The "Real Aerospace" Thread
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Re: The "Real Aerospace" Thread
Plus they do test it with more modern materials, and modern software.
They work more autonomously than previous missions of the sixties did (where it was due to a lack of computer power for the systems).
Basically, they need to redo all the steps, because those are steps in the direction of "We're here, and we're here to stay!".
And it finally put them on the wikipedia list of Timeline of space exploration.
It's also impressive to see how quick they evolve their abilities.
It's simply that most of the time since the collapse of the USSR, the push into space seemed very glacial. At best they sent some satellites to improve our knowledge of space, but actual hands-on space exploration was more or less halted.
Heck, to me the ISS seemed like a big thing, finally a new station in orbit again! But it was still a remade step, since we (Humans) have had space stations before already.
Man, I'd prefer to spend my paycheck part for military on space exploration instead. Or at least a very large part of it.
But then, space missions always have been lighthouse missions. A lot of money for rather inefficient results, but very visible to the masses (thus allowing PR to attract more money for more effective exploration of space).
They work more autonomously than previous missions of the sixties did (where it was due to a lack of computer power for the systems).
Basically, they need to redo all the steps, because those are steps in the direction of "We're here, and we're here to stay!".
And it finally put them on the wikipedia list of Timeline of space exploration.
It's also impressive to see how quick they evolve their abilities.
It's simply that most of the time since the collapse of the USSR, the push into space seemed very glacial. At best they sent some satellites to improve our knowledge of space, but actual hands-on space exploration was more or less halted.
Heck, to me the ISS seemed like a big thing, finally a new station in orbit again! But it was still a remade step, since we (Humans) have had space stations before already.
Man, I'd prefer to spend my paycheck part for military on space exploration instead. Or at least a very large part of it.
But then, space missions always have been lighthouse missions. A lot of money for rather inefficient results, but very visible to the masses (thus allowing PR to attract more money for more effective exploration of space).
Re: The "Real Aerospace" Thread
There are actually significant differences in geology and composition. The differences are enough that it's been seriously proposed that the moon formed from two intermediate bodies in a low-energy collision, the smaller one spreading across one hemisphere to form the far side highlands: https://www.nature.com/articles/nature10289.Arioch wrote:Sure, they're making progress. The Chinese space program is impressive, especially compared to Roscosmos who seems content to feed off the rotting 28-year-old carcass of the Soviet space program (I say that without meaning to offend any of our Russian readers... NASA was not in a much better state before the shift in focus to private commercial spaceflight). I think that anyone who can land a craft on the Moon has accomplished an impressive feat, but let's not pretend that the Chinese are breathing down our necks just because they accomplished something that Soviet Luna and US Surveyor craft did in 1966. I'm sure that the Chinese selected the far side of the Moon so that they could say they did something that no one has ever done, but no one ever did it because there was no compelling reason to. Aside from the need to establish relay satellites for communication, there's nothing particularly hard or special about landing on the far side of the Moon.
Russia still has excellent engines that would work great in a reusable system, launch sites from which they could easily set up downrange landing pads, and a history of doing far more with far less than others, and of interest in reusable boosters. They could have stayed on top of things. Instead Rogozin ridiculed US efforts while SpaceX steamrolled the commercial launch market with Falcon 9 and developed Crew Dragon, and while Russian capabilities decayed to the point where Soyuz spacecraft sometimes have extra holes drilled in the hull.
But I wouldn't stay that NASA's much better, even now. Apart from their repeated failures to successfully develop a launch vehicle (at least the long-delayed Angara's put a mass simulator in orbit), look at the sheer absurd scale of the delays and cost inflation of JWST, and the problems that have cropped up in testing. And while Commercial Cargo was a huge success, NASA went into full inflexible bureaucracy mode when it came to getting the Commercial Crew vehicles certified, to a degree that seems unlikely to be anything but deliberate stalling in an effort to get either Starliner or Orion flying first.
Re: The "Real Aerospace" Thread
hi hi
Congress is often times NASA's biggest enemy. They make absurd demands (I'm lookin' at you, space shuttle) and cancel projects that are just coming to completion.
