in the forward direction which is equal to the mass of the rock multiplied by the speed at which you flung it away (in physics, velocity is speed in a particular direction). Now, since you started off sitting still, the total momentum of the system wants to remain zero–which means that when you fling the rock away, you’re given the exact same amount of momentum in the opposite direction, so everything cancels out nicely. But even though the total momentum of the system is zero, you’re still moving, and you didn’t have to push against anything.

Now imagine that instead of having a single big-ass rock in your lap, you have a bucket of golf balls. Each one is a lot lighter, but if you throw enough of them fast enough, you’ll end up with the same amount of momentum.

Now keep going and imagine you have a rocket engine in your lap (get yer mind outta the gutter…). This time, you’re throwing gas molecules (and the other assorted crud that’s created by burning rocket fuel). Since the molecules are really small, you have to throw a helluva lot of them really fast–but that’s exactly what a rocket engine is designed to do. And you still end up moving backwards without pushing against anything.

This is, of course, overly simplified and probably pretty garbled (Physics I was a long time ago), but that’s the very basics of how it works. Actually moving in space is waaaay more complicated thanks to orbital dynamics (e.g., if you speed up, you can end up higher and slower than when you started).

4

The other way to explain it in a more Earth-like context is to imagine yourself standing on an icy pond. (On skates, for preference.)

You throw a rock to the north shore of the lake. But the momentum of your throw pushes you backwards, toward the south shore and into the mouth of a giant man-eating freshwater walrus. Okay, bad example.

Instead, you shoot a gun toward the north shore of the lake. The gun produces greater force than a thrown rock. On dry ground this force would normally be absorbed through your body and transferred to the soles of your shoes, which grip against the earth (friction). But you’re on ice, so you slide backwards.

And, as DrMatrix says, when the space shuttle (with or without skates) throws exploding fuel out the back of its engines, it throws the shuttle forward by the same force and into the mouth of a giant shuttle-eating space walrus.

There, clear now? If not, simply read the words of the Master. Trust in the power of the Straight Dope. :slight_smile:

5

To address the question of the shuttle’s speed, it goes quite a bit faster than Mach 1. It takes 8 or 9 minutes to get to orbit, and by that time it is moving at nearly 20,000mph. It surpasses the sound barrier in the first minute after liftoff and accelerates even faster as its altitude increases.

On a side note, Mach numbers vary with altitude and temperature, as the speed of sound is not constant throughout the atmosphere.

6

It’s called “reaction mass”. An easy way to see this is to take a basketball to a skating rink. (If you don’t have ice skates, a skateboard on a smooth floor will do.) While motionless on either of these low friction surfaces, throw the basketball forward. This will cause you to move backwards.

Now, I’m assuming you know enough basic physics that “for every action there is an equal and opposite reaction” is a simple concept. For the basketball to move forwards away from you, it must push off of something - you. This small “reaction push” (a term I just made up) is the source of your motion. Rockets do the same thing; except instead of spitting basketballs out the back, they spit out gasses (or water, or air, or anything with mass - hence “reaction mass”)

I hope that was simple and lucid enough. If not, plug “reaction mass” into google and search around a bit.

7

“I doubt it reaches mach one”

-Direct result of my lack of proper perspective about such things.

Thanks so much for the well explained answers, guys! I get it! Whee!

8

Just so you know, Deva (in case you ever find yourself taking a quiz in a class on Newtonian physics), anytime you see a word problem that mentions something like “smooth ice”, that’s code for “you can ignore friction when you do the equations.”

(Trivia for anyone who used the classic Halliday & Resnick intro-physics books: The guy who wrote the lab manual was my Physics III TA. He was seriously cool. Of course, Resnick was also known for giving a final exam entirely in limerick form.)

9

However, the giant man-eating freshwater walrus is unlikely to be a code for anything. I repeat, the walrus will not be on the final exam.

10

For every action there is an equal and opposite reaction. This is one of three lesser known laws of physics (Sorry that bit about lesser known was sarcasm, but meant in a friendly way) But consider when you let go of a balloon, (one you have blown up and not sealed) it flys all over the room. The air expelled out the back does not push against anything. Yet the ballon is nevertheless propelled. It is the mass (air molecules) escaping from the opening in the balloon that causes the balloon to move in a direction opposite from the opening.

