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I have no problem with starting up that sort of engine from Earth orbit, but not a ground launch. The antiproton catilized engines should have a lot less political fallout as they don't use bombs, have better performanc as well. I don't see a bomb driven Orion being built in reality.
Aerobraking would be possible only if the dV is low enough, but could be done for extreamly high payload mass cargo missions.
EPPP is only inefficient in realation to the total energy involved, with a fission system and a 10m size the Isp can be currently as high as 5000s (SCNTR max out at about 1000s) with a thrust level of 4 MN. While this is not as high as many electric engines, the thrust levels are far higher.
In reality the best bet for EPPP politically is using antiproton ignition systems with fission/fusion pellets rather then supercritical devices. With such systens (like ICAN) the Isp is about 13000s while the thrust levels are lower at 180 kN for a 10m ship. They are not availible now but will likely be ready before controled fusion rockets.
Defects can exist in any system, cracked pusher plates/thrust shell, exploding turbopumps, cracked fuel tanks, defective magnetic nozzels. In fact turbopumps for liquid fueled rockets are pobably more likely to fail due to their high stresses, and a failure is just as likely to ruin your day.
As for the plume while decelerating, in either form the debrei cloud is being accelerated away from the ship so I don't see the problem.
The time required to get to Mars relies on a number of factors, but I'll try to keep it simple. Using a external pulse plasma propulsion ship small enough to be lauched in pieces using a Saturn V class rocket (10m diameter, like the ships in NASA's old DRM 1.0), General Atomic calculated that flying an oposition class mission (long travel times, short 30 day stay) could be accomplished in 300 days, 180 days to get there, 30 days at Mars, 87 days back. Chemical rocket and nuclear thermal rocket powered missions from the same time period, mid to late 1960's, usually have total mission times between 450 and 680 days (the chemical rockets are more towards the higher end), with total ship masses far greater (ranging from 2-30 million pounds of propellant alone depending on flight time, method of returning to Earth, and engines used. the ORION weighed between 1.5-1.8 million pounds total). For example an all chemical ship flying a similar trejectory planned for 180 days to Mars, 60 days on the surface, and 330 days home.
Modern tech can improve the performace enough that if used for a conjuction class mission (long stay, like Mars Direct or the DRM) flight times would likely be down around 45-60 days carring 150 tonnes of payload. Around 30 days would likely be the upper limit of how short you can reasonibly make it, with a far lower payload fraction.
For those who want performance numbers the origonal figures for the 10m size were Isp of ~2000s and 3.5 MN of thrust, the modern numbers (taken from a 2000 report by J. A. Bonometti) have an Isp of 5000s and 4 MN of thrust. Bonometti's paper also talks about a smaller version capible of being launched by a Titan IV (~4-4.3m diameter I estimate, it doesn't say in the paper) with an Isp of 4000s and 2 MN of thrust, potentialy doing away with the requirement of a HLLV.
Fusion reactions are not easy to start. You would still need the fission reactor to get the reaction started. It would propably be safer to have a seperate power system running the magnetic field anyways.
That would really limit where you could go with the rocket, and those solar pannels would have to be HUGE. Even if you plan on trying to run a bimodal reactor (personaly I would just keep the fusion engine an engine, it would make life simpler)you still need a lot of power to get the reaction started, and will likely mean fission reactors.
Probably not more then a few weeks if even that.
It was still Lockheed-Martin last time I looked.
I see no reason why the same airframe could not be adapted to both a pure crew transport and a Progress-like automated cargo/supply ship. By seperating the two, the overall design is simplified.
Currently recycling waste is more expensive then mining new urainium barring ore, not including the cost of building the reprocessing plant. Eventually the cost of reprocessing will be cheeper, but not at the moment.
A NEP powered probe to Jupiter would be a great first step. Too many nuclear programs have faltered because they tried to move too far to fast (SP-100 for example). Once a succesful mission is acomplished, then we can move on to bigger and better things.
And after what Galileo brought back, a new mission could answer a lot of questions.
The main problem with Americium-242 is the cost of producing it, Americium is a man-made element, and its half life in the ground state is only 16.02 hours. This would make it difficult to use as a fuel.
The urainium, thorium and other radioactive elements natualy in the coal does. Globaly coal plants dump 10000 tons of radioactive waste into the atmosphere a year, nuclear plants release a negliglble amount. Coal makes up over 50% of US electrical generation. You get over three times the radioactive dose standing next to a coal plant than a nuclear plant. Plus coal has the fun side effects of CO2, SO2, acid rain, etc.
