Author Topic: Mass drivers for propulsion  (Read 1362 times)

Offline Nilof

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Mass drivers for propulsion
« on: 01/01/2018 04:37 AM »
...since mass drivers haven't gotten any discussion here in quite a while.

I was wondering if anyone had some hard numbers on the performance of mass drivers for use in propulsion?

The vast majority of papers on the subject that I've found seem to be behind paywalls. According to this really old article from 1984 (http://ieeexplore.ieee.org/document/1063089/ ), mass drivers could miniaturize to 1 cm length for accelerating a 2 gram ring to 5 km/s, which puts mass drivers in the range where they could be practically usable for electric propulsion in some regimes.

Mass drivers are fairly significant enablers for retrieving asteroids, since they can be loaded with asteroid rock for reaction mass. So it'd be interesting to know how well they could trade against more conventional SEP.
« Last Edit: 01/01/2018 04:37 AM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline QuantumG

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Re: Mass drivers for propulsion
« Reply #1 on: 01/02/2018 01:11 AM »
There's also light-gas guns - significantly higher performance for launch. As for retrieving asteroids, solar thermal (i.e., steam rockets) is probably still the winner if you have the volatiles.
Jeff Bezos has billions to spend on rockets and can go at whatever pace he likes! Wow! What pace is he going at? Well... have you heard of Zeno's paradox?

Offline A_M_Swallow

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Re: Mass drivers for propulsion
« Reply #2 on: 01/02/2018 01:30 AM »
There's also light-gas guns - significantly higher performance for launch. As for retrieving asteroids, solar thermal (i.e., steam rockets) is probably still the winner if you have the volatiles.

The volatile does not have to be water, just something that boils at a low temperature. A low latent heat of vaporization is also good.

Offline ncb1397

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Re: Mass drivers for propulsion
« Reply #3 on: 01/02/2018 06:39 PM »
There's also light-gas guns - significantly higher performance for launch. As for retrieving asteroids, solar thermal (i.e., steam rockets) is probably still the winner if you have the volatiles.

The volatile does not have to be water, just something that boils at a low temperature. A low latent heat of vaporization is also good.

Since we are talking parameters, you probably also want a good expansion ratio
water: 1600x = good
nitrogen: 694x = ok
etc.

But to get a little bit back on topic. The Navy built this rail gun that fires projectiles at 5600 mph or 2.5 km/s which translates to an ISP of 255. Not exactly horrible. Makes in space refueling pretty simple though.

« Last Edit: 01/02/2018 07:15 PM by ncb1397 »

Offline morganism

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Re: Mass drivers for propulsion
« Reply #4 on: 03/08/2018 08:04 AM »
You are also creating a space hazard, with projectiles spewing around the system. Now if you are throwing into a "catchers mitt", there are some papers out there on slinging stuff from Luna to LEO.

Offline Asteroza

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Re: Mass drivers for propulsion
« Reply #5 on: 03/08/2018 10:09 PM »
Isn't frozen gas (solid oxygen slugs) the preferred mass driver payload for a propulsion mechanism, since it will sublimate into a gas dispersion (which is only hazardous to moderate fractions of C spacecraft?)

Offline morganism

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Re: Mass drivers for propulsion
« Reply #6 on: 03/09/2018 07:52 PM »
Water is para-magnetic, and needs to be jacketed. You also don't want to throw away the fuel you can sell.

Scientists ignite aluminum water mix

https://m.phys.org/news/2014-07-scientists-ignite-aluminum.html

So if you use the railgun on Luna, you can loft aluminum regolith, jacketed with iron (from regolith also)

Online Coastal Ron

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Re: Mass drivers for propulsion
« Reply #7 on: 03/09/2018 08:13 PM »
Mass drivers are fairly significant enablers for retrieving asteroids, since they can be loaded with asteroid rock for reaction mass.

OK, so you take an asteroid that is relatively easy to see and not run into, and then you crush it up into small bits that are virtually untraceable and spew them out behind your spaceship into an orbit that will likely bisect other spaceships?

Quote
So it'd be interesting to know how well they could trade against more conventional SEP.

1. SEP ships don't make it likely that they will destroy spaceships that follow in their paths.

2. Once crushed, I doubt there would be economical ways to clean up all of the debris from mass driver engines in case they truly are recognized as a danger to navigation in space.

As you can see I'm not a fan of the idea. Maybe I'm wrong, but it just seems like it's a bad idea for general use.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

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