To paraphrase someone I know who works in putting spaceships into space, "It's amazing what you can do with total dictatorial control and the ability to exploit your workers." And it's no wonder that Space X grabs up people who know what they're doing when their projects get canned.
If anyone remembers from earlier discussions, I'm not very fond of Elon Musk, but I certainly will concede that he's good at marketing things and getting people to buy in. Which, given the way the world economy and big government budgets work these days, is an incredibly important thing.
Congress is often times NASA's biggest enemy. They make absurd demands (I'm lookin' at you, space shuttle) and cancel projects that are just coming to completion.
To paraphrase someone I know who works in putting spaceships into space, "It's amazing what you can do with total dictatorial control and the ability to exploit your workers." And it's no wonder that Space X grabs up people who know what they're doing when their projects get canned.
If anyone remembers from earlier discussions, I'm not very fond of Elon Musk, but I certainly will concede that he's good at marketing things and getting people to buy in. Which, given the way the world economy and big government budgets work these days, is an incredibly important thing.
Re: The "Real Aerospace" Thread
Not a fan of the whole of New Space, either, but if they can whip up Lunar and Martian rockets, superheavies etal, then it's a welcome addition. Governments will snap back in time anyway.
Re: The "Real Aerospace" Thread
So, I've just had a reeeeally crazy idea I've been mulling over for about two weeks, and I'd like to pass it by here.
Firstly, I started from the concept of the Nuclear Thermal Turbo rocket, an idea for a Air Augmented Nuclear Thermal Rocket, that uses air-breathing propulsion to lofts itself out of the lower atmosphere, transitioning from a ramjet type of system (powered by a turbofan spun by gasses leached from the reactor chamber), then to a scramjet (using the air directly by heating, and leaving the fanblades at idle angles of attack), then finally using standard NTR-type propulsion for the final ascent and orbital circularisation.
More on the ideea (and it's creator, John Bucknell) here:
https://forum.nasaspaceflight.com/index ... c=43344.20
http://www.projectrho.com/public_html/r ... rmal_Turbo
The kicker is the quite phenomenal promised (and I do stress "promised") mass fractions, that can be seen in the tables presented. This is due to the high average Isp of the thing (as air-breathing systems have Isp's in the thousands generally).
I thought, "great, but that pure rocket Isp is killing me. Looks so weak compared to the rest". So I started digging for more experimental types of NTR's. I stumbled onto MITEE, which uses a hybrid power system to dissociate some of the hydrogen into it's monoatomic state, thus achieving Isp of about 1350. There was also the NTER, the european concept that used hot gasses to drive what is essentially an electrothermal thruster.
But... then I realised that, since I already have a nuclear reactor onboard, how about going even more speculative...and mate it to this:
http://www.projectrho.com/public_html/r ... d-core_NTR
If it works (think I said that about half a dozen times) then I could literally get to Titan with an SSTO. And back (with the payload fractions in LEO plus a specific impulse of about 17,000 sec from the pulsed regime).
Finally, since I've got copious amounts of power onboard, why not use a more... exotic system to aerobrake for the return trip? Like the magshell:
https://forum.nasaspaceflight.com/index ... c=29912.80
https://selenianboondocks.com/2010/02/m ... rocapture/
There are MONSTROUS issues with the above concept:
1) making the turbo-fan airbreathing mode work (supersonic ducted fans are not really a thing).
2) transitioning between modes
3) getting rid of the excess heat from the pulsed NTR mode (radiators aplenty).
My initial vehicle would be almost entirely fuel, but that can be aleviated if one uses ISRU (pump the atmosphere of wherever it is you be landing into your tanks; NTR's don't care much what they propell out the nozzle as long as it doesn't gum up the works; water vapour, methane, ammonia, H2 produced by electrolisys and cracking, etc) and magnetoshell aerobraking.
Or you can split the mission, and use a Starship-esque upper stage that has the pulsed unit, along with a simpler airbreathing mode and a pumping station.
Monstrously complex, but, as you might know, I like thinking ludicrous...
