meiza - 30/12/2007 7:56 PMI'm not a nuclear engineer, but if there is so little material (or it's in a sparse matrix) that it can't even melt itself when made to a pile, (I assume it reaches a steady state where it radiates as much heat as it generates, glowing white hot) what will the hydrogen temperature actually be when it flows through the core. I mean, these seem as pretty opposite design parameters. You want a hot core to make hot hydrogen for high ISP and high thrust but you want a cool core for failure safety. The power is the same in both cases, for an engine that can not explode. Someone could easily calculate the loss of coolant accident temperature for various size cores with various power ratings with the Beer-Lambert law....
As far as I know, the nerva designs were not safe in this regard (and not in many other regards either). But I could be wrong.
Marsman - 30/12/2007 8:18 PMPerhaps you could use a nuclear stage to go from Earth orbit to Lunar orbit, and a dedicated chemical (or nuclear) lander. I could see a three launch architecture with existing EELVs- 1. Small Orion on Atlas 4012. LSAM Descent on DIV-H or Atlas H or 5513. LSAM Ascent (possibly with descent hydrogen) on DIV-H or Atlas H or 55140-50 mt EELV's could do a two launch architectureA big question that I have is why are people so afraid of nuclear in space? There is no danger of pieces falling back from a transfer back to Earth as previously explained, and the engine wouldn't even have enough material to go critical in the first place. The only danger that I can see is a nuclear upper stage ignited on a suborbital trajectory - if it doesn’t ignite, then it is coming back. But if it is ignited in orbit, there is no danger.
meiza - 30/12/2007 9:01 PMLet's calculate a bit.If we want a nuclear J-2 equivalent, that's 100 tons or 1000 kN of thrust and and ISP of perhaps 1000 s meaning v_ex 10 km/s. That means a kinetic power of 5 GW for the exhaust jet. I think the biggest state of the art stationary nuclear reactors are that size in thermal power.The hydrogen flow rate would be 100 kg/s. Heated from cryogenic to about 2600-2700 K or 2300-2400 degrees C. The core glows white hot.Now, what happens when this hydrogen flow is for some reason discontinued? Where does all the power go? Stay tuned for the next episode, aired in 2008...
mike robel - 30/12/2007 9:08 PMCrap, when I edit, it sticks the whole post into a single paragraph. How can I fix that?
I think everyone (NASA included) is still missing the point of "Return to the Moon" as part of VSE. It's supposed to be practice and a learning experience for "Mars Exploration". There's just no point in developing a Moon architecture that won't apply to Mars.Key point: A Mars mission is going to require a big, extended duration, habitat module. Including things like personal space, emergency radiation shelter, water & air recycling and probably some sort of centifugal, artificial gravity. This is NOT going to land on Mars. Ideally, it should return to Earth orbit or L1/L2 for servicing, resupply & reuse. The Moon architecture should incorporate this module so as to test it over shorter durations, and much closer to home. i.e. If something breaks, like air or water recycling, it's only a few days to home, not six months or more.
Similarly, TLI and TMI injection are of approximate size. A Mars Ascender has similar delta v to a Moon Lander/Ascender.
Design your Mars architecture, and test as much as possible by going to the moon.
kkattula2 - 30/12/2007 11:21 PMI think everyone (NASA included) is still missing the point of "Return to the Moon" as part of VSE. It's supposed to be practice and a learning experience for "Mars Exploration". There's just no point in developing a Moon architecture that won't apply to Mars.Key point: A Mars mission is going to require a big, extended duration, habitat module. Including things like personal space, emergency radiation shelter, water & air recycling and probably some sort of centifugal, artificial gravity. This is NOT going to land on Mars. Ideally, it should return to Earth orbit or L1/L2 for servicing, resupply & reuse. The Moon architecture should incorporate this module so as to test it over shorter durations, and much closer to home. i.e. If something breaks, like air or water recycling, it's only a few days to home, not six months or more. Similarly, TLI and TMI injection are of approximate size. A Mars Ascender has similar delta v to a Moon Lander/Ascender.Design your Mars architecture, and test as much as possible by going to the moon.
kkattula2 - 30/12/2007 11:21 PMIdeally, it should return to Earth orbit or L1/L2 for servicing, resupply & reuse.
