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Old 12-04-2017, 12:07 PM   #16
kreios
 
Join Date: Jul 2013
Default Re: [Blog] n-Body Politics

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Originally Posted by Humabout View Post
Again, an interesting analysis, but a few things to consider when discussing torches. While ISP is a measure of efficiency, it should be noted that rockets generally cannot be throttled down terribly far (iirc, the Merlin has the widest range at present and can be reduced to 30% max thrust). This means that a torch will not only provide constant acceleration (meaning high-dv transfers), but they also have higher thrusts (meaning even more dv consumption). The end result is that torches eat far more fuel than non-torches, but they do get you there faster.
As ericthered said, this is only an issue if we're assuming non-restartable engines. Which often happens in real-life - most rocket engines aren't restartable, and something like the Vinci engine with three or four ignitions is noted to increase Ariane 5's flexibility by quite a bit.

However, if our engine is restartable, a high-thrust engine is superior to a same-ISP low-thrust engine, since you can simply spend your dV over a shorter period of time. This also allows you to follow several trajectories which you otherwise couldn't follow (Hohmann orbits generally assume instant acceleration) and greatly simplify your orbital math.

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Why do we care? Economics. It actualy turns out that, at TL 11 (because that's the TL I did the analysis at a while back), the most economical engine for anything outside of cislunar space is a fusion torch, with advanced nuclear pulse drive a close runner up (im doing this from memory; those might be reversed. I'll see if i still have the spreadsheet for this). Inside cislunar space, an ion drive is cheapest. And beyond Neptune, you have to use a pion drive and it's going to wreck your budget.
Not unlikely. While the fusion pellets cost 25x as much as hydrogen, they also provide almost 8x as much dV, which means you should be cheaper at about... Interestingly, my back-of-the-envelope calculations give me 3000km/s dV needed (assuming no cost due to increased volume and mass), which is probably quite wrong.


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Also, recall that all of the above engines, excepting the ion drive, are probably too hazardous for populated space. This would indicate the presence of tugs that tow inyerplanetary ships into and out of port, so they can fire their engines safely. These tugs would likely use either high-g HEdM rockets or ion engines (again, cost).

The alternative to tugs are ion-engine-shuttles that transfer passengers and cargo from inhabited space to parking orbits further out where its safe to fire up a the kind of radioactive nightmare that is a ANPU drive or fusion drive.
The way it seems right now, one of the big lies in the setting will be a non-radiation-spewing fusion torch. I need my cheap spacelift.

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I've got my spreadsheet handy: The best ISP is the advanced Fusion pulse drive. Its got better ISP than the torch at all TL's. The non-advanced fusion pulse drive is slightly under the torch. Of course, the torch has 10 times the power of the non-advanced and 100 times the power of the advanced drives. Freighters probably use the advanced Fusion Pulse drives.
Most importantly, though, the fusion torch's propellant is also only $2000 (or $20 with water) per ton, compared to the $50,000 per ton for the pulse drives. That will probably make it more economical for almost all trade - if you can assume waystations, it will be more economical.
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