Another, simpler mission, is to use a molten salt thorium reactor to melt my way into Europa. The thorium would produce U233, which fissions and gives off neutrons... but also U232, which fissions into Th (Thallium) 228, which is a HARD gamma emitter (at 2.67 Mev, it's the third most potent one known). The reactor and the submersible that follows it down would be bathed in hard gamma, thus resolving the issue of how to sterilise the craft (the most complicated problem in this type of Europa mission). the reactor would be fueled so that it's no longer critical by the time it finishes melting, and just plunks into the abyss. The submersible could have it's own, separate, reactor or RTG to do long duration surveys (and would transmit data to the lander on the surface by optical cable). Before landing, the craft could use high-power ground penetrating radar (remember, plenty of power available) to map the ice, and determine ideal landing spots (not too thin, or the reactor will continue to be critical as it finishes melting, and kill whatever is in the general vicinity of it's landing spot when it hits the bottom).
Firstly, I started from the concept of the Nuclear Thermal Turbo rocket, an idea for a Air Augmented Nuclear Thermal Rocket, that uses air-breathing propulsion to lofts itself out of the lower atmosphere, transitioning from a ramjet type of system (powered by a turbofan spun by gasses leached from the reactor chamber), then to a scramjet (using the air directly by heating, and leaving the fanblades at idle angles of attack), then finally using standard NTR-type propulsion for the final ascent and orbital circularisation.
More on the ideea (and it's creator, John Bucknell) here:
https://forum.nasaspaceflight.com/index ... c=43344.20
http://www.projectrho.com/public_html/r ... rmal_Turbo
The kicker is the quite phenomenal promised (and I do stress "promised") mass fractions, that can be seen in the tables presented. This is due to the high average Isp of the thing (as air-breathing systems have Isp's in the thousands generally).
I thought, "great, but that pure rocket Isp is killing me. Looks so weak compared to the rest". So I started digging for more experimental types of NTR's. I stumbled onto MITEE, which uses a hybrid power system to dissociate some of the hydrogen into it's monoatomic state, thus achieving Isp of about 1350. There was also the NTER, the european concept that used hot gasses to drive what is essentially an electrothermal thruster.
But... then I realised that, since I already have a nuclear reactor onboard, how about going even more speculative...and mate it to this:
http://www.projectrho.com/public_html/r ... d-core_NTR
If it works (think I said that about half a dozen times) then I could literally get to Titan with an SSTO. And back (with the payload fractions in LEO plus a specific impulse of about 17,000 sec from the pulsed regime).
Finally, since I've got copious amounts of power onboard, why not use a more... exotic system to aerobrake for the return trip? Like the magshell:
https://forum.nasaspaceflight.com/index ... c=29912.80
https://selenianboondocks.com/2010/02/m ... rocapture/
There are MONSTROUS issues with the above concept:
1) making the turbo-fan airbreathing mode work (supersonic ducted fans are not really a thing).
2) transitioning between modes
3) getting rid of the excess heat from the pulsed NTR mode (radiators aplenty).
My initial vehicle would be almost entirely fuel, but that can be aleviated if one uses ISRU (pump the atmosphere of wherever it is you be landing into your tanks; NTR's don't care much what they propell out the nozzle as long as it doesn't gum up the works; water vapour, methane, ammonia, H2 produced by electrolisys and cracking, etc) and magnetoshell aerobraking.
Or you can split the mission, and use a Starship-esque upper stage that has the pulsed unit, along with a simpler airbreathing mode and a pumping station.
Monstrously complex, but, as you might know, I like thinking ludicrous...
Another, simpler mission, is to use a molten salt thorium reactor to melt my way into Europa. The thorium would produce U233, which fissions and gives off neutrons... but also U232, which fissions into Th (Thallium) 228, which is a HARD gamma emitter (at 2.67 Mev, it's the third most potent one known). The reactor and the submersible that follows it down would be bathed in hard gamma, thus resolving the issue of how to sterilise the craft (the most complicated problem in this type of Europa mission). the reactor would be fueled so that it's no longer critical by the time it finishes melting, and just plunks into the abyss. The submersible could have it's own, separate, reactor or RTG to do long duration surveys (and would transmit data to the lander on the surface by optical cable). Before landing, the craft could use high-power ground penetrating radar (remember, plenty of power available) to map the ice, and determine ideal landing spots (not too thin, or the reactor will continue to be critical as it finishes melting, and kill whatever is in the general vicinity of it's landing spot when it hits the bottom).
