Author Topic: Accelerating a starship with bullets  (Read 11687 times)

Offline goran d

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Accelerating a starship with bullets
« on: 01/29/2016 06:16 pm »
We could build a million killometer gauss gun to accelerate bullets at 10^6 m/s^2 and fire them at a starship.
 They could be magnetic and captured using magnetic field scoop. With s=10^9m and a=10^6, we get speed of bullets 4.5x10^7, which should be enough for the starship to reach 10% light speed.

Offline jabe

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Re: Accelerating a starship with bullets
« Reply #1 on: 01/29/2016 06:27 pm »
going to skip over lots of things why I don't think this will work.. but how will a magnetic field scoop work??  The size of the scoop  and the power needed to do it would be huge!!
jb
BTW I am assuming you mean the bullets will collide totally inelastically with the spaceship?

Offline goran d

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Re: Accelerating a starship with bullets
« Reply #2 on: 01/29/2016 08:08 pm »
yes, of course. There'sno way they could bounce back elastically. They would be vapourised into plasma inside the magnetic scoop. The outer parts of the scoop could be a net out of carbon nanotubes with flowing current.
« Last Edit: 01/29/2016 08:10 pm by goran d »

Offline jabe

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Re: Accelerating a starship with bullets
« Reply #3 on: 01/29/2016 08:20 pm »
So this is a reverse rocket engine equation.  :)
∆v = Vexh*ln(mass_f/mass_I). Ignoring other technical issues how many bullets will it need to "catch"  to reach your required speed?
Jb

Offline Robotbeat

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Re: Accelerating a starship with bullets
« Reply #4 on: 01/30/2016 12:00 am »
The bullets could be very small. They could be accelerated as charged dust particles and neutralized on the way out, then vaporized somehow as you describe. So-called dust accelerators already exist and are used to simulate impacts from interplanetary (or interstellar) dust on spacecraft and stuff.

I've thought of this same idea, and I think something like it is a far more efficient way to accelerate to a significant fraction of c (like 10%) than using lasers.
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Offline Bob Shaw

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Re: Accelerating a starship with bullets
« Reply #5 on: 01/30/2016 12:50 am »
There's a lot to be said for any interstellar propulsion system which keeps the operational gubbins centrally located - not least because one 'engine' can keep multiple vehicles in flight. I worry, however, lest we are falling into the old 'rockets won't work outside the atmosphere because they have nothing to push against' fallacy, which certainly isn't an issue with propulsion schemes which keep an energy source located at home. 'Feeding' an engine with mass, while powering said engine with electromagnetic energy is attractive - but let's not go down the something-for-nothing rabbit hole.

In terms of keeping pre-Singularity biological entities in charge for at least a limited time in inner solar systems, this idea also has serious attraction, though I, for one, would wish to be among the first to welcome our new, and entirely digital, overlords.

Offline The Amazing Catstronaut

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Re: Accelerating a starship with bullets
« Reply #6 on: 01/30/2016 02:07 am »
Why not locate the dust accelerator on the moon? Ample material for the projectiles and no atmospheric losses of note.
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Offline Hanelyp

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Re: Accelerating a starship with bullets
« Reply #7 on: 01/30/2016 02:54 pm »
- A bullet hitting a magnetic field will see part of that as an electric field proportional to velocity.  A high enough electric field would break down even a ceramic bullet.  Still, a spacecraft design where the bullet path passed through a clear opening is enticing.

- Being an inelastic drag mechanism, a simple current loop magnet should work well if the bullet can present enough electric cross section.

- A bullet in the form of a conductive ring could interact strongly with the catcher magnetic field (provided it hits with a reasonable attitude), an induced current vaporising it.

- Jabe: an inelastic collision need not result in the spacecraft collecting the bullet mass.  A more likely scenario is bullet vapor leaving a trail behind the starship that following bullets must pass through.  Alternately, if the bullet vapor doesn't pass all of its momentum it may pass in front of the starship, clearing the way of debris.

- Precision aim is essential.  For longer distance it may require a "smart" bullet.

- Relativistic bullet = heavy weapon.  Maybe even WMD.

Offline dror

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Re: Accelerating a starship with bullets
« Reply #8 on: 01/30/2016 04:03 pm »
What do you do with the bullet after you catch it as plasma?

You can use it as reaction mass for an ion propulsion, thus not growing the ships mass.
Use a few, slightly tilted, ion engines so that the beams won't interfere with the incoming bullets.
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Offline Rocket Science

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Offline Stormbringer

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Re: Accelerating a starship with bullets
« Reply #10 on: 01/31/2016 07:28 am »
does the scoop have to be made of solid matter? couldn't it be a M2P2 type magneto-plasma bubble if the bullets are small enough (e.g; dust particles?) Such a bubble can be kilometers across which would put the catching "surface" a satisfying distance away from the ship's aft. As an added bonus the M2P2 itself is a propulsion system that can propel the ship at the speed of the stellar wind and is also a radiation shield and is not very massive.
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Offline Stormbringer

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Re: Accelerating a starship with bullets
« Reply #11 on: 01/31/2016 07:31 am »
In the case of a magnetic bubble as a bullet catcher the dust/plasma goes to help maintain the bubble plasma which is a little leaky otherwise.
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Offline Hanelyp

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Re: Accelerating a starship with bullets
« Reply #12 on: 01/31/2016 02:51 pm »
What do you do with the bullet after you catch it as plasma?

