Re: Cannon in space, how much more propellent efficent are they than missiles?

On Jan 14, 12:53 am, IsaacKuo <mech...@xxxxxxxxx> wrote:
On Jan 12, 9:33 pm, Michael Price <nini_...@xxxxxxxxx> wrote:

On Jan 12, 6:07 pm, IsaacKuo <mech...@xxxxxxxxx> wrote:
If the cannon uses gunpowder, then it's pretty much always
better to use pure missiles.
  The idea is to minimise propellant weight, so we're using the best
propellant available which is probably the same as rocket fuel.

If the desired "muzzle velocity" as any decent fraction of the
rocket fuel's exhaust velocity, then you minimize propellant
weight with a rocket.  A gas gun only reduces propellant weight
if the muzzle velocity is a small fraction of the exhaust velocity.
If your desired muzzle velocity is on the order of 500m/s, then
a gas gun might use less propellant than rockets.

Yes and the reason you have that pressure is because the gas hits
the surface as I described.  It does work the way I said.  There is
pressure on all sides but I only care about pressure on the back
of the projectile for purposes of muzzle velocity.  Pressure on the
gun is the same as pressure on the back of the projectile.

It is not.  There's more pressure on the breech block than
there is on the base of the bullet.

You're right, I assumed a stationary projectile when I did the math
on
the collision with the back of the projectile.

 This is not a big deal
when the muzzle velocity is much lower than the speed of
sound in the working gas, but it's an increasingly big deal
as the bullet velocity approaches this speed.

By my math each doubling of the barrel length adds a constant amount
of energy for a give charge (once it's all burned).
Once it's all burned, there's a particular amount of energy
in the system and it's not going to get any greater.
  But that energy is in the form of pressure that expands the volume.

There's still a limited amount of it.

But that limited amount is approached asymptotically so you can add
energy
by adding barrel length forever. So the

The pressure never drops to zero no matter how much you
expand the volume because P = k/V (where k is a constant for each shot),

The pressure goes down faster than that.  You seem to be
assuming constant temperature, which is wrong.  As the
volume expands, the temperature goes down.

So how much of the kinetic energy comes from pressure and how much
from
temperature?

 Your math has gone wrong somewhere because it implies you
can add an unlimited amount of extra energy just by
making the barrel longer.
 And that's right, with an infinite barrel you could.

That violates conservation of energy.

I mispoke. You can add energy infinitely but you can't add infinite
energy.

First, it should be clear that your conclusion violates conservation
of energy.  From that undeniable problem, you can then work
backwards to see where you went wrong.  It seems that you
have naively applied Boyle's law, forgetting that it requires the
temperature to remain constant.

You're right.

Not really, but generally a rocket will be much more efficient
than a gun in the vacuum of space.  A gun has an unavoidable
inefficiency as the burnt "exhaust" leaves the barrel at high
velocity along with the projectile.  A missile, in contrast, tends
to leave the exhaust behind it at low velocity.
  Err.... no.  If your missile is leaving low velocity exhaust
it's using a lot of really bad fuel.

It leaves the rocket at a high velocity in the reference frame
of the rocket.  But in the frame of reference of the launching
ship, the exhaust's velocity is reduced by the rocket's own
velocity.  This means that the actual velocity of the exhaust
is always lower than the exhaust velocity--up until the rocket's
velocity starts exceeding twice the exhaust velocity (which is
far greater than a gun can even reach).  The kinetic energy
which isn't going into the exhaust is going into the rocket.

 The exhaust from a gun would be lower
velocity because it would hit the back of the projectile
several times, slowing down each time before it left the barrel.

The exhaust from the gun will never be at a lower velocity
than the projectile,

But it will be lower than the exhaust from a rocket.

since it can't push the projectile to go any faster than it's
going.  It will actually leave the gun at
an even faster velocity (typically MUCH faster).

Where's your evidence for this?

Much more of the chemical energy is converted into kinetic energy of the
missile.

The kinetic energy is mostly in the velocity of the fuel itself
leaving the back of your rocket.

This is wrong.  Up until a delta-v significantly larger than the
exhaust velocity, rockets efficiently put the kinetic energy
into the rocket, not the exhaust.

The momentum is the same right? So unless the fuel is heavier than
the rocket the kinetic energy of the fuel is a lot more than the
increase
in KE of the rocket.

Look at it this way.

You're looking at it the wrong way.  Kinetic energy is highly
dependent on the reference frame.  The reference frame
of the projectile is the WRONG reference frame to use.
The correct reference frame to use is the reference frame
of the launching ship.

The advantage of guns isn't expense (since the ammunition has
to be guided anyway for anything but extremely short range/low
acceleration targets.

The guns in my game are only used for extremely short range
(there are no low acceleration targets in this game).  These guys
are in a desperate situation with very limited resources, so they
use what they have available even though it's not very good.

If you're that close you're dead against anything with decent
missiles.

A pulse detonation rocket thruster is easy to convert into a
LOX/LH2 "spud gun" (and it's even still usable as a rocket thruster).
And blasting a shower of point blank range shot during an
extremely close pass is preferable to suicide-ramming even
though the Pk is much lower.  (They're desperate, but they're not
THAT desperate...yet.)

The advantages are that you use less propellant for a given
delta v

Wrong.

Prove it.

and that over short ranges they get to the target faster.

This can be true, but rockets can accelerate up to speed in
a VERY short time.

Isaac Kuo

.

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