Fusion may be the future, but that doesn't mean we shouldn't 'green' up our energy by going nuclear fission until it becomes a reality.
It would probibly be easier and more effective to just spin the ship itself rather then work with large, bulky centrafuge systems.
Some see it as waste, but spent fuel can be reprocessed in fast reators into more fuel. Nuclear fission power is zero emmision and works well in the large scale, something few other low or zero emission power souces can claim. And I have seen the geology behind sites like Yucca Mountain, if it needs to be disposed of, it can be done safely.
In space disposal isn't a problem, the ship can always be shot into an solar trajectory on its final mission. Nuclear fission power systems and propulsion systems offer advanteges beyond conventional systems.
We would all love to use fusion power, but it won't exactly be ready within the next 30 years for space or land power purposes.
The fact is that most of the spacecraft cannot be decently shielded without rasing its mass dramaticly. Because of this most plans only contain a small 'storm shelter'. And thick shielding can actually make the damage from cosmic rays worse. The majority of the radiation risk is accumulated in transit, on Mars the atmosphere and local materials can be used to produce adaquit shielding. Space is a high radiation environment. I would not be surprised if the Energia plan racks up 150-200 rems for the crew, nearly four times what one would accumulate on a Mars Direct style mission, and 30-40 times the yearly limit in a Govt. facility.
Yes, I do support using technology to shorten trip times without increasing mass signifegently. There is no need for a huge rocket to put a little capsule on Mars.
Zero G or not, you also have nearly two straight years exposed to comic radiation. Shielding helps, but exposure racks up outside Earth's protective fields.
A ton is usualy a US Short ton equal to 2000 pounds.
A tonne can be a US long ton equal to 2200 pounds, but is usually used to refer to a metric ton equal to 1000 kilograms, or 2202.6 pounds. No one uses long tons anymore.
Energia's plan is just too risky in my opinion. Subjecting the crew to more then one year of zero G before getting to Mars and a year more on the return. Anything that slow should be deligated to cargo carring not human transport.
Hall thrusters are really too weak for manned spaceflight, with 15 MWe of power they could run a much higher power megnetoplasma dynamic thruster of some sort and get there a lot quicker.
For rutine flights to LMO I would use compressed CO2 fueled NTR "shuttles". Fueling time is restained only by power constraints, it requires 84kWh to compress 1 ton of Martian atmosphere, so each shuttle could fly every couple of days if hooked up to a ground power source.
If we are still sticking to a Mars Direct scenario, the use of a 45000 lbf SCNR as a "third stage" to an Ares class vehicle could be used to reduce flight times to 80-120 days (From Zubrin's origonal 1990 Mars Direct report). The stage would not start up until it reached a 700km orbit.
I agree, the longer they are in 1g, the better. A cable will probably be able to be spun in and out easier then a truss, so long lengths are easy to accomidate.
I wonder, what are you going to use as a counterweight for the return trip? And how exactly were they planning on deploying a 22m truss. I havn't seen any mention in Zubrin's technicle papers.
I think an 180 day trip is streatching it, at least reduce it to 120 days, that does not require exotic propulsion systems as sub 100 day transits would. The less time in space the better for the crew.
If the cost of launching went down enough more companies may want to invest in space assets. Ariane V already lofts multiple satellites at once. The question is whether there is enough want in the lower orbits likely used by a Shuttle-C, which probably won't diliver to GEO.
12m would be a rather bulbous payload for a shuttle derived HLLV. The external tank has a diameter of about 7.5m(?) I think, so 12m is a little less then twice this. The origonal Shuttle-C had an internal diameter of about 4.6m, so would be unable of fitting a 12m object inside.
We should take advantage of NASA's current nuclear friendly administration, Goldin was extreamly hostile to the thought of even RTG's, let alone propulsion. He frequently demanded non-nucler option be used, even if it ment the project could't be done.
VASMIR makes a good stop-gap measure until we can utilize something better, and it is not intrinicly linked with nuclear(although manned missions would use reactors) so will be easier to develope politically. VASMIR, however, still suffers from the curse of all known electric rockets, it has to slowly spiral out of orbit. This adds time not included in the transit times shown in reports. VASMIR is better then most adding only ~30 days to the outbound leg and ~10 days to the inbound one. I have not seen figures for capture times, so I am not sure how long they would be.
I think NASA was waiting for a space station before looking into a serious demonstration of artificial gravity in space, and that quest is taking a little longer then they suspected it would back in the shuttle era.
Spin it, but not before you lay on the coal. That would help solve the rest fo the medical hazards of interplanetary flight, like radiation. Flank speed straight to Mars!