New Mars Forums

6 km/s over 18 ft. implies 340,000 g's. (!) I've heard that when you drive a really high current through whatever is pushing your payload, it can turn into a plasma. This isn't a problem for condunctance, since plasma conducts well, but I don't know how well the plasma can push the payload...?

Sorry, that IS seconds^2 (at least I got it right the second time! ) . You don't have to find the the Gravitational Constant, the number is given to you. So all you have to do is plug all the numbers (G, M, and r) into the equation for v. You don't substitute anything for the units kg, m, or s. Those are just units, and as you can see in the equation. The m^3/m cancels to m^2, the kg/kg cancels to 1, and the 1/s^2 stays as it is. That leaves m^2/s^2 under the square root, so when you take the square root, you are left with m/s, or meters/second, which is the units of velocity.

When it comes to Mars, I think that when we're at the colonization stage that we shouldn't want fast travel times. This doesn't mean we don't want advanced propulsion. I think we should be maximizing the number of people that can go instead of maximizing the speed. (this assumes that whatever advanced propulsion tech. has a high enough thrust).

I agree, though, that we need to go much faster than what chemical propulsion offers today when it comes to the outer planets.

Interstingly enough, there's a rocket that attempts to address a few of the issues in your (Stanley's) post. I think the design is called AIM-star. It's an antimatter initiated fusion rocket, and it uses a storage ring to store the antimatter (so it's in a plasma state, and I would think they need to put positrons in there too, but maybe it's not necessary?). The ring uses magnetic confinement. The heat removal system is interesting. What I've seen for systems like NTR is several 'fins' surrounding the engine that radiate the heat away. In AIM-star, there's a liquid drop radiator. They pump the engine coolant (or transfer the heat to some radiating fluid first) and send it to the front of the ship. Then it's expelled in droplet form along a sort of pipe that's perpendicular to the acceleration of the ship, so that the droplets accelerate to the back of the ship in a sheet-like formation, where they are collected by a similar 'pipe.' They even have quantitative stuff on this.

There's an address for a paper on this rocket, but I checked out Penn State's physics site and it says they're reorganizing stuff. Here's the link anyway in case it starts working:
http://antimatter.phys.psu.edu/Papers/AIMStar_99.pdf

Worry not, I don't know enough about critical mass for that to matter! My nuclear engineering experience is about one month in an introductory class so far. However, I also took a freshman research project class, which was just writing a paper under the supervision of the Fusion Professor, and I wrote this paper. I was just a freshman, so don't blame me if the model is a little sketchy. I'm doing physics, nuclear engineering is just a minor. As for your questions, all I can say is, deuterium is kind of expensive as I understand it, so as long as you don't need a large fuel tank full of it, it would be ok.

(standard disclaimer: Since writing this paper I realized that a liquid hydrogen propellant is unnecessary and it is far more efficient to just increase the thickness of the shell. This greatly increases the thrust while only partly decreasing the specific impulse because the increased shell thickness allows for more fusion to take place. This effect is illustrated in one of the graphs at the bottom of the paper. Also reflected in another graph is a correction to the calculated optimal specific impulse for tin (to about 7.2 x 10^5 s at a shell thickness of 0.027 cm and f=27%). The other metals also have correspondingly higher specific impulses. These changes along with a slower Mars mission would lower the helium-3 requirement significantly.)

The textbook Introduction to Nuclear Engineering by Lamarsh and Baratta (2001) claims that "the estimated mass of a chemical rocket required for a manned mission from a stationary parking orbit to an orbit around Mars is approximately 4,100,000 kg. The mass of a nuclear rocket for the same mission is estimated to be only 430,000 kg." Whatever! This is in the introduction as one of the reasons why nuclear engineering is useful. With Mars Direct, of course, the vehicle that carries the astronauts is more like 130 tonnes (?). Maybe they assumed some opposition class monstrosity?

The first rocket doesn't lift the ribbon on takeoff. Accoring to the site, "The NIAC work laid out a detailed description of a possible space elevator program. Initially, a small, carbon-nanotube-composite ribbon (10 to 20 cm wide and microns thick) capable of supporting 990 kg payloads would be deployed from geosynchronous orbit using four rockets and a magnetoplasmadynamic upper stage. Climbers (230) are sent up the initial ribbon (one every 3 to 4 days) adding small ribbons alongside the first to increase its strength. After 2.3 years a ribbon capable of supporting 20,000 kg climbers would be complete."

I can't find out where it says how thick the final ribbon is. Also, would it be tapered like other designs?

It would make sense if they could put translife in the ISS's Centrifuge module, which is already designed to simulate gravity > 0g (such as 0.38g), but to my knowledge that has been either axed or delayed.

Shadows from the moons (especially Io) would definitely be visible; any amateur astronomer could see them from Earth. As for manned landings, you'd have to stay out of the radiation belts as far as near future technology is concerned (which applies to both moons and an orbital path that takes you into dangerous areas).

If I remember right, Zubrin says in The Case for Mars that (at a distance roughly between Earth and Mars) that the cosmic radiation is about equal to the solar radiation dose. Is that right?

Papers by Terry Kammash and others seem to suggest that the fusion based PDEs can direct the explosion with a magnetic nozzle[1] since you end up with a plasma. This is an important difference between MICF and the project orion-style systems of the past, which I assume used a chunk of high explosives to ignite the fissile material. Ideally, an antiproton beam is used, but if that cant be made precise enough, a single ion or laser beam will do(but a 2-3 megajoule laser or ion driver is much more massive).

By my calculations, you get a specific impulse of approximately 7e5 seconds using a tin shell with d-helium-3, or about half that with a tungsten shell (other authors tend to use tungsten...tungsten might be necessary for other reasons), so it's something that deserves more attention!

[1]Kammash, Terry. "Pulsed Fusion Propulsion Systems for Rapid Interstellar Missions." Journal of Propulsion and Power, 16.6 (2000)

p.s. theoretical designs tend to fall in the 200-300 tonne range.