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Science types pleas Help.

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Wed, 2010-09-15 13:02

OneTrikPony OneTrikPony's picture

Science types pleas Help.

I'm writing a story about Fresh Kills, the scum statin at EML5 and I've run into an interesting technical problem. Presumably stations get lots of traffic from ships with Fusion Rocket drives. My understanding of the EP tech is that these are He3-D fusion reactors with hydrogen propellant. doesn't this engine produce--in technical terms--"A shitload" of gamma radiation? So every ship arriving and departing is pointing a really big gamma ray gun at the station? I can think of several solutions to this problem but I can't analyse them because I don't know how to figgure out how much gamma an accelerating ship produces. If anyone could put me on the right track I'd be very gratefull. This is what I understand so far. In EP He3-D rockets have an Isp of 100,000 (seconds?) this gives a Ve of 981,000m/s the thrust of the ship is .49m/s squared which you can convert to Newton/seconds by deviding the mass of the ship (kg) by .49 about this point I'm unsure but; Newton/sec = Joules = watts ? So obviously at this point I've screwed up somewhere because all of the energy of the reaction has become thrust and none of it has become gamma so I'm stuck. What I need to figgure out is how many joules of gamma are produced so I can figgure out how far they have to tug the ships before they start their engines or how thick the tungsten shielding on the station has to be ect. I've tried to find the answer on this page http://en.wikipedia.org/wiki/Nuclear_fusion But I can't make sence of it and shortly after I read words like "Bremsstrahlung" I find my mind has wandered off to go look at porn :D If anyone could start me on the right path I'd be very thankful.

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Mea Culpa: My mode of speech can make others feel uninvited to argue or participate. This is the EXACT opposite of what I intend when I post.

Wed, 2010-09-15 20:56

Arenamontanus Arenamontanus's picture

Re: Science types pleas Help.

Isp gives how much your momentum changes per unit of propellant. What we want to find out is the energy output of the reactor. The energy output is equal to P=0.5m'v^2 where m' is the mass ejected per second and v its velocity relative to the ship. v= 981,000m/s. m' can be found from that the thrust F = m'*v and Newton says that F=ma (where m is the spaceship mass). m'=ma/v. So if a = 0.49 m/s^2 and we assume a 1 million kilogram spaceship the mass flow is 0.499 kg/s. So the energy output is around 240 gigawatts! Not all of that is gammas, of course. Most will be heat (= fast moving protons and neutrons in the exhaust, also bad news). But suppose only 0.1% is gammas, and you still get 240 megawatts of gamma. At a distance d you will experience about P/ 4 pi k d^2 Watt per square meter, where k is a factor denoting how directional the exhaust is (k is likely between 1/8 and something near zero, let's use 1/10). So at a distance of 1 km the intensity is 190 W/m^2 - not quite sunlight strength, but enough to heat up lead shields a few degrees per second (and likely lethal within a few seconds). At 10 km the intensity is 1.8 W/m^2 - unhealthy, but likely easy to shield with the normal habitat shielding. Note that antimatter rockets are much worse: a perfect rocket will radiate *only* gammas. In practice they use the AM to accelerate hydrogen to extreme velocity, turning the whole exhaust into pretty much a proton beam.

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Thu, 2010-09-16 00:59

kylleran

Re: Science types pleas Help.

One thing to keep in mind regarding spacecraft docking with habitats is that they are probably using only maneuvering rockets close in to the stations and not using their main drives except once they are well away or for a flip around and final burn far out for deceleration.

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Brian Cross
Posthuman Studios

Thu, 2010-09-16 04:49

OneTrikPony OneTrikPony's picture

Re: Science types pleas Help.

Thankyou thankyou ! :D this helps alot. Very cool dude, I'd give you r-rep ++ but I don't have any. so I'm thinking the directionality of the gamma component of the exhaust is closer to 0 since the gamma will shine everywhere the ship's shadow shield isn't. And I just need to study the theory's of helium fusion a little more to get a closer idea of what percentage of the energy will end up as gamma. I've found some reference to the number 5.5MeV I know now that as a rule smaller, lighter ships will be able to light-off closer to the habitat than heavier ones. Regardless, there's quite a bit of distance required between docking and starting your orbit transfer burn. Since Gamma radiation isn't a natural condition of space habitats don't usually deploy a lot of dense material shielding. Also this makes it seem likely that "Tugs" will be used. Given equal cost of rocket fuel to tugs and freighters the freighter would probably rather pay the tug to manuver around the dock than pay to push a mass of rocket fuel across to another planet. Seems that my Tug pilot/vac worker/smuggler character is going to work! :D

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Mea Culpa: My mode of speech can make others feel uninvited to argue or participate. This is the EXACT opposite of what I intend when I post.

Thu, 2010-09-16 05:44

Arenamontanus Arenamontanus's picture

Re: Science types pleas Help.

It is tricky to make the gamma perfectly directed, since a lot of it is going to be bouncing around in the exhaust. But a k=1/100 factor doesn't sound at all extreme.