Re: The "Real Aerospace" Thread
The basic problem with any nuclear-powered aerospace vehicles is that such vehicles inevitably crash. A nuclear powerplant turns a routinely tragic loss of life and property into a full scale disaster.
As for the other proposal, it seems to me that the only reason to explore below the ice on Europa is to search for life, and I don't see how a probe can do that if it's radioactive.
As for the other proposal, it seems to me that the only reason to explore below the ice on Europa is to search for life, and I don't see how a probe can do that if it's radioactive.
Re: The "Real Aerospace" Thread
Well, it IS an idea... never said it was a good one
.
To quote Bucknell himself:
"Exploding is not a thing with a monopropellant rocket (how often do aircraft explode?). As for fuel containment/reactor structure – if the rocket were to have a RUD event the core/shielding is largely metallic with a tungsten carbide gamma shield (six of the seven tons of core mass). It would put a large dent in anything it landed on, but that is a risk with any rocket (ie why launches are over water).
The radiation shielding included in the rocket is probably overkill – it keeps exposure to both payloads and launch site below terrestrial background radiation (0.2 rad/y) if less than five flights a year are flown. Gamma shielding is fully surrounding the reactor (tungsten carbide), and lithium hydride for neutron fore for payload protection. Details in the paper."
For details: https://www.nextbigfuture.com/2017/07/j ... tions.html
As for the Europa mission, the reactor is just a big dumb hulking piece of red-hot metal. Could be molten salt, but some sort of pebble bed might be better. It's physically separated from the submersible trailing behind it (wich can unspool a lenght of optical as it goes down. The ice refreezes as the melter/sub pair go deeper). Once the melter melts it's way through, it drops (it would be heavy) and sinks, but the submersible is then free to roam about, away from the sinking neon-blue metallic deathball. Radiation is probably sufficient to kill stuff within tens of meters of where the thing lands on the bottom (some tens to hundreds of km from the hole in the ice is, depending on depth), but water is decent at shielding from gamma rays (massive enough). The submarine, as stated, is independent, and has it's own power source. The interior should be sterilised by the on-board source plus whatever gamma makes it in while it is near the melter as they both descend. The exterior is sterilised by the far more powerful rays from the gamma (needs a vehicle which exposes it's surface as much as possible to do that efficiently).
![Razz :P](./images/smilies/icon_razz.gif)
To quote Bucknell himself:
"Exploding is not a thing with a monopropellant rocket (how often do aircraft explode?). As for fuel containment/reactor structure – if the rocket were to have a RUD event the core/shielding is largely metallic with a tungsten carbide gamma shield (six of the seven tons of core mass). It would put a large dent in anything it landed on, but that is a risk with any rocket (ie why launches are over water).
The radiation shielding included in the rocket is probably overkill – it keeps exposure to both payloads and launch site below terrestrial background radiation (0.2 rad/y) if less than five flights a year are flown. Gamma shielding is fully surrounding the reactor (tungsten carbide), and lithium hydride for neutron fore for payload protection. Details in the paper."
For details: https://www.nextbigfuture.com/2017/07/j ... tions.html
As for the Europa mission, the reactor is just a big dumb hulking piece of red-hot metal. Could be molten salt, but some sort of pebble bed might be better. It's physically separated from the submersible trailing behind it (wich can unspool a lenght of optical as it goes down. The ice refreezes as the melter/sub pair go deeper). Once the melter melts it's way through, it drops (it would be heavy) and sinks, but the submersible is then free to roam about, away from the sinking neon-blue metallic deathball. Radiation is probably sufficient to kill stuff within tens of meters of where the thing lands on the bottom (some tens to hundreds of km from the hole in the ice is, depending on depth), but water is decent at shielding from gamma rays (massive enough). The submarine, as stated, is independent, and has it's own power source. The interior should be sterilised by the on-board source plus whatever gamma makes it in while it is near the melter as they both descend. The exterior is sterilised by the far more powerful rays from the gamma (needs a vehicle which exposes it's surface as much as possible to do that efficiently).