tankmodeler - 31/12/2007 11:33 AMI suspect that isn't going to happen until there is enough traffic to Mars to make it economical to retain the MM. Remember, to retain a Mars MM you have to brake it into Earth orbit, which means you have to take the fuel for that all the way to Mars and back, which puts you on the up-spiral in weight for the entire system. Paul
clongton - 31/12/2007 11:39 AMQuotetankmodeler - 31/12/2007 11:33 AMI suspect that isn't going to happen until there is enough traffic to Mars to make it economical to retain the MM. Remember, to retain a Mars MM you have to brake it into Earth orbit, which means you have to take the fuel for that all the way to Mars and back, which puts you on the up-spiral in weight for the entire system. PaulIf the spacecraft is NEP or SEP powered and the start/return point is EML2, then the returning spacecraft would be thrusting almost all the way home. By the time they arrive in the vicinity of EML2, they could probably be captured there with simple thruster maneuvers. -Reusable interplanetary spacecraft!
tankmodeler - 1/1/2008 1:48 PMYes, that's true, but you're still carrying the fuel all the way there & back.
vanilla - 1/1/2008 3:18 PMQuotetankmodeler - 1/1/2008 1:48 PMYes, that's true, but you're still carrying the fuel all the way there & back.The fuel load isn't nearly as bad as with a chemical rocket.
libs0n - 31/12/2007 5:17 PMI have a different point of view. I believe that a moon mission should be designed solely around getting to the moon. ...
clongton - 29/12/2007 3:15 PM{snip}Ross outlined an EELV Heavy Lift a little ways above that was better than the Ares. It beats the Ares hands down - more powerful, less costly and fielded much sooner. And most important - sustainable. On top of that, the Orion would fit on the unenhansed core of that launcher and get the US back into manned space within 2 years of Shuttle retirement. What's not to love? LM and Boeing are perfectly capable of proposing this launch vehicle and then they could do what I suggested above; sell the customer the “equivalent” of the “Mack” truck. But apparently, the EELV company CEO’s do not want to play. They don’t have the balls to compete. Oh well. Then let them stew in it. That is a sad commentary from my perspective. I would “LOVE” to see that competition against STS. I am for STS because it's the only heavy lift option offered. If LM & Boeing would get off their scardy-cat lazy butts and compete - well it could be a totally different ball game and I'm not so sure STS would win a fair competition like that. But apparently they are too cheap to spend the cash to put the proposal in place. They have been feeding at the government cow's teet for too long. It's time to wean them and make them earn their business, just like everyone else. They could produce such an awesome launch vehicle family for this nation if only their leaders would grow a little hair. I know that the designers and engineers that work for them are just itching to do this because they know beyond a doubt that they can. Those employees have more faith in themselves, their abilities and their companies than their leaders do and that is just plain sick. It turns my stomach. So I guess as far as EELV is concerned, it's 25mT pop-guns or nothing.
kkattula2 - 2/1/2008 8:11 AMQuotelibs0n - 31/12/2007 5:17 PMI have a different point of view. I believe that a moon mission should be designed solely around getting to the moon. ...I agree that the moon is worth going to for its own sake, and for long term occupation you would want an optimized transportation system. But that's not the clearly stated objective in the VSE. Mars is the goal. The Moon is just practice. Until & unless that changes, ESAS is not implementing the VSE. Whether it should change is another question.I would like to see NASA gradually develop more and more capability. It needn't be all in one go:1) Develop a crew launcher to LEO2) Put a long duration habitat (HM) in LEO (no propulsion, but designed to handle interplanetery injection stresses), test with crew3) Put a propellant depot (PD) in LEO, and start filling it4) Put a propulsion module (PM) in LEO (similar to an EDS), practice filling at depot5) Test PM on loop around the Moon, Lunar Orbit insertion, LEO return6) Use PM to send PD 's to Mars Orbit, Lunar orbit, L1 etc,7) Use PM to send crewed HM on various Lunar, NEAR & Mars tests Put Moon or Mars specific landers in LEO, dock with HM, test9) Use PM to send HM and lander to Moon, Mars etcAlmost all of this could be done with EELV's, with minimal "assembly", but quite a few dockings.Heavier lift would make it a bit simpler though.Things like docking and propellant transfer will have to become routine one day. Now is the time they should be developed. "...we choose (to do these things) not because they are easy, but because they are hard." anyone?
minimal "assembly", but quite a few dockings.