OneTrikPony wrote:
And I just need to study the theory's of helium fusion a little more to get a closer idea of what percentage of the energy will end up as gamma. I've found some reference to the number 5.5MeV

That is the energy of the gammas. The exact percentage is likely very hard to calculate properly,since it depends on how much gamma gets absorbed by the propellant, re-radiation and lots of things. http://www.projectrho.com/rocket/rocket3c2.html has a formula for engine temperature that suggests that the engine has be to about 1-10 million K. This allows us to calculate the blackbody spectrum. The peak of the energy is in the hard UV range - it is bad to even look at the exhaust. Looking at the percentiles http://en.wikipedia.org/wiki/Planck%27s_law we still get a very small fraction as pure gammas, less than 0.01% - good news, but can easily be eaten by a low k. Low k however implies that you know pretty well what you will hit when you burn, so moving near habitats become a bit safer.

Quote:
I know now that as a rule smaller, lighter ships will be able to light-off closer to the habitat than heavier ones. Regardless, there's quite a bit of distance required between docking and starting your orbit transfer burn. Since Gamma radiation isn't a natural condition of space habitats don't usually deploy a lot of dense material shielding.

In the inner system and near gas giant radiation belts you will have a lot of shielding to deal with the solar wind, CMEs and cosmic rays. Gammas are actually among the most benign, since particles like protons and alphas do much more damage to tissue. Still, while particles can be stopped with enough low atomic-mass stuff like a few centimeters of plastic or water, gammas require a few cm of heavy metal (halving thickness of lead is 1 cm, for steel 2.55 cm)

Quote:
Also this makes it seem likely that "Tugs" will be used. Given equal cost of rocket fuel to tugs and freighters the freighter would probably rather pay the tug to manuver around the dock than pay to push a mass of rocket fuel across to another planet. Seems that my Tug pilot/vac worker/smuggler character is going to work! :D

Yup. Tugs also have the benefit of knowing the lay of the land.

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Thu, 2010-09-16 08:00

GJD

Re: Science types pleas Help.

Can you use magnetics to shield against or deflect the high energy particles (it's all done with magnets)? Another thing to consider is that a good pilot will want to use the drives as little as possible, choosing orbits and trajectories that don't require long, re-mas expensive burns. It's a balance between economy and speed - ideally you can slot your ship into an orbit that requires you to just give it a nudge now and then. I like the tug idea "Marcus Garvey, she be a tug, mon." G.

Thu, 2010-09-16 08:08

Arenamontanus Arenamontanus's picture

Re: Science types pleas Help.

GJD wrote:
Can you use magnetics to shield against or deflect the high energy particles (it's all done with magnets)?

Given the coronal shields in Sunwards, I guess so. In practice it requires a pretty hefty magnetic field, but that is probably OK for a habitat to generate for protection. Shunting exhaust particles away from the hab leaves the gammas. My guess is that the standard navigational rule is to brake using a zig-zag path that avoids pointing the exhaust straight at the target. Lots of traffic control issues when many ships are moving around: they must all avoid each other's directions.

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Thu, 2010-09-16 09:01

OneTrikPony OneTrikPony's picture

Re: Science types pleas Help.

Alpha particles and neutrons aren't really that big a concern. Neither are x-rays. As far as I understand you can use fairly light materials to shield yourself from particles. Hydrogen, water, stuff you're going to have alot of on your station anyhow. X-ray shielding can be accomplished by the metalic foam you built your hull out of. I think it's the gamma that you really have to be concerned about because it needs Dence material like lead or tungstin or posably some refractory metal of the future. Gamma shielding is the most expencive in terms of mass and manufacturing. Distance is the easiest way to protect yourself from gamma but attaining that distance uses up time and chemical fuel. (I wish someone would come up with an estimate of how much a Kilo of metalic hydrogen costs.)

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Mea Culpa: My mode of speech can make others feel uninvited to argue or participate. This is the EXACT opposite of what I intend when I post.

Thu, 2010-09-16 09:07

GJD

Re: Science types pleas Help.

Have a look at the ASTEN design document. It talks about using polyeurathane sheeting impregnated with exotic salts for radiation shielding (plus it's a nice hab design anyway) http://www.nss.org/settlement/nasa/Contest/Results/2009/ASTEN.pdf G.

Thu, 2010-09-16 09:47

#10

root

Re: Science types pleas Help.

root@Science types please Help [hr]

OneTrikPony wrote:
Very cool dude, I'd give you r-rep ++ but I don't have any.

You don't need to have any particular reputation to gift it. If we needed a standing with a reputation faction in order to hand it out, who hands out the first point of reputation? You get the chicken and the egg problem (in Spaaaaaace!).

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[ @-rep +1 | c-rep +1 | g-rep +1 | r-rep +1 ]

Thu, 2010-09-16 10:25

#11

Arenamontanus Arenamontanus's picture

Re: Science types pleas Help.

GJD wrote:
Have a look at the ASTEN design document. It talks about using polyeurathane sheeting impregnated with exotic salts for radiation shielding (plus it's a nice hab design anyway) http://www.nss.org/settlement/nasa/Contest/Results/2009/ASTEN.pdf

This kind of habitat also looks like a perfect sketch for what a scum barge originally looked like. Now the whole structure has been overgrown with additions, clunky extra modules, balloon bubbles and nanowebs. The old, neat grid remains as a skeleton filled with holes where hab modules have been removed or replaced with conduits linking to new docking systems.

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Thu, 2010-09-16 11:08

#12

GJD

Re: Science types pleas Help.

Indeed. I was most impressed that the design was done by a high school student! G.