Re: The "Real Aerospace" Thread
NTRs are actually heavily reliant on having liquid hydrogen. Water drops you down to barely better than hydrolox chemical, while leaving you with the poor mass ratios and T/W of nuclear systems, and requiring you to deal with an oxidizing working fluid. High temperature CO2's also oxidizing, and well into chemical rocket performance territory. Methane's not oxidizing, but decomposes at the temperatures required for decent performance. Ammonia's the most workable, but least available (basically only Mars, Titan, Earth, and stuff out at the distance of Pluto and Neptune have enough nitrogen) and still involves throwing away most of your performance. You're realistically stuck with liquid hydrogen and its various problems.
As for air breathing: the rocket equation is: deltaV = Ve*ln(Mo/Mf).
Augmenting this for air breathing gives: deltaV = (Ve/2)*ln(Mo/Mf)/(1 + 1/(L/D*A/g)).
The high specific impulse of air breathing engines is exaggerated by a factor of 2 to begin with, in short because it's only that high at low speeds and falls off roughly in inverse proportion to speed. On top of this, you need both a good hypersonic L/D ratio and good acceleration (high T/W) or you lose most of it to increased drag losses. Air breathers and nuclear engines both have poor T/W ratios, and hypersonic L/D ratios are never that great and require compromises in vehicle geometry. All this means that the apparent gains of air breathing are nowhere near as significant as the specific impulse number makes it appear, and it involves a major increase in complexity and dry mass.
The main drivers of cost are complexity and dry mass. Propellant cost is in the rounding error. Staging allows for enormous increases in payload fraction and allows separating the propulsion and structural requirements for the two very different flight regimes encountered, while adding little cost and complexity. Adding on lots of complexity and dry mass...exceptionally expensive dry mass containing fission power systems and heavily optimized to minimize mass to orbit...with the goal of reducing propellant mass and eliminating staging is rather misguided.
As for air breathing: the rocket equation is: deltaV = Ve*ln(Mo/Mf).
Augmenting this for air breathing gives: deltaV = (Ve/2)*ln(Mo/Mf)/(1 + 1/(L/D*A/g)).
The high specific impulse of air breathing engines is exaggerated by a factor of 2 to begin with, in short because it's only that high at low speeds and falls off roughly in inverse proportion to speed. On top of this, you need both a good hypersonic L/D ratio and good acceleration (high T/W) or you lose most of it to increased drag losses. Air breathers and nuclear engines both have poor T/W ratios, and hypersonic L/D ratios are never that great and require compromises in vehicle geometry. All this means that the apparent gains of air breathing are nowhere near as significant as the specific impulse number makes it appear, and it involves a major increase in complexity and dry mass.
The main drivers of cost are complexity and dry mass. Propellant cost is in the rounding error. Staging allows for enormous increases in payload fraction and allows separating the propulsion and structural requirements for the two very different flight regimes encountered, while adding little cost and complexity. Adding on lots of complexity and dry mass...exceptionally expensive dry mass containing fission power systems and heavily optimized to minimize mass to orbit...with the goal of reducing propellant mass and eliminating staging is rather misguided.
Re: The "Real Aerospace" Thread
Just for fun:
I don't know if some of you are also reading the Freefall webcomic.
The creator published todays comic as a short notice to explain that he made an error.
![Image](http://freefall.purrsia.com/ff3300/fc03261.png)
In the link to the powererror, simply states
I don't know if some of you are also reading the Freefall webcomic.
The creator published todays comic as a short notice to explain that he made an error.
![Image](http://freefall.purrsia.com/ff3300/fc03261.png)
In the link to the powererror, simply states
Yes, indeed. While cartoonists and Sci-Fi authors are necessary to make us look forward and to the stars and see the possibilities, we still need engineers to make it happen.http://freefall.purrsia.com/ff/powererror.txt wrote:This is why NASA uses real engineers instead of cartoonists for their rocket equations.
