A Revelation Or Two I Had On Scifi Spaceflight

[Bamax]

ISRU On A Spaceship For Propellant Is For Beginner Space Civilizations


I say this because while worldbuilding a non-overpowered space drive I realized this.

Let's say you have a rocket drive that can expend it's fuel of 90 tons in 70 minutes at 3g max acceleration or longer at lower accelerations.

To get that kind of rocket efficiency you need crazy high exhaust velocities as well as high energy combustion/detonation chamber temperatures higher than rockets we currently use... basically scifi territory unless you count NSWR drives which are radioactive death plume rockets.

If you want to last longer than 70 minutes at 3g you just carry 180 tons of fuel instead and either increase the amount of engine nozzles for higher thrust or otherwise lighten the ship's mass so that it will be similar to what you started with so you do not have to raise thrust to accomodate extra weight.



You do not need to carry ISRU equipment... you just have a mission plan and stick to it so you do not run out of propellant.

Want more wiggle room for scifi day of the week adventures?

Use my other non-overpowered space drive:

Impulse Warp Drive: Warps space ahead of your vessels that it moves in the opposite direction your vessel is currently accelerating with main engines... at the same rate of acceleration.

So what you do is accelerate at max acceleration for a moment and engage impulse warp, then shut off main rocket engines. Impulse warp will continually warp space past you at 3g until you shut it down... allowing you to traverse vast distances without needing a torch drive.

Why is it not overpowered? Well.. the ship's ACTUAL momentum/velocity never changed. If it hit something while at warp it will be just the same as if it was not at warp.. since it's actual velocity and momentum vecotor would all reveal itself on impact.

Because of this using warp can be used to drop into low orbits to increase speed for free, then warp back and do it again and again.

This is the easiest way to make a relativistic kill vehicle but it is time consuming and any ships or probes nearby would notice what you are doing.



Conclusion: ISRU for propellant is best reserved for space stations or space or planet colony bases... as they can be better optimized to do a better job than spaceship can and thus better serve spaceships with propellant.



EDIT: That was a revelation I just had. Even with super efficient high thrust rocket engines spaceship mass to propellant ratios matter, as well as how many rocket nozzles you have.

With a single nozzle you need all the thrust and heat to come out a single nozzle, which may be too much for a single engine to pull off, so making multiple engines with nozzles is optimal.

And adding more or extra propellant to a vessel than what it was designed to carry WILL lower it's maximum thrust unless you increase the engine's maximum efficiency which won't happen unless you reduce the ship's mass since increasing performance is not possible if it is already at the maximum OR increase the number of engine nozzles so you can burn more fuel in less time making it less efficient but more thrusty.

[Demarquis]

I think you just rediscovered the "Rocket Equation" (https://www.planetary.org/articles/2017 ... ion-part-1). See also (https://www.nasa.gov/mission_pages/stat ... yanny.html)

[Mithramuse]

I think you just rediscovered the "Rocket Equation" (https://www.planetary.org/articles/2017 ... ion-part-1). See also (https://www.nasa.gov/mission_pages/stat ... yanny.html)

Alternately, try https://what-if.xkcd.com/21/.

--Mithramuse

[Demarquis]

Much better explanation!