Phobos Railgun

[wnderer]

The US Navy has got this new railgun. They say it can launch a 23 lb missile up to speeds of mach 7. If you put these on Phobos and instead of shooting missiles; shoot pieces of Phobos, would you be able to get Phobos into a synchronous orbit? ( A synchronous orbit for Phobos would allow a space elevator between Phobos and Mars.) How much of Phobos would be left once it got to synchronous orbit?

[Ralathon]

Using basic orbital mechanics you can figure out that you'd need 547.5 m/s of dV to get Phobos to a mars synchronous orbit.

dV1 = sqrt(6.42e23*6.67e-11/9.376e6)*(sqrt(2*1.7e7/(9.276e6+1.7e7))-1) = 293.9 m/s

dV2 = sqrt(6.42e23*6.67e-11/1.7e7)*(1-sqrt(2*9.276e6/(9.276e6+1.7e7))) = 253.5 m/s

dVtot = 293.9 + 253.5 = 547.5

Mach 7 means your railgun has an exhaust velocity of 7*340 = 2380 m/s.

Phobos initially weighs 1.07e16 kg.

Lets toss this into Tsiolkovsky and solve:

m(empty) = m0/e^(dV/Ve) = 1.07e16/e^(547.5/2380) = 8.5e15 kg.

So you'd need to use up slightly more than 20% of phobos to alter its orbit into a synchronous mars orbit if you use a railgun like that.

However, the limiting factor here isn't the mass fraction, but time. Say you use that gun to fire a kilogram of matter per second. That means it'd take you 69.8 million years to complete the transfer. To do it within a century you'd need about 700 thousand railguns, each firing a kg of matter every second of every day for a century. Not very feasible

[Norpo]

-snip, basically saying "it'd take a long time"-

Geez, at that rate, it'd be more feasable to just figure out how to make more efficient rockets, and go there in a couple centuries.

What about Deimos? It's smaller, weighs less, and it's closer to mars stationary orbit to start with (I think) Not sure if it's density would make it effective for the railgun technique, though. It'd take a really long time still probably, but it'd certainly be easier.

[Skyler4856]

The lack of an atmosphere on Phobos would probably allow for speeds far higher than mach 7

[wnderer]

Thanks Ralathon. That's what I wanted to know.

Geez, at that rate, it'd be more feasable to just figure out how to make more efficient rockets, and go there in a couple centuries.

What about Deimos? It's smaller, weighs less, and it's closer to mars stationary orbit to start with (I think) Not sure if it's density would make it effective for the railgun technique, though. It'd take a really long time still probably, but it'd certainly be easier.

Maybe, but Phobos gets in the way of the space elevator. If you want to build a space elevator on Mars, you have to do something about Phobos.

[Whirligig Girl]

Thanks Ralathon. That's what I wanted to know.

Maybe, but Phobos gets in the way of the space elevator. If you want to build a space elevator on Mars, you have to do something about Phobos.

Blow it up with a nuke?

[Camacha]

However, the limiting factor here isn't the mass fraction, but time. Say you use that gun to fire a kilogram of matter per second. That means it'd take you 69.8 million years to complete the transfer. To do it within a century you'd need about 700 thousand railguns, each firing a kg of matter every second of every day for a century. Not very feasible

Not to mention the weight of the railguns, power generation, crew and other equipment themselves!

[Ralathon]

Not to mention the weight of the railguns, power generation, crew and other equipment themselves!

Not to mention that 550 dV is an estimate based on a Hohmann transfer. This would be a spiral transfer, which means it needs closer to 1100 dV. So twice the time as well.

[wnderer]

Blow it up with a nuke?

Then little bitty pieces of Phobos get in the way of everything in orbit of Mars.

[Ralathon]

Then little bitty pieces of Phobos get in the way of everything in orbit of Mars.

If you're building a space elevator on Mars you don't need to worry about Phobos. Phobos has a 1 degree inclination to the martian equator. So there are only 2 points where it could possibly collide. It's also only a few km's in diameter, so you can just do a minor plane change maneuver with your space elevator if a collision is imminent. Wouldn't take much fuel at all. They do the same with the space station to dodge orbital debris.

[xcorps]

Bah. Build the space elevator out of adamantium, use some elven rope from Lothlorien to lasso Phobos, Pull that sucker really close with Mumaks and use it as a waystation for refueling the Sunjammers before they head off to bring down the Aslan. But make damn sure you don't open the portal...otherwise you're gonna need a badass space marine to beat back the demon hordes. I suppose you could station Elric and Hawkmoon there to guard it.

<grumble>

[NFUN]

Bah. Build the space elevator out of adamantium, use some elven rope from Lothlorien to lasso Phobos, Pull that sucker really close with Mumaks and use it as a waystation for refueling the Sunjammers before they head off to bring down the Aslan. But make damn sure you don't open the portal...otherwise you're gonna need a badass space marine to beat back the demon hordes. I suppose you could station Elric and Hawkmoon there to guard it.

<grumble>

...

The spice must flow!

[Idobox]

The lack of an atmosphere on Phobos would probably allow for speeds far higher than mach 7

I don't think so. Friction of the projectile with the rails is probably more significant than air resistance at those speeds.

Given the time scales, an ion engine would be much better. You could also imagine putting solar sails. They might have terrible TWR, but you could use mass from phobos to build them, and over thousands of years, they'll provide a significant deltaV.