Steve Jackson Games - Site Navigation
Home General Info Follow Us Search Illuminator Store Forums What's New Other Games Ogre GURPS Munchkin Our Games: Home

Go Back   Steve Jackson Games Forums > Roleplaying > GURPS

Reply
 
Thread Tools Display Modes
Old 05-19-2012, 07:04 AM   #31
thrash
 
thrash's Avatar
 
Join Date: Aug 2004
Location: traveller
Default Re: TL9 Antimatter pion rockets

Quote:
Originally Posted by Anthony View Post
To get a lightsail up to 0.1c requires 4.5e+15 J/kg (of spaceship mass), divided by the efficiency of your photon source; for an antimatter-pion drive, at the quoted Isp of 0.69c, requires 7e+15 J/kg, divided by the efficiency of your antimatter production.
I trust you meant to say exhaust velocity, and the rest of the figures are correct.

Quote:
The efficiency of light sources can plausibly exceed 10% (90% for a klystron, though that forces use of long wavelengths and extremely large focusing elements), even extremely optimistic assumptions on antimatter production are on the order of 0.01%, so we're dealing with 3-4 higher orders of magnitude.
Energy, yes, but what about power? The energy to drive the lightsail has to be delivered continuously at full power and at a single point while the ship is accelerating. The energy required to produce antimatter can be spread over as long a time and as many sources as the storage technology permits and delivered to the ship in packages.

Quote:
In addition, the above AM-pion rocket requires storing 0.078 kg of antimatter per kg of ship mass, which is a storage efficiency we have no reason to think is possible.
This argues for a lower final velocity, but doesn't invalidate the design altogether. What is the effective storage density implied by Spaceships?

Quote:
These are some pretty challenging numbers, but compared to the AM-pion they're trivial.
A primary advantage of the antimatter pion rocket over lightsails is that once launched it is self-contained. Both technologies require a strong social organization to build and maintain the supporting infrastructure. The rocket needs an enormous investment up front, but carries all that investment with it (in the form of reactants) when it departs. The lightsail, however, needs constant attention throughout the acceleration phase: if there is a breakdown in the supporting social network or simply a loss of interest, the project fails.

Moreover, investing in the infrastructure to produce the required quantities of antimatter pays immediate benefits, in the form of less-energetic antimatter reaction drive options. A network of lasers or masers to drive a lightsail is less flexible overall, and again will be tied up throughout the acceleration phase rather than becoming fully available immediately upon departure.

Social engineering requirements may favor an antimatter rocket over a lightsail, even given the differences in efficiency.
thrash is offline   Reply With Quote
Old 05-19-2012, 08:23 AM   #32
dataweaver
 
dataweaver's Avatar
 
Join Date: Aug 2004
Default Re: TL9 Antimatter pion rockets

How do magnetic sails stack up? My understanding is that they'd be less dependent on social engineering, as you put it; there's also the fact that the force that they generate falls off more slowly than it does for lightsails, making them more attractive for outer-system maneuvering. The only downside I know of is that they can't exceed the speed of the solar wind, putting an absolute upper cap on how fast they can travel through interstellar space.

(I'm not quite so fond of plasma sails; while they apparently get better performance characteristics than magnetic sails, the need to replenish the plasma that leaks out pretty much kills them as a "fuelless propulsion system". But maybe I'm wrong?)
dataweaver is offline   Reply With Quote
Old 05-19-2012, 08:35 AM   #33
Fred Brackin
 
Join Date: Aug 2007
Default Re: TL9 Antimatter pion rockets

Quote:
Originally Posted by dataweaver View Post
How do magnetic sails stack up? )
Any time the local charged particles aren't going at least as fast as your ship and in the direction you want, you start losing speed as momentum transfers from your sail to those particles rather than the other way around.

The mag sail generally makes a better brake than an accelerator. Places whre there is a convenient solar wind are about the only places it does make even a little sense.
__________________
Fred Brackin
Fred Brackin is offline   Reply With Quote
Old 05-19-2012, 08:39 AM   #34
dataweaver
 
dataweaver's Avatar
 
Join Date: Aug 2004
Default Re: TL9 Antimatter pion rockets

Right; but don't all stars have "a convenient solar wind" blowing in an outward direction?
dataweaver is offline   Reply With Quote
Old 05-19-2012, 01:11 PM   #35
Anthony
 
Join Date: Feb 2005
Location: Berkeley, CA
Default Re: TL9 Antimatter pion rockets

Quote:
Originally Posted by dataweaver View Post
How do magnetic sails stack up?
It's not really possible to focus a driver beam at the required ranges. If you could, it would be somewhat more efficient, because you can use a driver beam at 0.2c or so. The solar wind is much much too slow to be relevant to this discussion.
Anthony is offline   Reply With Quote
Old 05-19-2012, 05:50 PM   #36
Fred Brackin
 
Join Date: Aug 2007
Default Re: TL9 Antimatter pion rockets

Quote:
Originally Posted by dataweaver View Post
Right; but don't all stars have "a convenient solar wind" blowing in an outward direction?
I do not believe that the solar wind extends past the heliopause. If you don't go beyond the heliopause you certainly won't be doing much in terms of interstellar travel.

