11-29-2017, 12:13 PM | #11 | |
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Re: [Blog] n-Body Politics
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11-29-2017, 03:41 PM | #12 |
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Re: [Blog] n-Body Politics
A note on your lift-off requirements and acceleration: as you decrease acceleration, the delta-v needed to attain orbit increases, thanks to gravity drag. Thus, blasting off at 3g takes less delta-v worth of fuel than doing it at 1.1g. To that end, if you correct for this, you'll find your assessment of boosters is off. You'll also (correctly) find that SSTO is just a dream for hlox, kerolox, and metholox engines.
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12-03-2017, 12:00 PM | #13 | ||
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Re: [Blog] n-Body Politics
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Speaking of which: Next post! Quote:
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12-04-2017, 09:58 AM | #14 |
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Re: [Blog] n-Body Politics
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.
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. This analysis assumed single stage craft. Beyond Neptune might be achieved more cheaply with staged fusion engines/anpu. If this doesn't square with the FTL drive requirements, ships may need a set of auxiliary high-G engines, or the FTL drive might need tweaking to "just work". As for combat, again, a set of high-g engines might be useful for maneuvering. This does address two issues at once. 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.
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12-04-2017, 10:36 AM | #15 | ||
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Re: [Blog] n-Body Politics
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12-04-2017, 12:07 PM | #16 | ||||
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Re: [Blog] n-Body Politics
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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. Quote:
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12-04-2017, 12:27 PM | #17 | |
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Re: [Blog] n-Body Politics
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It still leaves fusion rockets outperforming the torches, but its a narrower margin (x4 instead of x7). Actually, you probably won't strain credulity any more than the torch already does if you let one engine switch freely between rocket and Torch mode. Though that may just complicate your math more. Are you trying eliminate space only craft as an element?
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12-04-2017, 01:27 PM | #18 |
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Re: [Blog] n-Body Politics
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12-04-2017, 08:11 PM | #19 |
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Re: [Blog] n-Body Politics
The issue, Eric, is that constant acceleration transfers naturally achieve higher delta-vs than impulse transfers. And the higher your acceleration, the more delta-v such a transfer takes. So if you have a 0.1g acceleration drive running nonstop, it eats more delta-v than a 0.01g accel drive between The Same Orbits. Thus, lower accel drives tend to use less fuel, regardless of isp.
Kreios, I never said anything about restartability. I addressed throttle-ability. Modern rocket engines can't operate below about 30% max thrust. They can turn off and fire back up. Heck, JPL is looking at using the auxiliary thruster on Voyager 1 for attitude control after its been mothballed for 40 years. But they can't run it at 10% thrust. The reason that matters is that you can't dial back a 1g drive down to 0.005g to get a lower delta-v transfer that doesnt take over a decade to get to Saturn (Hohhman). This is inherent to the drives and presents an unintuitive challenge when analyzing costs. Ultimately, you want to cost transfers by ($/ton)(ton/dv) or ($/dv). Thus, the more dv, the more money, and then you can balance transit time (crew wages, maintenance costs, etc.) against the cost of fuel to get a ballpark transit time and cost per transit. Spreadsheets are your friend here.
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12-05-2017, 11:43 AM | #20 | |
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Re: [Blog] n-Body Politics
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At the extreme end, we can look at the real-life example of the SMART-1 mission, which expended ~3.9km/s dV from a geostationary transfer orbit to a low lunar orbit. However, an optimal trajectory (such as produced by a high-thrust system) would only need ~1.5km/s dV for such a transfer. Of course, the ion engine was far more efficient in terms of propulsion (since its Isp is more than four times a chemical rocket's) - but if they would've had a high-thrust system with those numbers available, they would've used it. |
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