HEPlaR (once more with a feeling)

[TJP]

I've been away from these boards for some time, so apologies if you've discussed this to death already, but I did try a quick forum search and it didn't turn up the answer, so here goes.

Back in the day, I remember the HEPlaR thrusters of TNE getting a lot of flak, because they were ridiculously efficient. I did a few back of an envelope calculations and got results that give a stupendously effective rocket for sure, but not one that is completely beyond plausibility given high-tech shenanigans and all that. Maybe I did my math wrong (not a rocket scientist, you see). Hopefully someone here could take a look at my math and tell me where I went wrong.

According to FF&S the HEPlaR thruster produces 20 metric tons of thrust per MW and consumes 0.25 cubic meters of liquid hydrogen per MW. By my calculations, it should have an ISP of approx. 4,065,041 seconds and thus an exhaust velocity of circa 39,878 km/sec. These are *extremely* good values, but not completely out of the question in the realm of science fiction. Or am I misunderstanding this? At least the exhaust is not faster than light, which was one complaint I remember from way back when.

I calculated the ISP of the HEPlaR as follows: Thrust (in Newtons) = g * ISP * Mass Flow Rate (in kg/s)
So, plugging in the values known, we get: 20,000 kg * 9.81 m/s2 = 9.81 m/s2 * ISP * 0.00492 kg/s --> 196,200 = ISP * 0.0482652 --> ISP = 4,065,041 sec

So, did I do my math wrong (wouldn't surprise me) or is the ISP (above) correct?

 

[Timerover51]

I've been away from these boards for some time, so apologies if you've discussed this to death already, but I did try a quick forum search and it didn't turn up the answer, so here goes.

Back in the day, I remember the HEPlaR thrusters of TNE getting a lot of flak, because they were ridiculously efficient. I did a few back of an envelope calculations and got results that give a stupendously effective rocket for sure, but not one that is completely beyond plausibility given high-tech shenanigans and all that. Maybe I did my math wrong (not a rocket scientist, you see). Hopefully someone here could take a look at my math and tell me where I went wrong.

According to FF&S the HEPlaR thruster produces 20 metric tons of thrust per MW and consumes 0.25 cubic meters of liquid hydrogen per MW. By my calculations, it should have an ISP of approx. 4,065,041 seconds and thus an exhaust velocity of circa 39,878 km/sec. These are *extremely* good values, but not completely out of the question in the realm of science fiction. Or am I misunderstanding this? At least the exhaust is not faster than light, which was one complaint I remember from way back when.

I calculated the ISP of the HEPlaR as follows: Thrust (in Newtons) = g * ISP * Mass Flow Rate (in kg/s)
So, plugging in the values known, we get: 20,000 kg * 9.81 m/s2 = 9.81 m/s2 * ISP * 0.00492 kg/s --> 196,200 = ISP * 0.0482652 --> ISP = 4,065,041 sec

So, did I do my math wrong (wouldn't surprise me) or is the ISP (above) correct?

I would have to get out my book Thrust Into Space by Maxwll Hunter (which looks like it was re-issued a few years ago), but that specific impulse sounds about right for the higher end of possibility in 1966.

 

[TJP]

Okay, that's good. I doubt the higher end of possibility has changed that much from 1966, really. Besides, I was just trying to see if it was at least somewhat plausible or if it was completely "handwavium". Don't know why there was so much crying about the HEPlaR back in the day, maybe people just didn't want reaction drives (rockets) in OTU, or something. However, I kind of like the HEPlaR and was thinking of using it in my next Traveller campaign for which I'm wanting a bit grittier feel. Not trying to be any harder sci-fi as such, well maybe a bit, but really just wanting a different feel from the clean and easy reactionless drives of not-TNE OTU.

 

[mike wightman]

I like HEPlaR too, I particularly like having to balance your fuel use between maneuver and jump.
The one change I make is to go back to the CT standard of 10% of hull volume per jump number.