# A rail gun as a thruster

## Summary:

if one once of Iron was accelerated to 80% of the speed of light, could this be use of as a means of thrust for an interplanetary probe?
if one once of Iron was accelerated to 80% of the speed of light, could this be use of as a means of thrust for an interplanetary probe? where the probe has a continuous expulsion of Iron. and ignoring the need massive amount of power.

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anorlunda
Staff Emeritus
Yes, throwing an object in space gives you a reaction force that moves you in the opposite direction.

If you don't care about the weight of the probe or the massive use of power, do you care how many billions of years it takes to get to the destination?

Baluncore
2019 Award
If used as a thruster, will it direct the beam of projectiles towards the Earth and satellites?
A railgun must fire a sequence of discrete projectiles, each must be sequentially loaded.
The rails must connect electrically to the projectile or sabot. How will the plasma “brushes” needed to complete the circuit be provided in space without an atmosphere? At near light speed, the damage to the rails will be significant.

...80% of the speed of light...
If the release speed is low (below the escape velocity of the solar system) you must aim very accurately to have something big to safely stop them somewhere - this limits the directions available for the acceleration.
If the release speed is (very) high, then you have more freedom (unless somebody from outside comes to complain, of course ) but you need insane amount of energy provided in very short time - the time spent in the 'gun' will be short (this also means insane wear on the 'gun').

In short - in theory it is possible: in reality there are limits: but still, it is just not practical.

russ_watters
Mentor
if one once of Iron was accelerated to 80% of the speed of light, could this be use of as a means of thrust for an interplanetary probe? where the probe has a continuous expulsion of Iron. and ignoring the need massive amount of power.
If you remove the "r", you're describing a thruster that already exists.

• • Janus
Staff Emeritus
Gold Member
Summary: if one once of Iron was accelerated to 80% of the speed of light, could this be use of as a means of thrust for an interplanetary probe?

if one once of Iron was accelerated to 80% of the speed of light, could this be use of as a means of thrust for an interplanetary probe? where the probe has a continuous expulsion of Iron. and ignoring the need massive amount of power.
In this case, your reaction mass and energy source don't come from the same thing (unlike typical rocket fuel where your fuel provides both, and adding more fuel means adding both reaction mass and energy supply.)

So you need an external energy source to accelerate your iron.
This brings up an interesting issue: Assuming a fixed available amount of energy, you get a "bigger bang" in terms of ship velocity with lower reaction mass velocity.
Simple example: Assume a energy store of 1000 J, and a ship mass of 1000 kg
We will compare two scenarios:
Using a 1 kg hunk of iron as reaction mass
Using a 10 kg hunk of iron as reaction mass

with 1000 J of energy, you can accelerate a 1kg mass up to 44.72 m/s, and a 10 kg mass up to 14.14 m/s

Now we use the rocket equation:

V = ve ln(Mi[\sub]/Mf)
V is the velocity change for our ship
ve = the exhaust velocity ( calculated above)
Mi[\sub] is the total initial mass (ship mass+ reaction mass)
Mf is the final mass (ship mass alone)

In the first scenario we get
V= 44.72m/s * ln(1001 kg/1000 kg) = 0.0447 m/s for our ship velocity.

In the second scenario, we get:
V= 14.14 m/s * (1010 kg/1000 kg) = 0.141 m/s for our ship.

In other words, you get better than 3 times the ship velocity accelerating the larger mass to a lower exhaust velocity with the same energy budget.

• • Tghu Verd and Torbert
mfb
Mentor
If you remove the "r", you're describing a thruster that already exists.
Not at 80% the speed of light, however, that would need a big particle accelerator and it would be very inefficient. Ion thrusters have typical exhaust velocities of tens of kilometers per second, or ~0.01%-0.02% the speed of light.

• russ_watters