Re: The "Real Aerospace" Thread
hi hi
It's been a while since I read Freefall. Not since that really awkward moment on the forums where the author basically said a thread about having intercourse with domestic canines was more on topic than a thread about civil rights in the context of AI having to prove that they're worthy of civil rights. (which, at the time, seemed relevant to the comic.)
It's been a while since I read Freefall. Not since that really awkward moment on the forums where the author basically said a thread about having intercourse with domestic canines was more on topic than a thread about civil rights in the context of AI having to prove that they're worthy of civil rights. (which, at the time, seemed relevant to the comic.)
Re: The "Real Aerospace" Thread
Hmmm... I've never been on the forums. Never found the link on the comic's page.
So I presumed there is no official forum...
But that indeed sounds weird.
The question of dog/human relations may be interesting, especially since Winston/Florence is a thing.
But the AI-rights is a continuous relevant topic for this story too.
It's allabout artificial intelligence rights...
So I presumed there is no official forum...
But that indeed sounds weird.
The question of dog/human relations may be interesting, especially since Winston/Florence is a thing.
But the AI-rights is a continuous relevant topic for this story too.
It's allabout artificial intelligence rights...
Re: The "Real Aerospace" Thread
So...is Bucknell wrong? :Mjolnir wrote: The high specific impulse of air breathing engines is exaggerated by a factor of 2 to begin with, in short because it's only that high at low speeds and falls off roughly in inverse proportion to speed. On top of this, you need both a good hypersonic L/D ratio and good acceleration (high T/W) or you lose most of it to increased drag losses. Air breathers and nuclear engines both have poor T/W ratios, and hypersonic L/D ratios are never that great and require compromises in vehicle geometry. All this means that the apparent gains of air breathing are nowhere near as significant as the specific impulse number makes it appear, and it involves a major increase in complexity and dry mass.
http://www.projectrho.com/public_html/r ... nttr12.jpg
http://www.projectrho.com/public_html/r ... nttr13.jpg
I'm an aero engineer, but avionics is my specialty. Aerodynamics is something I never felt I could fully grasp. My understanding is that, for airbreathers, ISP is dependent upon exhaust velocity and mass fraction used for fuel. In an atmosphere, since an SSTO uses no fuel, it should have far higher ISP. Eventually, there comes a point where the vehicle can no longer accelerate, due to drag. Where that point is depends on many factors, such as materials used, design, optimal regime, etc. The idea seems sound enough: use no propellant while getting to an altitude that gets you above 80 percent of the atmosphere (which doesn't have to be that fast; your burn time is not dependant on what you have onboard, but by the materials of the reactor), then tilt the thing and try to accelerate (using an engine with higher Isp than conventional ones), at first in feeble airbreathing mode, then in NTR rocket mode. This study gives an airbreathing SCRAMJET isp of about 1000 (which is neither low altitude, nor low velocity):
http://mragheb.com/NPRE%20402%20ME%2040 ... ulsion.pdf
I do think he's optimistic about his transition. A Mach 15 airbreather is no joke
As for the propellant, I was simply pointing out that it could be done. While a lower ISP is not very good, with, say, nitrogen, it WILL still allow you to reach orbit of Mars or Titan. Also, just crack the hydrogen out before launch, or let the molecules dissociate during engine run. This paper cites the ISP numbers assuming some dissociation:
https://ntrs.nasa.gov/archive/nasa/casi ... 001880.pdf
Re: The "Real Aerospace" Thread
Bucknell's engine sounds incredibly complex and difficult to design and the airbreathing feature seems a bizarre addition to something that flies a vertical trajectory. Getting above the atmosphere isn't the hard part of getting to orbit. Atmospheric drag losses tend to be ~100 m/s. The Saturn V only lost 40 m/s to aerodynamic drag. Gravity losses are an order of magnitude worse, but still an order magnitude short of the delta-v required to reach orbit. Air breathing can't make a really major difference without atmospheric flight at speeds that produce major heating and structural problems. It's really only relevant to SSTO vehicles that can barely get to orbit, and turning them into TSTO systems with a booster is a much simpler way of improving their economics.Mr.Tucker wrote:So...is Bucknell wrong? :Mjolnir wrote: The high specific impulse of air breathing engines is exaggerated by a factor of 2 to begin with, in short because it's only that high at low speeds and falls off roughly in inverse proportion to speed. On top of this, you need both a good hypersonic L/D ratio and good acceleration (high T/W) or you lose most of it to increased drag losses. Air breathers and nuclear engines both have poor T/W ratios, and hypersonic L/D ratios are never that great and require compromises in vehicle geometry. All this means that the apparent gains of air breathing are nowhere near as significant as the specific impulse number makes it appear, and it involves a major increase in complexity and dry mass.
