Rocket Propulsion -- For Margamatix

[Kiwi]

margamatix said:

When the module descended, braking by firing its rockets, what did the thrust from these rockets react against, if there is no atmosphere?

alasdhair said:

I thought you knew all about Newton's Third Law of Motion? The thrust doesn't need to "react" against anything: ejecting the gases of the exhaust plume out of the nozzle is of itself sufficient to propel the rocket in the opposite direction.

margamatix said:

Only in an atmosphere. There needs to be something against which the thrust can act. Otherwise it would simply dissipate without any braking effect. That's how rockets work.

It is amazing that someone who considers himself expert enough to criticise the Apollo missions and claim they were faked should make such an amusing error when the correct information is so freely available. Perhaps this explains why Margamatix thinks the lunar module couldn't have landed on and taken off from the moon -- it didn't have wings.

The Encyclopaedia Britannica, 1970, Volume 19, page 418, "Fundamental Principles of Rocket Propulsion":
The thrust does not result, as sometimes erroneously presumed, from the jet pushing against the surrounding air.

Collins Concise Encyclopedia, 1977, page 482:
rocket, projectile driven by its reaction to stream of hot gases it produces by burning propellant. By carrying its own source of oxygen, it operates independently of Earth's atmosphere and can be used in outer space. Propellants used incl. liquid hydrogen and liquid oxygen.

Hutchinson Softback Encylopedia, 1991, page 710:
rocket projectile driven by the reaction of gases produced by a fast-burning fuel. Unlike the jet engine, which is also a reaction engine, the rocket carries its own oxygen supply to burn its fuel and is totally independent of any surrounding atmosphere. As rockets are the only form of propulsion available which can function in a vacuum, they are essential to the exploration of outer space.

[Bob B.]

A rocket actually work better in a vacuum. The basic thrust equation is,

F = qV_e+(P_e-P_a)A_e

where q is the rate of the ejected mass flow, Pa the pressure of the ambient atmosphere, Pe the pressure of the exhaust gases and Ve their ejection speed.

As you can see, F is greatest when Pa is equal to zero.


edited to make easier to read

[Kiwi]

More quotes. These, along with those in post No. 1, are from some of the reference materials I have at home. It's so easy and cheap to own many of them nowadays compared with in the past that there's little excuse for not having them. Some of the CD-ROMs came free with hardware, with other software, or with magazines, and the books are easily bought new or second hand.


Science and The Universe, Mitchell Beazley Encyclopaedias Limited, London, 1979, page 268:
The atmosphere does not extend upwards for very far on a cosmic scale and in space, where there is no air, a vehicle needs an engine that does not "use" air, that is, a rocket motor. In a rocket, a stream of gas emitted from the exhaust of the vehicle provides the motive power needed; it has been said that a rocket "kicks against itself" and the presence of a resisting atmosphere is actually a hindrance.

Microsoft Bookshelf '95 CD-ROM:
rocket, any vehicle propelled by ejection of the gases produced by combustion of self-contained propellants. Tremendous pressure is exerted on the walls of the combustion chamber, except where the gas exits at the rear; the resulting unbalanced force, or thrust, on the front interior wall of the chamber pushes the rocket forward.

Microsoft Bookshelf '95 CD-ROM:
rocket engine noun: A reaction engine that contains all the substances necessary for its operation and is not dependent on substances such as atmospheric oxygen, drawn from the surrounding medium, and thus is capable of operating in outer space. Also called rocket motor.

Microsoft Encarta 96 CD-ROM:
Rocket, general term for a jet propulsion device propelled by the expulsion of gases generated in a combustion chamber (see Jet Propulsion). Because the combustible propellants contain both fuel and an oxidizer, a rocket develops thrust independent of its surroundings, unlike other types of jet engines that utilize oxygen from the atmosphere to burn fuel carried aboard (see Combustion). A rocket engine, therefore, is self-contained and is the only type of device suitable for flight propulsion in outer space.

Compton's Interactive Encyclopedia 2000 CD-ROM:
...A rocket moves in accordance with Isaac Newton's third law of motion, which says that for every action there is an equal and opposite reaction (see Mechanics). Burning gases are ejected forcibly and violently from an exhaust nozzle of a rocket. The opposed reaction to this exhaust blast, and not the exhaust blast itself, moves the rocket forward. The greater the speed of the exhaust blast, the greater the reaction against the forward end of the rocket and the greater its speed. Since the exhaust blast need not push against anything at the rear, a rocket can operate even better in empty space, or vacuum, than in the atmosphere, where the exhaust blast is slowed down by the air. (See also Jet Propulsion.)

[Retrograde]

bob said:

A rocket actually work better in a vacuum. The basic thrust equation is,

F = qV_e+(P_e-P_a)A_e

where q is the rate of the ejected mass flow, Pa the pressure of the ambient atmosphere, Pe the pressure of the exhaust gases and Ve their ejection speed.

As you can see, F is greatest when Pa is equal to zero.


edited to make easier to read

Hi Bob B.,

Just to clarify, A_e is the surface area across the engine nozzle? Or do I misunderstand?

Thanks,

Nick

[Bob B.]

kos said:

Just to clarify, A_e is the surface area across the engine nozzle? Or do I misunderstand?

Oops, I left that out didn't I? You are correct, Ae is the area of the nozzle exit.

[Retrograde]

bob said:

kos said:

Just to clarify, A_e is the surface area across the engine nozzle? Or do I misunderstand?

Oops, I left that out didn't I? You are correct, Ae is the area of the nozzle exit.

Very good. Thanks.

Nick

[Glom]

LOL! Was that really said? I've been away too long.

