I seem to remember years ago that an article on NK claimed that the Delta IV CBC had the ability to orbit several hundred kilograms as an expendable SSTO.
Quote from: douglas100 on 01/19/2017 05:05 pmI seem to remember years ago that an article on NK claimed that the Delta IV CBC had the ability to orbit several hundred kilograms as an expendable SSTO.And here we are still talking about the feasibility more than 30 years later. I thought that paper of mine was forgotten.
Quote from: HMXHMX on 01/21/2017 11:25 pmQuote from: douglas100 on 01/19/2017 05:05 pmI seem to remember years ago that an article on NK claimed that the Delta IV CBC had the ability to orbit several hundred kilograms as an expendable SSTO.And here we are still talking about the feasibility more than 30 years later. I thought that paper of mine was forgotten.Any chance we could get a link to the NK article or, better, still, the paper itself?
Correction: the latest version of Merlin 1D has a thrust to weight ratio of about 200 due to thrust increases.And if we're talking state of the art, that rules out every nuclear rocket.Anyway, I was explicitly talking about what is possible, not just what we can do today. My point is that you can make huge improvements to mass fraction, so the fact that chemical has lower Isp does NOT mean that what I described can't be done.Merlin 1D has performance that once would've been ridiculed as absurd and beyond the state of the art as well.
Quote from: DarkenedOne on 01/20/2017 02:39 amQuote from: Robotbeat on 01/19/2017 07:17 pmAnd the theoretical basis for why NTR is fundamentally limited is due to the propellant, hydrogen. It has terrible density. And there simply is no possible material that could make the nuclear lightbulb work.First of all, density is irrelevant. Your spacecraft could be the size of a blimp.Density does matter. A vehicle the size of a blimp will suffer large drag losses. Any given mass fraction will be more difficult to achieve with a propellant of lower density, because the tanks, the plumbing and the engines will all need to be bigger. And if we're talking hydrogen, insulation needs to be factored in too. John Whitehead wrote a nice paper (attached to this post) about SSTO mass budgets that illustrates the drawbacks of hydrogen (albeit lox-hydrogen, rather than hydrogen-NTR).
Quote from: Robotbeat on 01/19/2017 07:17 pmAnd the theoretical basis for why NTR is fundamentally limited is due to the propellant, hydrogen. It has terrible density. And there simply is no possible material that could make the nuclear lightbulb work.First of all, density is irrelevant. Your spacecraft could be the size of a blimp.
And the theoretical basis for why NTR is fundamentally limited is due to the propellant, hydrogen. It has terrible density. And there simply is no possible material that could make the nuclear lightbulb work.
Quote from: Robotbeat on 01/20/2017 04:44 amCorrection: the latest version of Merlin 1D has a thrust to weight ratio of about 200 due to thrust increases.And if we're talking state of the art, that rules out every nuclear rocket.Anyway, I was explicitly talking about what is possible, not just what we can do today. My point is that you can make huge improvements to mass fraction, so the fact that chemical has lower Isp does NOT mean that what I described can't be done.Merlin 1D has performance that once would've been ridiculed as absurd and beyond the state of the art as well.Alright lets see what falcon 9 rocket with a mass faction of 150 would look like with the Merlin 1D assuming the mass faction of 200. According to wikipedia the mass of the Merlin D is 630, a thrust of 845 metric tones, and a mass of 630 kg. Nine Merlin 1Ds would weigh 5670 and have be able to lift 1134000 kg. For the entire rocket to achieve a mass ratio of 150 the dry mass of the rocket could therefore not exceed 7560 kg. Subtracting the total weight minus the weight of the engines you get 1890 kg for the rest of the rocket. The tanks, the avionics, the landing gear, the heat shield, the payload fairing, and the payload itself would have to weigh less than a Ford Edge.I cannot prove that it is impossible. There is no law of physics that says it cannot be done, but it is easy to prove that it is not feasible as it is far beyond the state of the art. As far as nuclear thermal and other nuclear technologies we cannot necessarily say that they can do better, but we do know that the limitations on what the technology can do are far higher. If you just took the prototypes for nuclear thermal engines that were tested in the 60s and updated them with modern materials you would get likely get significantly higher T/W. In my opinion SSTO spaceships (not RLVs) are much more achievable with advanced nuclear thermal than with chemical rockets with insane mass ratios.Alright lets see what a rocket with a mass faction of 150 would look like with the Merlin 1D assuming the mass faction of 200. According to wik, a thrust of 845 metric tones, and a mass of 630 kg. Nine Merlin 1Ds would weigh 5670 and have a thrust of 7605000 kg. For the entire rocket to achieve a mass ratio of 150 the
If you just took the prototypes for nuclear thermal engines that were tested in the 60s and updated them with modern materials you would get likely get significantly higher T/W.
As far as nuclear thermal and other nuclear technologies we cannot necessarily say that they can do better, but we do know that the limitations on what the technology can do are far higher. If you just took the prototypes for nuclear thermal engines that were tested in the 60s and updated them with modern materials you would get likely get significantly higher T/W.
In my opinion SSTO spaceships (not RLVs) are much more achievable with advanced nuclear thermal than with chemical rockets with insane mass ratios.
Alright lets see what a rocket with a mass faction of 150 would look like with the Merlin 1D assuming the mass faction of 200. According to wik, a thrust of 845 metric tones, and a mass of 630 kg. Nine Merlin 1Ds would weigh 5670 and have a thrust of 7605000 kg. For the entire rocket to achieve a mass ratio of 150 the
Quote from: DarkenedOne on 01/24/2017 03:13 pmIf you just took the prototypes for nuclear thermal engines that were tested in the 60s and updated them with modern materials you would get likely get significantly higher T/W.I don't think so, in large part because we've already looked into it--in the late 80's and early 90's, project TIMBERWIND took a look at NERVA and tried to optimize it, and the best they could do was a TWR of 30:1*. We could probably push that a little bit higher, but to make it viable for a SSTO we'd need to push it a lot higher. Not to mention the political infeasibility of NTR's, especially as a first stage.EDIT:*And this may be highly optimistic--I don't think they ever obtained real-world thrust on this magnitude, but I could be wrong.