1 G acceleration engine ISP and Thrust

[TeeGee]

Hi everyone,

Quick inquiry about the Nuke engine... what ISP and thrust would it need to have to maintain a 1 G thrust to and from DUNA while pushing 15-25 tons? 

The reason I am asking is because I wanted to start tweaking the nukes cfg file to simulate what interplanetary travel would be like if we had access to a 1 G acceleration engine. How long it would take to and from our duna destination etc.

Thanks for your contribution!

[Dman979]

It would need ~200 kN of thrust to maintain a ~10 m/s^2 acceleration. If I understand your question correctly, you want it to have 10 tons of fuel to draw from. It would need ridiculous Isp to maintain 1G acceleration for the entire Duna trip, and I can't do that math. I feel like the length of time you would be firing it would require calculus to solve.

[Guest]

You'd need a TWR on Kerbin (or, basically, thrust to mass ratio) of 1 of achieve 1G of acceleration.  So, for, say, twenty tons, we'd want an engine with a thrust of (20,000kg * 9.807) / 1000 = 196kN.  Now remember, that's not your final figure as you also have to include fuel, which depends on your isp.  If you postulate a 1G interstellar craft it's probably going to have an exhaust velocity somewhere near lightspeed, maybe figure 100,000,000m/s, which comes to about 10,193,680s?  Light speed is  299,792,458m/s, for reference.

As for how much fuel you would need?  Couldn't tell you, but you'll want continuous operation for the entire trip, and that should be fairly easy to calculate.

Edited January 21, 2016 by regex

[Gaarst]

Did some very quick caculations for getting to Duna:

• How quick ? About 11 hours minimum (at Duna's closest to Kerbin, 24h at its furthest)  if you travel in a straight line: considering the travel time, it's a reasonable approximation.
• What Isp ? About 80,000 s (or 784,800 m/s effective exhaust velocity) with constant thrust and mass consumption, if you fire your engines with 200 kN for 11 hours and have 10t fuel to burn.

Of course, you can do some overcomplicated calculus with everything changing at the same time (acceleration, speed, mass, orbit...) to get exact results, but these figures should be rather close to the actual numbers (I rounded them up a bit).

Oh, and this doesn't include the braking when you get to Duna: you are just going to get past it at ludicrous speed with these figures (about 370 km/s).

Edited January 21, 2016 by Gaarst

[Guest]

1 minute ago, Gaarst said:

What Isp ? About 80000 s with constant thrust and mass consumption, if you fire your engines with 200 kN for 11 hours and have 10t fuel to burn.

Gaaaaah, interplanetary, not interstellar.  'scuse my earlier post.

[TeeGee]

30 minutes ago, Gaarst said:

Did some very quick caculations for getting to Duna:

• How quick ? About 11 hours minimum (at Duna's closest to Kerbin, 24h at its furthest)  if you travel in a straight line: considering the travel time, it's a reasonable approximation.
• What Isp ? About 80,000 s (or 784,800 m/s effective exhaust velocity) with constant thrust and mass consumption, if you fire your engines with 200 kN for 11 hours and have 10t fuel to burn.

Of course, you can do some overcomplicated calculus with everything changing at the same time (acceleration, speed, mass, orbit...) to get exact results, but these figures should be rather close to the actual numbers (I rounded them up a bit).

Oh, and this doesn't include the braking when you get to Duna: you are just going to get past it at ludicrous speed with these figures (about 370 km/s).

AWESOME! Actually I was hoping to do a 9.8m/s/s constant accel to about halfway to Duna then go retrograde and burn 9.8 m/s/s until I reach orbit. 

[Gaarst]

31 minutes ago, TeeGee said:

AWESOME! Actually I was hoping to do a 9.8m/s/s constant accel to about halfway to Duna then go retrograde and burn 9.8 m/s/s until I reach orbit. 

Then you take the time needed to get halfway, and multiply it by 2: you spend half the trip accelerating, and the other half decelerating at the same rate.

After a bit of maths, you find that the trip would be √2 longer than in the first case (only accelerating).

• Time taken is then 15h30 to get there minimum, 34h maximum
• And Isp needed is 110 ks (1.1 Mm/s) minimum, 250 ks (2.5 Mm/s) maximum, again for 10t of fuel to burn and 200kN of thrust

Edit: you can apply the same reasoning to further destinations in KSP without worrying about relativistic effects and c as KSP doesn't simulate it. Also, if you would like to see the math behind these numbers so that you can replicate these results for other contexts, feel free to ask.

Edited January 22, 2016 by Gaarst

[TeeGee]

12 minutes ago, Gaarst said:

Then you take the time needed to get halfway, and multiply it by 2: you spend half the trip accelerating, and the other half decelerating at the same rate.

After a bit of maths, you find that the trip would be √2 longer than in the first case (only accelerating).

• Time taken is then 15h30 to get there minimum, 34h maximum
• And Isp needed is 110 ks (1.1 Mm/s) minimum, 250 ks (2.5 Mm/s) maximum, again for 10t of fuel to burn and 200kN of thrust

Wow... 

[Temstar]

1G acceleration for 34 hours is a honest-to-Heinlein touchship, we're talking a ship with 120km/s of delta-V in its tank.

1 hour ago, Gaarst said:

Isp needed is 110 ks (1.1 Mm/s) minimum, 250 ks (2.5 Mm/s) maximum

That exhaust is a weapon of mass destruction.

Edited January 22, 2016 by Temstar

[NathanKell]

@Temstar No, using an engine to touch someone is Niven, not Heinlein.

(Though you did go there after the quote, so...maybe it was a pun after all?)

[Guest]

10 minutes ago, NathanKell said:

@Temstar No, using an engine to touch someone is Niven, not Heinlein.

(Though you did go there after the quote, so...maybe it was a pun after all?)

I do believe Temstar correctly invoked both authors, including the Kzinti Lesson.

[Temstar]

12 minutes ago, NathanKell said:

@Temstar No, using an engine to touch someone is Niven, not Heinlein.

(Though you did go there after the quote, so...maybe it was a pun after all?)

Nah Heinlein came up with the name:
https://en.wikipedia.org/wiki/Torchship

The touch here doesn't refer to using the engine as a weapon (although it's certainly beefy enough to be used as a weapon) but rather to the fact that the ship has an engine that's unreasonably powerful:


In fact in most of his books the spaceship crew are divided into two types: torchers who fly torchships and rocketeers who fly rockets (chemical or NTR). The torchships never land on planets and rely on rockets to transfer people and cargo between the torchship and the planet. Landing and taking off from a planet's surface as the above picture depicts will cause environmental catastrophe.

[NathanKell]

@Temstar Yes. But you said touchship not torchship. Hence my post, and my wondering if you were intentionally punning.