Bob B., do you have a derivation on that thrust equation? I'm unclear why, once you include the rate of momentum, you need the pressure term.

[gwiz]

The rate of momentum is equivalent to the pressure force on the inside of the nozzle, but the pressure term in the equation is for the atmospheric pressure on the outside of the nozzle, which is independent.

[Glom]

gwiz said:

The rate of momentum is equivalent to the pressure force on the inside of the nozzle, but the pressure term in the equation is for the atmospheric pressure on the outside of the nozzle, which is independent.

But that term doesn't disappear in a vacuum. In a vacuum, the equation becomes.

F = qV_e+P_eA_e

So the force is the sum of the rate of momentum and the force of the exhaust at the nozzle exit. But surely the second is due to the first.

[gwiz]

Perhaps I was a bit too glib. The rate of momentum is reduced by the downstream pressure in the exhaust, so there is a trade between the pressure and momentum terms.

Consider a nozzle with a removable extension. With the extension in place, the flow is further accelerated down to ambient, so no pressure term, but you now have a higher velocity. This is the best you can do, so without the extension you are losing more from the lower exhaust velocity than you gain from the pressure term. The pressure term isn't a bonus, but without it you'd get the wrong answer for the loss of thrust due to removing the extension.

[Retrograde]

kiwi said:

Perhaps this explains why Margamatix thinks the lunar module couldn't have landed on and taken off from the moon -- it didn't have wings.

I guess they could have put wings on it, to be extended once the lunar module begins its descent. If they were made of thin paper or something like that, they wouldn't weigh very much...

Didn't one of the asteroid films from a few years back have the spacecraft landing on the asteroid like an airplane?

[Bob B.]

glom said:

Bob B., do you have a derivation on that thrust equation? I'm unclear why, once you include the rate of momentum, you need the pressure term.

I have a derivation for momentum thrust, qVe, in my web page. Pressure thrust, (Pe-Pa)Ae, is simply the result of unbalanced pressure forces at the nozzle exit.

Pressure thrust is usually small in comparison to momentum thrust. In fact, the best engine design is one where Pe=Pa, a condition called optimum expansion. The more a nozzle is extended the lower Pe becomes but the higher Ve becomes. Thus, extending a nozzle increases momentum thrust while decreasing pressure thrust. Total thrust increases until Pe=Pa; after which pressure thrust decreases faster than momentum thrust increases, so total thrust declines.

This is why engines designed to work at low altitude (first stage engines) have smaller expansion ratios and lower specific impulses than those designed to work at high altitude or in a vacuum (upper stages).

[JayUtah]

In practice, in a vacuum, you can never achieve optimum expansion because the nature of fluid dynamics ensures you will always have some static pressure at the exit.

I got into trouble by trying to draw too sharp a distinction between the dynamic and the static effects of the gas. I'll try to remedy that. Gas pressure, in every case, is simply the effects of molecules colliding with something and transferring kinetic energy to them through collision mechanics. What we think of as "static" gas pressure is still molecules hitting things, but the effect is isotropic. That's a fancy way of saying that it occurs in all directions simultaneously at the same intensity. A container of gas will suffer collisions on all its interior surfaces equally per unit area. The gas molecules are in motion and thus have individual momentum, but the collective momentum of the gas cancels to zero.

In a rocket engine we first get the gas molecules moving very fast by heating them to very high temperatures. But then we also want to get them all moving in the same direction. If we can do that, we create a situation where that mass of gas has a collective momentum that makes Sir Isacc happy. That's what the nozzle does. I used to tell laymen that a nozzle converts pressure to velocity. That's true enough, but in fact the real description of the behavior is much simpler: the geometry makes it easier for the gas to expand in one direction than in another.

But you never fully succeed at doing that. Ideally the gas molecules would stream out of the engine in exactly the same direction at exactly the same speed. But in fact slight variations in velocity vectors and slight variations in speed create "sideswipes" and "rear-end" collisions that cascade into a tendency toward turbulence.

So what it means is that the gas momentum at the engine exit plane will be anisotropic but still largely directed along the engine axis.

[Bob B.]

I just started reading John D. Clark’s book, Ignition! An informal history of liquid rocket propellants (1972). On the first page of the book, Mr. Clark writes about an article that appeared in a Russian scientific journal in 1903. The article, titled Exploration of Space with Reactive Devices, was authored by Konstantin E. Tsiolkovsky (1857-1935). Clark summarizes the substance of the article as follows:

1. Space travel is possible.
2. This can be accomplished by means of, and only by means of, rocket propulsion, since a rocket is the only known propulsive device which will work in empty space.
3. Gunpowder rockets cannot be used, since gunpowder (or smokeless powder either, for that matter) simply does not have enough energy.
4. Certain liquids do possess the necessary energy.
5. Liquid hydrogen would be a good fuel and liquid oxygen a good oxidizer, and the pair would make nearly the ideal propellant combination.

(emphasis mine)

Tsiolkovsky was a true visionary and pioneer of astronautics, and he realized over a century ago something that margamatix, and HBs like him, can’t seem to be able to grasp.

[Data Cable]

Heck, I learned this basic principle of rocketry (and by extension, Newton's 3rd law) from Tennesse Tuxedo and his Tales when I was no more than 5 or 6 years old.

Tennesse: That exhaust coming out the back of the rocket pushes against the air, and it goes... right, Mr. Whoopie?

Mr. Whoopie: Wrong, Tennesee. The air has nothing to do with it. In fact, rockets work better in space where there is no air to resist it.

That show, BTW, was produced from 1963 to 1969, though I wasn't produced until the middle of the following decade.