You can use it as reaction mass for an ion propulsion, thus not growing the ships mass.
Use a few, slightly tilted, ion engines so that the beams won't interfere with the incoming bullets.
I expect catching in the sense of bringing to a relative stop will be a lot easier than collecting so ion engines can then use it.

Offline Vultur

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Re: Accelerating a starship with bullets
« Reply #13 on: 01/31/2016 07:29 pm »
It's physically possible, but I believe we can't yet get the relevant velocities with railguns/coilguns. The articles I've seen on the current Navy railgun work talk about speeds of Mach 7 or so, which isn't that fast in space terms.

Also, there are issues with the railguns being damaged by the firing, IIRC.

Someday, it might be useful, but not near-term.

(And a million-kilometer gauss gun sounds harder than reaching 10% of light speed with advanced fusion propulsion or laser-pushed lightsails.)


Offline Lars-J

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Re: Accelerating a starship with bullets
« Reply #14 on: 02/01/2016 05:54 am »
It's physically possible, but I believe we can't yet get the relevant velocities with railguns/coilguns. The articles I've seen on the current Navy railgun work talk about speeds of Mach 7 or so, which isn't that fast in space terms.

Also, there are issues with the railguns being damaged by the firing, IIRC.

Someday, it might be useful, but not near-term.

(And a million-kilometer gauss gun sounds harder than reaching 10% of light speed with advanced fusion propulsion or laser-pushed lightsails.)

Yep... And suddenly laser propulsion (lightsails) start to sound better and better.

Offline qraal

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Re: Accelerating a starship with bullets
« Reply #15 on: 02/01/2016 07:16 am »
The M2P2 doesn't work. In about 2005 several different researchers pointed out flaws in the plasma physics. The researchers couldn't answer the critics and let the idea die.

In the case of a magnetic bubble as a bullet catcher the dust/plasma goes to help maintain the bubble plasma which is a little leaky otherwise.

Offline qraal

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Re: Accelerating a starship with bullets
« Reply #16 on: 02/01/2016 07:29 am »
Clifford Singer first proposed this concept in a 1979 paper. It's available online, but most of it didn't copy very well and so the scan is unreadable in most places.

Here's the JBIS reference:

“Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107–115.

Followed by the sequel here:

“Questions Concerning Pellet-Stream Propulsion,” JBIS 34 (1981), pp. 117-119.

The bad scan is available here: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/11/515/11515450.pdf

Gerald Nordley has worked on the idea, as Singer has moved on to other concerns, and has a few papers on it. His webpage is well worth a visit:

http://www.gdnordley.com/

With a magnetic mirror, the bullet plasma can be reflected reasonably efficiently, which significantly reduces the mass of bullets required. If the bullet speed is twice the desired final speed of the vehicle, then the total mass of bullets needed is half the vehicle mass - assuming perfect reflection.

If the speed of the bullets is always *twice* the vehicle speed, and they're perfectly reflected, the transfer of energy to the vehicle is 100% efficient. Of course that means a lot of bullets if the initial speed is low (it's actually the vehicle mass times the natural log of the ratio of the initial and final velocities - a kind of reverse rocket equation.) If we start at 30 km/s, then accelerate to 30,000 km/s, the total mass of bullets is LN(1000) ~ 6.9 times the vehicle mass.

We could build a million killometer gauss gun to accelerate bullets at 10^6 m/s^2 and fire them at a starship.
 They could be magnetic and captured using magnetic field scoop. With s=10^9m and a=10^6, we get speed of bullets 4.5x10^7, which should be enough for the starship to reach 10% light speed.

Offline gospacex

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Re: Accelerating a starship with bullets
« Reply #17 on: 02/01/2016 11:03 am »
It's physically possible, but I believe we can't yet get the relevant velocities with railguns/coilguns. The articles I've seen on the current Navy railgun work talk about speeds of Mach 7 or so, which isn't that fast in space terms.

Those railguns are very short, some tens of meters long only.

Offline Nilof

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Re: Accelerating a starship with bullets
« Reply #18 on: 02/01/2016 11:57 am »
I think the idea is more readily applied to transport within the solar system. Reusable reaction mass and 100% energy efficiency would make interplanetary travel a lot easier.
« Last Edit: 02/01/2016 11:57 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 Vultur

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Re: Accelerating a starship with bullets
« Reply #19 on: 02/02/2016 12:13 am »
The M2P2 doesn't work. In about 2005 several different researchers pointed out flaws in the plasma physics. The researchers couldn't answer the critics and let the idea die.

In the case of a magnetic bubble as a bullet catcher the dust/plasma goes to help maintain the bubble plasma which is a little leaky otherwise.

Could you provide a source please? M2P2 sounded interesting but it's hard to find that much info...

And would these problems apply to the magnetosphere aerobraking thing?

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