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My Rockets
© Copyright 1997, Jim Loy
One night, in Viet Nam, I built some
rockets. I started with some aluminum foil, which I wrapped around a pencil. I
wrapped the bottom end around a pin, to make a small nozzle. I made the nozzle
that small, so that the fuel would not pour out. I figured that the burning
fuel would expand the nozzle, when the time came. I fashioned small fins at the
bottom, out of excess aluminum foil. Then I filled the aluminum foil tube with
gun powder, from an M16 bullet. I twisted the top into a nose cone. And I had a
rocket. I leaned it against a little wire frame that I made. And, I held a
lighted match under the rocket. Nothing happened. The gun powder would not
ignite.
So, I poured the gun powder out, and scraped some phosphorus from a
match head, and poured that into the rocket. Then I refilled the rocket with
the gun powder, and twisted the top again. I held a lighted match under the
rocket, and it took off. It flew about a foot in the air. Well, the Wright
brothers' first flight wasn't much better.
I decided that I needed to make my rocket lighter. So, I peeled aluminum
foil from chewing gum wrappers. And, I made several rockets out of this. And
some of them flew 5 to 10 feet into the air.
Then I decided to see if I could make a two-stage rocket. I made two
rockets, fitting the top one into the top of the tube of the bottom one. I
figured that the heat of the burning gun powder, of the bottom stage, might
ignite the match head phosphorus of the top stage.
I held a burning match under my two-stage rocket, and whoosh, it flew
off into the night sky. The bottom stage was on the ground, about a foot from
the launch pad. At first, I couldn't find the second stage. I opened the gate
of the "Crypto Compound" where I worked, and searched outside the fence. And,
there it was, about 60 feet from the launch pad (I stepped off the
distance).
All right! Success! Now I'm a rocket scientist, unlike most people.
Multiple stage rockets are the secret of getting a rocket into outer
space. As a rocket's fuel burns, the expended fuel tanks (the lower stages)
become dead weight. The rocket's efficiency improves tremendously if these fuel
tanks are merely discarded, making the rocket much lighter. This is all
logical. But, seeing the results (a rocket that flies many times farther) can
be very dramatic.
On the right, we see me sitting on a frictionless ice
surface. I throw a brick to the left, and I slide to the right. That is rocket
propulsion, pure and simple. A rocket would be throwing many gas molecules to
the left, instead of bricks. And the rocket would move to the right.
Rockets, of course, work just fine in outer space. They don't need air
to push against. Imagine me throwing bricks, as I float in space, no problem.
In fact, the air pushing against the nose cone decreases the efficiency of a
rocket. A rocket works much better in outer space. Besides, near the earth, the
rocket usually has to work against the force of gravity; so it works better in
outer space for that reason too.
The hot gasses (from the burning fuel) in the combustion chamber push
against the front end of the combustion chamber, pushing the rocket forward.
And a rocket can even fly faster than the speed of the hot gasses going out the
bottom end.
Disclaimer: OK. You were going to build rockets, like I did, and
burn your fingers, and sue me. Right? Give me a break. Not that a jury won't
believe you, although you might have trouble finding a jury of my peers, booga
booga. But, you can't get much money out of me, not enough to make it worth
your while. OK. Take my computer. You deal with Windows and pull your hair out.
Then you can sue Bill Gates, which you should have done all along. How's that
for a disclaimer?
You might want to go with the Estes rockets.
Estes
1295 H St.
Penrose, CO 81240
They are a lot of fun. Their rockets (mostly kits) are often found in
hobby stores. Some of them can fly right out of sight. They range from $3.19 to
over $30. Most come with a parachute. Engines (the smallest are $4.49 for a
package of four) come with igniter wire, which must be hooked up to a power
source, like a car battery (they sell switches). A cheap launch pad is handy.
And, a vertical launch rod (fits into a soda straw glued to the side of your
rocket) is a necessity ($5.29), to keep the rocket from zooming off
sideways.
See Estes Rockets. Also see
Apogee Rockets.
Addendum:
Let's do a little arithmetic, using Newton's
F=ma (force=mass times acceleration). Let's say that I have the rocket
on the left, in a completely frictionless, gravity-free environment. I have ten
bricks for fuel. And the rocket (with me included) weighs two bricks, without
fuel. And let's say that I can heave a brick to the left (in the diagram) at 1
m/sec. When I throw the first brick overboard, the brick goes backward with
respect to the rocket, and the rocket begins moving to the right at 0.09 (or
1/11) m/sec. Let's throw another brick overboard. Now we are going 0.19 m/sec.
A third brick gives us 0.30 m/sec. A fourth gives 0.43 m/sec. Then 0.57, 0.74,
0.94, 1.19, 1.52, 2.02, and we are out of bricks. A few of things should be
observed here:
- The acceleration is not constant; it keeps increasing. The
force remains the same, but the mass of our rocket keeps decreasing as we used
up fuel.
- We can easily go faster than our fuel speed. Above, we ended
up going twice as fast as the fuel speed.
- If we can double (or multiply by x) the fuel speed, we double (or
multiply by x) the rocket speed. If we threw the bricks twice as hard (2
m/sec) all of the above numbers would be doubled.
How about if we decrease the weight of our rocket? Let's say it (with me
included, but without fuel) only weights one brick. Then the numbers are: 0.10,
0.21, 0.34, 0.48, 0.65, 0.85, 1.10, 1.43, 1.93, 2.93. By cutting our weight
(without fuel) in half, we have multiplied our final speed by almost 1.5. So,
we have a fourth important observation:
4. The lighter the rocket (without fuel) the better.
In fact, weight is so important that we should throw away useless
portions of our rocket (empty fuel tanks in the case of real rockets) as they
become available. This is the idea of the multi-stage rocket. The lower stages
of a rocket (mostly empty fuel tanks) are discarded as they become useless.
That gives us way more acceleration. And most rockets are extremely light. Most
of their weight is fuel.
In a real rocket, the molecules of burned fuel are much smaller than
bricks. But the principle is the same. The numbers change in a smoother
fashion. With a little elementary calculus, the speeds, accelerations, and
changes in acceleration can be easily calculated.
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