Re: Coolant [Spaceships]
 

Found a site with worked numbers. https://space.stackexchange.com/ques...-get-you-there

These are all from earth

The Moon / Luna:
Closest to Earth (Supermoon): 356,577 km
Travel time (at 9.80665 m/s2, no deceleration): 2h 22m 12s
Travel time (at 9.80665 m/s2, decelerating halfway): 3h 20m 24s

So turnover at 1h 40m gives you 42m to stop it. Should be doable if it isn't a warship with armor and point defenses.

Mars:
Closest to Earth: 65 million km
Travel time (at 9.80665 m/s2, no deceleration): 1d 7h 58m 5s
Travel time (at 9.80665 m/s2, decelerating halfway): 1d 21h 13m 1s
Turnover at 23h, gives about 9 h to stop it.

That all sounds like you can with procedures and pre planned interceptors deal with it.

Lots of possibility for complacency, budget cutting and such to make it a PC problem that they have to make do.

Re: Coolant [Spaceships]
 

What about the case where the rogue ship doesnt degenerate?

Re: Coolant [Spaceships]
 

Thing is, you can't 'stop' it - you have to either deflect it or blow it into bits too small to harm whatever they're going to hit. This what what I mean about 'draconian penalties', and 'being thorough' - you need to make the call and then act swiftly, and the correct action is to blow the ship and everyone in it into teeny tiny pieces.

Re: Coolant [Spaceships]
 

That's an important part of the rocket equation, and especially the rocket equation's tyranny. The tyranny comes from the fact that both momentum and energy are conserved, but energy increases with the square of velocity, while momentum increases linearly.

the coolant drops the energy part of the equation. Its odd, but I can see settings where it is the right option.

Re: Coolant [Spaceships]
 

Earth moves a lot in the ~58 minutes between the two scenarios, meaning you'd need to have a different trajectory between the two. When the computers plug your trajectory into the appropriate calculations, they're going to spit out that the only way you can reach Earth is if you do so at a rather unsafe speed - in other words, they will detect that you're using your vessel as a weapon roughly as soon as you launch. If you start with a "safe" trajectory, you'll have to use extra time (and delta-v) to put yourself onto a hostile trajectory in order to impact the target, and they'll detect the problem as soon as you start to deviate from the safe trajectory. So, they'll have a lot of time to stop you. If they can stop you before you complete your acceleration, whatever "stop" method they use will mean you miss your target entirely (your remains will be moving too slowly to intercept). If you've completed your acceleration, simply blowing you up will still leave the target to deal with your expanding debris cloud; even a planet with sufficient atmosphere to tank said cloud would have the problem of having satellites get destroyed (which could result in a cascade). With enough time before impact, they can probably nudge you off course (then destroy you if you have the means of getting back on course) using lasers or the like to make certain you miss the target entirely.

"You need to find a way to stop the attack before the authorities do, or their methods are going to vaporize the (VIP/MacGuffin/PC's) on board" can also work.

Re: Coolant [Spaceships]
 

...I don't know what you think the "tyranny of the rocket equation" is, but I reiterate that the consumable coolant literally results in the rocket equation being followed and the OP explicitly observed that fact.

The reactionless+coolant causes conservation laws to be breached, certainly, but doesn't actually give the ships different performance from a rocket. (Potentially a very good rocket, but nonetheless.)

Also, kinetic energy increasing as the square of velocity doesn't actually pose any problem for rockets, just for people getting confused looking at them. You can derive the rocket equation without making any reference to conservation of energy.

Re: Coolant [Spaceships]
 

In general, if you're using reactionless drives, you're throwing realism out the window, and they also don't actually do a great job of emulating most fictional genres (there are edge cases like The Forever War), so I find it simplest to just remove reactionless drives, and replace with techs that do a better job. The usual candidates are:

• Ether Drive: the drive works by pushing on the ether, or something similar. This means it has airplane-like performance -- deceleration and turning are relatively easy, but acceleration drops sharply as velocity increases (power requirements are equal to the dot product of force and velocity). Obviously this is throwing relativity out the window.
• Planetary Drive: similar to an ether drive, but instead of pushing on the ether, it pushes on nearby large masses. This adds an additional limit on acceleration, as it is also reduced if there are no nearby large masses (a reasonable cap is some multiple of local gravity).
• Sublight Warp Drive: it's just the setting's FTL drive, tuned down to be slower than light. There are reasonable energy arguments for FTL drives having a max speed that is inversely proportional to local gravity. This has the useful side benefit that speed is much lower in the areas that are most interesting to be in -- a drive that can get anywhere on Earth in an hour can get just about anywhere in the solar system in eight hours (oddly, Mercury is the hardest to get to).

Re: Coolant [Spaceships]
 

The simplest way to prevent a fast ship from being a WMD is to remove it's kinetic energy entirely. Pull out the psuedo-velocity design switch. This allows you to get from place to place without long periods of down time, but you can't wind a ship up to relativistic speed and wipe out cities.

In my own in-development setting, I'm toying with the idea that the superscience behind reactionless drives is very sensitive to relativistic effects, and their performance drops off drastically as speed increases, before just not working at all at some point. I haven't put anything down on paper (divide by some power of the Lorentz factor, mayhaps?) yet.

But, as people have pointed out, there's no real way around the fact that something SM +6 or bigger at orbital velocity is already carrying enough energy to do some real damage.

Re: Coolant [Spaceships]
 

It's understandable but linguistically funny to talk about being "sensitive to relativistic effects" in a way that is clearly discarding the lack of a privileged reference frame that is the nominal bedrock of relativity.

Re: Coolant [Spaceships]
 

Umm. Speed in what frame of reference? If you are about to discard the Principle of Relativity, it's best not to draw attention to the Theory of Relativity right at the point where you do it.

My earnest advice is that an inertialess, reactionless, pseudovelocity, or FTL drive cannot be made convincing, and the more you try the more you draw attention to the exact spot that you want players to look away from. Get over heavy ground as light as you can. Tell players the operational characteristic of your superscience as briefly and baldly as possible, flush the pseudoscientific rationalisation, and pass quickly to some other point of interest, such as technoninjas, pirate catgirls, and the Neo-Confucian Space Empire. Players don't worry about physics problems while their techo-youxia are flirting with the guards on the air-gangway of a pirate-catgirl space-cruiser.