Quote from: InterestedEngineer on 01/27/2023 05:15 amAn NTR based on NERVA specs can't get to Mars with any less fuel mass requirement than a fully refueled Starship from LEO.This is because of a larger dry mass and no aerobraking on Mars.NTR doesn't make sense in the era of low cost refuels to LEO.To halve the transit time to Mars an NTR would require an ISP of 1750. That'd be the lightbulb gas fuel design. A fantasy right now.https://docs.google.com/spreadsheets/d/1ycrQlKql9fft9oLDoW1uP-rotK-Y5yVtH4T3S2ZKgk4Well more or less as you said it does make sense if they (NASA/DARPA) design a NTR with a better characteristics than what was proposed for project TIMBERWIND - no need for 1700+ ISP but a 1000-1100 ISP with a T/W higher than 20 ( or as you said a single digit T/W but with 1700+ ISP ) .
An NTR based on NERVA specs can't get to Mars with any less fuel mass requirement than a fully refueled Starship from LEO.This is because of a larger dry mass and no aerobraking on Mars.NTR doesn't make sense in the era of low cost refuels to LEO.To halve the transit time to Mars an NTR would require an ISP of 1750. That'd be the lightbulb gas fuel design. A fantasy right now.https://docs.google.com/spreadsheets/d/1ycrQlKql9fft9oLDoW1uP-rotK-Y5yVtH4T3S2ZKgk4
Quote from: edzieba on 01/27/2023 02:42 pmQuote from: InterestedEngineer on 01/27/2023 01:22 pmAs far as aerobraking being an arbitrary restriction, you design a large LH2 tank and radioactive engine that has to be as far away from the cargo that can aerobrake. I doubt it is possible, and if it is possible, it's impossibly hard and expensive to test and iterate. But you are welcome to try. Do keep in mind basic aerodynamic rules such as center of mass lining up with center of drag.Failure of imagination is not a physical constraint. Architectures from inflatable decelerators to side-braking (gee, better not just dismiss that one out-of-hand!) to engine-first with an articulated heatshield, etc, are hardly new or novel designs. Incidentally, centre of mass and centre of drag do not need to line up, and generally don't even for existing capsules, as lifting entries are the norm to allow for steering.Actually precise atmospheric entry of a vehicle with two comparable masses at two ends would require significant new development.
Quote from: InterestedEngineer on 01/27/2023 01:22 pmAs far as aerobraking being an arbitrary restriction, you design a large LH2 tank and radioactive engine that has to be as far away from the cargo that can aerobrake. I doubt it is possible, and if it is possible, it's impossibly hard and expensive to test and iterate. But you are welcome to try. Do keep in mind basic aerodynamic rules such as center of mass lining up with center of drag.Failure of imagination is not a physical constraint. Architectures from inflatable decelerators to side-braking (gee, better not just dismiss that one out-of-hand!) to engine-first with an articulated heatshield, etc, are hardly new or novel designs. Incidentally, centre of mass and centre of drag do not need to line up, and generally don't even for existing capsules, as lifting entries are the norm to allow for steering.
As far as aerobraking being an arbitrary restriction, you design a large LH2 tank and radioactive engine that has to be as far away from the cargo that can aerobrake. I doubt it is possible, and if it is possible, it's impossibly hard and expensive to test and iterate. But you are welcome to try. Do keep in mind basic aerodynamic rules such as center of mass lining up with center of drag.
Quote from: edkyle99 on 01/27/2023 01:36 pmPeople get mad about all sorts of things these days. I think we can predict that many will not like this idea.Just ignore them, what are they going to do? Chain themselves on CCAFS' gates?
People get mad about all sorts of things these days. I think we can predict that many will not like this idea.
Quote from: sebk on 01/28/2023 12:23 amQuote from: edzieba on 01/27/2023 02:42 pmQuote from: InterestedEngineer on 01/27/2023 01:22 pmAs far as aerobraking being an arbitrary restriction, you design a large LH2 tank and radioactive engine that has to be as far away from the cargo that can aerobrake. I doubt it is possible, and if it is possible, it's impossibly hard and expensive to test and iterate. But you are welcome to try. Do keep in mind basic aerodynamic rules such as center of mass lining up with center of drag.Failure of imagination is not a physical constraint. Architectures from inflatable decelerators to side-braking (gee, better not just dismiss that one out-of-hand!) to engine-first with an articulated heatshield, etc, are hardly new or novel designs. Incidentally, centre of mass and centre of drag do not need to line up, and generally don't even for existing capsules, as lifting entries are the norm to allow for steering.Actually precise atmospheric entry of a vehicle with two comparable masses at two ends would require significant new development. New != impossible. In fact, there's a chemical architecture cited in this very thread with the assumption that it is the superior one, that mounts the engine mass out at one end of the vehicle and crew section at the other...
Even if DRACO succeeds, NASA’s involvement appears to be limited to technical lead. The requirements and budget still come from DARPA, and they seem focused on rapid orbit and plane changes for military smallsats, not the scale, environment, or reliability required for human deep space exploration. NASA may resurrect some expertise in the general technical area, but DRACO won’t produce a nuke thermal stage for humans to Mars.FWIW...
So inform the group exactly what the differences are between the DARPA DRACO design as proposed and what it takes for a nuke thermal stage for humans to Mars.
