Steve Jackson Games Forums

Steve Jackson Games Forums (http://forums.sjgames.com/index.php)
-   GURPS (http://forums.sjgames.com/forumdisplay.php?f=13)
-   -   [Blog] n-Body Politics (http://forums.sjgames.com/showthread.php?t=153002)

kreios 11-29-2017 12:13 PM

Re: [Blog] n-Body Politics
 
Quote:

Originally Posted by https://nbodypolitics.gitlab.io/2017/11/29/interface_propulsion.html
Last time, we talked about FTL travel. However, to be able to FTL-travel, we still need to leave that pesky gravity well behind - and at roughly 9.4km/s, itís a doozy. Just looking at todayís rockets, mass fractions and propellant densities make multi-stage rockets necessary. But how does it look like in the future?

Continue reading...

Humabout 11-29-2017 03:41 PM

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.

kreios 12-03-2017 12:00 PM

Re: [Blog] n-Body Politics
 
Quote:

Originally Posted by Humabout (Post 2139504)
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.

A good point. It's just a rough estimate, but the conclusion stays valid in my opinion: Forget launching anything with conventional rockets if you want to remain economical.

Speaking of which: Next post!



Quote:

Originally Posted by Space Propulsion
Now weíre moving to the interesting topic of space propulsion, and which one to use. Since GURPS Spaceships offers a large number of them, Iím only going to look at the best ones - some, like ion drives or chemical rockets, are simply going to lack in either thrust or efficiency to be useful.

Continue reading...

Humabout 12-04-2017 09:58 AM

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.

ericthered 12-04-2017 10:36 AM

Re: [Blog] n-Body Politics
 
Quote:

Why do we care? Economics. It actually 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.
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.

Quote:

Originally Posted by Humabout (Post 2140537)
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.

I don't think the math there follows. You can't burn a rocket at 10% of total thrust, but you can burn a rocket for 10% as long, assuming its not a solid booster and you have reasonable tolerances.

kreios 12-04-2017 12:07 PM

Re: [Blog] n-Body Politics
 
Quote:

Originally Posted by Humabout (Post 2140537)
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.

Quote:

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.


Quote:

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.

Quote:

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.

ericthered 12-04-2017 12:27 PM

Re: [Blog] n-Body Politics
 
Quote:

Originally Posted by kreios (Post 2140573)
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.

I missed that. For some reason I thought you'd be using raw hydrogen to fuel the pulse drives. That certainly makes the torches cheaper than the pulse drives.

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?

kreios 12-04-2017 01:27 PM

Re: [Blog] n-Body Politics
 
Quote:

Originally Posted by ericthered (Post 2140581)
Are you trying eliminate space only craft as an element?

On the contrary - I find it strains my disbelief to see gigantic spacecraft (without superscience, of course) land. For me, it'll probably be shuttles and drop pods. And elevators, of course.

Humabout 12-04-2017 08:11 PM

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.

kreios 12-05-2017 11:43 AM

Re: [Blog] n-Body Politics
 
Quote:

Originally Posted by Humabout (Post 2140664)
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.

Actually, this is wrong. The most efficient transfer is (depending on the ratio between the two orbits) is either a Hohmann or a bi-elliptic transfer. Both assume instant velocity changes, meaning you can never actually achieve them. However, the higher your acceleration, the closer you can get (and therefore need less dV). Assuming you're able to restart engines (bringing us back to that topic, but which means you can actually to those required two burns), a high-thrust engine is always more efficient than a low-thrust one to change orbits.

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.


All times are GMT -6. The time now is 11:07 PM.

Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2019, vBulletin Solutions, Inc.