- #1

CitronBleu

- 13

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My question refers to

**how rocket thrust works in a vacuum**. I know this question has been posted before, but I am still confused about how it works.

My understanding is that rockets do not move by pushing against air but move by pushing "

**against their own fuel**," as is stated in Newton's Third Law according to which "for every action there is an equal and opposite reaction."

The oft repeated analogy to explain how this works is throwing a ball off the stern of a small boat and the resulting forward movement of the boat, or the recoil energy resulting from firing a gun.

Ok. So a rocket moves by

**recoil**energy? Is this correct?

Here is an analogy of the recoil generated by a a gun, in this case an

**M16**, compared to the mass and force involved in a rocket, in this case a French rocket

**Ariane 5**. (The analogy is not mine.)

I would appreciate your comments.

Weight of M16 RIFLE:

**4kg**https://en.wikipedia.org/wiki/M16_rifle

Weight of an M16 bullet:

**3.6g**http://www.defencetalk.com/forums/army-security-forces/m16-vs-ak-47-a-4686-7/

Weight of an Ariane 5 rocket:

**760,000Kg**http://en.wikipedia.org/wiki/Ariane_5

Recoil Force involved in firing M16 bullet: "A bullet fired from an M16 rifle has approx

**1763 Joules**of kinetic energy as it leaves the muzzle,

**but the recoil energy exerted on the gun is less than 7 Joules.**" http://en.wikipedia.org/wiki/Recoil

The kinetic energy exerted on the rifle is thus equal to

**250 times less**than the energy exerted on the bullet .

If we multiply the weight of the M16 by

**190,000**, we get the weight of an Ariane 5 (760,000Kg).

Continuing the analogy, if we multiply the weight of the M16 bullet by 190,000, we get

**684Kg**(3.6g x190,000).

If the giant M16 were ejecting three giant bullets a seconds (

**2,054Kg**), then this would approximately equal the fuel mass ejected by Ariane 5 every second (

**2,000Kg**).

Multiplying (by the 190.000 factor) the known kinetic energy exerted on an M16 rifle by a bullet exiting its muzzle (about 7 joules) we get (7X190.000=

**1.330.000 joules**).

The giant M16 rocket will therefore produce a kinetic energy thrust equal to 7 x 190,000 x 3 =

**3,990,000 joules**, or approximately

**4 million joules**.

Four million joules is approximately equal to

**5,400 Horsepower**.

**How can 5,400 Horsepower be sufficient to lift off a 760,000Kg rocket into Low Earth Orbit?**