Elon got it right—fuel is cheap, so optimize for a huge fuel load, develop in-space refueling, and stop worrying about maximizing Isp.
"With respect to space, I think there’s really just one problem, which is a fully and rapidly reusable orbital rocket. This is the holy grail," Musk said, speaking with Lt. Gen. John Thompson at the Air Force’s Space Pitch Day event."SpaceX has made some progress in reusing the booster," Musk said. But that’s still only part of the rocket. As Musk said, "It’s absolutely profound to have a reusable rocket.""A giant reusable craft costs much less than a small expendable craft," Musk said.
Quote from: GuessWho on 01/29/2023 07:48 pmSo inform the group exactly what the differences are between the DARPA DRACO design as proposed and what it takes for a nuke thermal stage for humans to Mars.Isp is different. The DRACO BAA has a requirement of 700s. Assumptions for NASA human Mars missions start around 900s.Thrust will be different. DARPA wants to move satellites, not crew modules and landers.Mission duration will probably be different. Unknown what DARPA is looking at, but it’s probably not 2-3 years like conjunction- and opposition-class Mars missions.Heat rejection may be different, depending on exactly where DARPA is looking to operate.Etc.If, unlike its predecessors, DRACO gets to flight, it will resurrect some competency at NASA in nuclear thermal propulsion. But it won’t produce a nuclear thermal stage, or even engine, that NASA can use to send astronauts to/from Mars. That will require a lot more investment in a larger and more capable engine/stage.Regardless, I’m skeptical that DRACO gets to flight. Like its predecessors, there’s no pressing need for this capability on the defense side. (And if there was, it would be a black program.) And the DRACO BAA assumes no ground testing for the engine, which is unrealistic. The spacecraft and launch will be expensive enough that someone will want to know it works before putting it in orbit. But the environmental costs of ground testing are prohibitive for a DARPA project. At some point, they’ll run into that reality if the high cost versus weak need conundrum for space nuclear systems doesn’t get them first.FWIW...
Quote from: su27k on 01/28/2023 01:32 amQuote from: edkyle99 on 01/27/2023 01:36 pmPeople get mad about all sorts of things these days. I think we can predict that many will not like this idea.Just ignore them, what are they going to do? Chain themselves on CCAFS' gates?Lawsuits and ballots for starters. I don't think it will be the usual anarchy crowd leading the complaints about this one. It'll be people with lawyers and money. A reactor is a different deal than an RTG. Very few have been launched, the last in 1988. One that reentered was a giant costly headache - and not for the country that launched it. Another released a giant radioactive cloud in orbit, which is still a concern. I'm a space supporter, but I don't want another one of these flying over my head, and I don't want it launched by something with 0.98 reliability, or 0.99 reliability, which encompasses all known launch vehicles. That's not good enough. Also, if the U.S. does this, other countries will follow. China, a country that lets giant CZ-7 core stages reenter uncontrolled, already has a program for a big nuclear thermal reactor. Fun, right? - Ed kyle
Quote from: edzieba on 01/28/2023 08:20 amQuote from: sebk on 01/28/2023 12:23 amQuote from: edzieba on 01/27/2023 02:42 pmQuote from: InterestedEngineer on 01/27/2023 01:22 pmAs far as aerobraking being an arbitrary restriction, you design a large LH2 tank and radioactive engine that has to be as far away from the cargo that can aerobrake. I doubt it is possible, and if it is possible, it's impossibly hard and expensive to test and iterate. But you are welcome to try. Do keep in mind basic aerodynamic rules such as center of mass lining up with center of drag.Failure of imagination is not a physical constraint. Architectures from inflatable decelerators to side-braking (gee, better not just dismiss that one out-of-hand!) to engine-first with an articulated heatshield, etc, are hardly new or novel designs. Incidentally, centre of mass and centre of drag do not need to line up, and generally don't even for existing capsules, as lifting entries are the norm to allow for steering.Actually precise atmospheric entry of a vehicle with two comparable masses at two ends would require significant new development. New != impossible. In fact, there's a chemical architecture cited in this very thread with the assumption that it is the superior one, that mounts the engine mass out at one end of the vehicle and crew section at the other...The chemical architecture is testable. The nuclear one is not.
Isp is different. The DRACO BAA has a requirement of 700s. Assumptions for NASA human Mars missions start around 900s.Thrust will be different. DARPA wants to move satellites, not crew modules and landers.Mission duration will probably be different. Unknown what DARPA is looking at, but it’s probably not 2-3 years like conjunction- and opposition-class Mars missions.Heat rejection may be different, depending on exactly where DARPA is looking to operate.Etc.
700s man that's like nothing - like no material or architecture advancements since TIMBERWIND ( SNTP ) ?
DARPA has a minimum requirement of 700 seconds, doesn't say anything about what the design wants to be or could be. It could be 800, 900 or 1000 seconds. And if NASA and DARPA are collaborating, why would the ISP be different between the two?
I didn't see any requirements on thrust level. NASA has always looked at multiple engines so no way to tell how many DARPA engines it might take to meet a NASA mission design. Could be one or ten. So that argument fails .....
So no details on mission design or life so that means they must be different - gotcha ..... Very scientific.
Heat rejection is a spacecraft issue, not an NTP issue so not sure how that argument applies either.
From everything I have read, DARPA never planned to do a ground test. Too expensive and would take too long. That is a NASA mentality. So not a hurdle that DARPA has to jump.