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Let's say I have a space ship which uses a bussard ramjet to achieve relativistic speeds. Something catastrophic happens and it breaks beyond repair.

I have read "Tau Zero" by Paul Anderson (years ago) and I think that it just kept collecting more and more fuel and accelerating more and more. However, in my scenario everything about the propulsion method is broken: the particle collection mechanism and the reactor itself.

So what happens to the ship? There is no friction in space, right? So does the ship just continue at whatever velocity & trajectory it happened to be at when the catastrophe occurred? Does it eventually slow down?

What would it take to render such a ship "dead in the water"?

Thanks.

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    $\begingroup$ For all practical purposes, it will fly forever. The space is not empty, though. Although there is very little matter in interstellar space, there is some - the hydrogen you collect using the bussard ramjet, for example. If it can not sustain the velocity using th ramjet, collisions with hydrogen atoms would slow it down eventually. If it would hit some nebula, the drag could be increased a little, but it is probably still a very small amount. Then there are more dense objects - planets, stars. If the ship could not maneuver, it could hit some of the natural obstacles to stop rather abruptly. $\endgroup$
    – p4ulie
    Commented Feb 2 at 13:45
  • $\begingroup$ I should note that a Bussard Ramjet can only be used to slow down, trying to use it to speed up doesn't work: m.youtube.com/watch?v=MCMiguu_WQI So if you care about realism you should just use a regular fusion engine that doesn't have the same inherent problems. $\endgroup$
    – Vakus Drake
    Commented Feb 3 at 0:47

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After a very, very, very long time, it will stop.

Bussard ramjets operate on the principle of collecting huge amounts of interplanetary/interstellar gas using magnetic scoops and fusing it all together. Normally, this adds a top speed because of the drag on the engine produced by this gas, but then again, it’s not the ship that produces the drag - it’s the huge magnetic scoop that’s drawing in the fuel.

If this magnetic scoop is shut off, then the effective area of the ship goes way down. Eventually, yes, the ship will begin to slow down, but interstellar gas is practically a vacuum and Bussard ships move extremely fast.

Also consider that, on most Bussard ships, the engine is the power source, so your crew/probes are without power. If your probes don’t have another source of power, they will die, and if your crew can’t find another way to turn on the life support… they will die long before the ship appreciably slows down.

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    $\begingroup$ It would still take a long time, but if the magnetic scoop is still running and is large enough I can see a situation in which the ship could start to slow down appreciably before the crew/probes die. $\endgroup$
    – controlgroup
    Commented Feb 2 at 14:13
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    $\begingroup$ Without the scoop, the ship is also directly exposed to the interstellar medium, which at relativistic speeds will then be impacting the ship as ionizing particle radiation. This barrage of relativistic particles will kill the crew and fry any complex technology long before it slows the spacecraft. $\endgroup$
    – Christopher James Huff
    Commented Feb 2 at 15:09
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    $\begingroup$ Also true! The crew is not surviving this in any scenario it seems. Better keep that engine online. $\endgroup$
    – controlgroup
    Commented Feb 2 at 15:12
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    $\begingroup$ @trubliphone: The magnetic scoop takes a lot of electrical power. The ship won't be able to continue running it for very long without power from the engine. $\endgroup$
    – John Dallman
    Commented Feb 2 at 18:02
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    $\begingroup$ Perhaps the magnetic field is generated by superconductors. Once the current in the superconductors is activated, the current won’t decrease until the superconductors heat up (even in the absence of a power source). Granted, this will take some time and will eventually happen, but it’s a plausible reason why the magnetic field might not shut off immediately. $\endgroup$
    – controlgroup
    Commented Feb 2 at 18:27
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A Bussard ramjet would probably not work because of the resistance of the scoop. The scoop collects the few atoms that there are in the space ahead of it, which means it will experience a backwards force from those atoms. The ship then has to convert those atoms into a forwards force, using the atoms both as fuel and as a reaction mass. Without some fusion magic to do this efficiently, the net effect is going to slow you down. The outer parts of the scoop will not necessarily require any power to keep working. Maybe only the inner parts that concentrate the atoms to the densities needed by the motor are the only parts that are broken.

As you approach a star system, you will find more atoms. Eventually, you will have the stellar wind against you. If your scoop can stand the extra stresses, it will act as a brake.

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