As for the suggestion that Mach changes with increase in elevation, disregard it. Mach 1 is the speed of sound at sea level, it is a constant velocity. Mach 2 is twice the speed of sound at sea level. Just think how hard it would be to make accurate air speed indicators if they had to constantly recalibrate for altitude changes. While it is true that the speed at which sound travels does vary Mach does not.

11

If all you have to do is expel mass, why bother burning it? :confused:

12

That turns out not to be the case. From Wikipedia: “Since the speed of sound increases as the temperature increases, the actual speed of an object traveling at Mach 1 will depend on its altitude and the atmospheric conditions.”

Several other websites state the same.

Something that may be of interest to the OP, this site says that the shuttle has reached a speed of Mach 23.3 when its main engines are shut off.
http://www.aerospaceweb.org/question/atmosphere/q0126.shtml

13

To increase the velocity.

14

You don’t have to, but you have to accelerate it.

There is a wonderful movie on the subject. If you can ever get hold of Road to the Stars (1958) , a soviet educational sci-fi film from the very early days of space exploration, watch it. Since it could not assume any prior knowledge in the audience it addresses all the classic questions that come up from time to time (What does a rocket push against? Then, how should I imagine it? What about escape velocity? Weightlessness vs no gravity…and even Do you have to burn anything?) From time to time the question is raised how much this movie influenced the space scenes in 2001 - a Space Odyssey.

The scene that describes this has Konstantin Tsiolkovsky in a little boat, very enthusiastic about explaining the basics of propulsion. In order to demonstrate the concept he stands up and throws both oars away from his boat as hard as he can…

15

I know it’s not in the spirit of the OP, but in a sense, that “chucked mass” is what is being pushed against.

16

I commend you on your tact. You were right I was wrong yet you did not boorishly rub it in my face you merely pointed out my error. That demonstrates you are a man of class. I found the equation, here it is :

Mach number: Mach number is a quantity that defines how quickly a vehicle travels with respect to the speed of sound. The Mach number (M) is simply the ratio of the vehicle’s velocity (V) divided by the speed of sound at that altitude (a).

:slight_smile:

17

So presumably Mach numbers are a useless concept once you are outside the atmosphere? No air means no sound, so nothing to measure it against, right?

18

Prior to Johnathan D pointing out my error I would have disagreed with you I was under the impression that Mach was a constant, the speed of sound at sea level. Aparrently I was mistaken. That being the case I guess you are correct. Since sound has no speed in outerspace then I guess Mach is a meaningless term beyond the atmosphere. (but I still am not entirely convinced that Mach is variable, that would make it basically meaningless) :confused:

19

If you could stand on a perfectly frictionless surface (of which no such animal exists, really), the only way you could move would be by spitting. However, I suppose other bodily functions would propel one’s self as well… :wink:

Say, didn’t you ever wonder what keeps a child on a swing, swinging? (No, the answer is not spitting nor potty accidents. Guess again, but with a different answer!) - Jinx :smiley:

20

At my last job (I was a rocket scientist!) we would talk about the speed of an object at the 1000km “reference altitude,” to that discussions of its Mach speed would be meaningful. When I gave a slide presentation to some senior officers, I gave the speed of an object in km/s and was asked what Mach number that equated to (I had done the math ahead of time, knowing that some of the officers were former fighter pilots). My reply was “about Mach 21, but at those altitudes, Mach numbers are essentially meaningless.” I don’t know the altitude at which the shuttle turns off its engines, but I would hazard a guess that “Mach 23” is a polite fiction - they’ve done the math, and there’s a little bit of air up there, so you can technically calculate what Mach 1 is, but what they’re really saying is, “You know how fast Mach 1 sounds? Well this is going like twenty times FASTER, dude! That sucker is REALLY MOVING!”

I think the most accurate statement they could make about the shuttle is that “when its engines shut off, it is moving significantly faster than Earth’s escape velocity,” but that’s neither very helpful nor very illustrative.