I also seem to real that te solar wind slows down as it goes outward so you'd probably stow your mag sail even before you got to the heliopause.

That leaves you accelerating at a slow rate for only a very small period of your trip. Hard to make that look efficient.

I know of soem designs that accelerate by another means that use a mag sail to brake with. It is better at that than acceleraing.
__________________
Fred Brackin
Fred Brackin is offline   Reply With Quote
Old 06-12-2012, 10:45 PM   #37
David L Pulver
AlienAbductee
 
David L Pulver's Avatar
 
Join Date: Aug 2004
Location: In the UFO
Default Re: TL9 Antimatter pion rockets

Quote:
Originally Posted by thrash View Post
http://www.technologyreview.com/blog/arxiv/27847/
http://arxiv.org/abs/1205.2281

Beamed Core Antimatter Propulsion: Engine Design and Optimization
Ronan Keane, Wei-Ming Zhang
This paper seems to suggest that non-superscience antimatter pion rockets are possible at TL9. Would David Pulver or one of the original playtesters be able to comment on whether the Isp would improve as well?

Of course, antimatter production is always the most difficult part of this technology. Even so, this points to the possibility of hard sf interstellar missions much sooner than previously thought.
If the assumptions are correct, then increasing the exhaust velocity as specified would approximately double the Isp (and in Spaceships give a value of about 6600 mps per fuel tank, I guess).

I'd think increasing the exhaust velocity would quite likely reduce the acceleration, though, by at least half and possibly more. (High-impulse antimatter pion rocket?)

On TL, while making antimatter is a big problem, it can be dealt with with appropriate engineering projects (cover mercury with solar collectors, whatever). But the biggest problem is the storage of antimatter.

A pion drive relies on 1-1 ratio mix of matter-antimatter. However, storage facilities for antimatter currently mass a great deal to hold even sub-nanogram levels, and even the future generation versions quoted are things like "mass of a space shuttle tank to hold a few micrograms."

Since half your fuel is antimatter, if your fuel tank ends up weighing vastly more, or even a little more, than your fuel, you are utterly out of luck. You'll have to get the fuel tank mass from a future speculative "tons per microgram"
down to "micrograms per microgram" if you don't want your starship to have the acceleration of a flashlight. (There are possible ways to do this, maybe, like antihydrogen ice storage, but the road to them is probably TL10+).
.
Also, it's not just the storage - you have to find a way of moving the antimatter from whatever tank you have to the engine, which means the engine has to incorporate this technology. The most sensible approach I saw had the antimatter hydrogen stored as solid pellets and the hydrogen liquid.

(Incidentally, while the 3e version was TL9 - but in 3e TL scale. Most 3e technologies specified as TL9 became either TL10 or TL11 in 4e (blasters, for example)).
__________________
Is love like the bittersweet taste of marmalade on burnt toast?
David L Pulver is offline   Reply With Quote
Old 06-13-2012, 12:46 PM   #38
Trachmyr
 
Trachmyr's Avatar
 
Join Date: Nov 2008
Location: Florida
Default Re: TL9 Antimatter pion rockets

@ David Pulver: So when's the next Spaceships book coming out ;-), I got the $$ right here!

Seriously, I would love a Spaceships treatment on Current & Expected Tech in the next few decades, and fiddly numbers on things like Vasmir, M2P2, NERVA/LANTRN... broken down in Experimental through Mature TECH crunchy numbers.

Anyways, got the $$$ right here... just sayin' :)
Trachmyr is offline   Reply With Quote
Old 06-13-2012, 12:48 PM   #39
dataweaver
 
dataweaver's Avatar
 
Join Date: Aug 2004
Default Re: TL9 Antimatter pion rockets

That strikes me as being in the domain of the Vehicle Design System rather than the Spaceships series.
dataweaver is offline   Reply With Quote
Old 06-13-2012, 01:57 PM   #40
Flyndaran
Untagged
 
Join Date: Oct 2004
Location: Forest Grove, Beaverton, Oregon
Default Re: TL9 Antimatter pion rockets

I just can't see why anyone would do it when it would be infinitely cheaper and safer to use a multi-stage fusion drive ship.
__________________
Beware, poor communication skills. No offense intended. If offended, it just means that I failed my writing skill check.
Flyndaran is offline   Reply With Quote
Reply

Tags
antimatter, hard sf, spaceships

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Fnords are Off
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -6. The time now is 01:19 AM.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2024, vBulletin Solutions, Inc.