http://www.projectrho.com/public_html/r ... nttr12.jpg
http://www.projectrho.com/public_html/r ... nttr13.jpg
I'm an aero engineer, but avionics is my specialty. Aerodynamics is something I never felt I could fully grasp. My understanding is that, for airbreathers, ISP is dependent upon exhaust velocity and mass fraction used for fuel. In an atmosphere, since an SSTO uses no fuel, it should have far higher ISP. Eventually, there comes a point where the vehicle can no longer accelerate, due to drag. Where that point is depends on many factors, such as materials used, design, optimal regime, etc. The idea seems sound enough: use no propellant while getting to an altitude that gets you above 80 percent of the atmosphere (which doesn't have to be that fast; your burn time is not dependant on what you have onboard, but by the materials of the reactor), then tilt the thing and try to accelerate (using an engine with higher Isp than conventional ones), at first in feeble airbreathing mode, then in NTR rocket mode. This study gives an airbreathing SCRAMJET isp of about 1000 (which is neither low altitude, nor low velocity):
http://mragheb.com/NPRE%20402%20ME%2040 ... ulsion.pdf
I do think he's optimistic about his transition. A Mach 15 airbreather is no joke
"Mr. Bucknell notes that the only practical method of dramatically bringing down the cost of boosting payloads into low Earth orbit (LEO) is to lower investment and realize a large return on that investment. The implication is you want a low dry mass Single Stage to Orbit Resuable Launch Vehicle with a high payload mass fraction."
Bucknell's observation is correct, but the conclusion is bonkers. To maximize RoI, the last thing you want is a SSTO vehicle where the entire dry mass of the vehicle has to be highly mass-optimized and where you carry huge amounts of excess propulsion and extra hardware all the way to orbit instead of paying payload. You get far better return on your investment with staging. And you really do not want the immense costs of building and operating a nuclear system that operates 5 times a year, as Bucknell proposed when discussing the radiation issues...and his estimates on those seem really low.
Bucknell sounds like a propulsion guy who sees everything in terms of a propulsion problem. He's concocted a supertech propulsion system to solve the problem of space lift, but it's not a propulsion problem. He should think more on why Skylon failed...note that Falcon 9 is already cheaper than Skylon would have been able to achieve, despite not using anything like the list of exotic ultra-high technologies needed for Skylon. Reaction Engines doesn't even mention Skylon on their website any more, just vague mentions of the possibility of using SABRE for launch systems, including staged systems.
The most credible attempts at reusable launch vehicles now are using methane fuel, which doesn't even match kerosene in impulse density and falls well short of hydrogen in specific impulse. It's used because it has other advantages that outweigh its lower performance as a propellant. Insufficient specific impulse isn't what's made space expensive, that's been due to high operational complexity, low flight rates, and building-sized pieces of disposable aerospace-grade hardware.
If you're going to the trouble of involving a nuclear reactor, you want better return than "it'll allow you to reach orbit of Mars or Titan". You don't even need an especially high performance (and vastly cheaper and easier to operate) chemical propellant system to do that.Mr.Tucker wrote:As for the propellant, I was simply pointing out that it could be done. While a lower ISP is not very good, with, say, nitrogen, it WILL still allow you to reach orbit of Mars or Titan. Also, just crack the hydrogen out before launch, or let the molecules dissociate during engine run. This paper cites the ISP numbers assuming some dissociation:
https://ntrs.nasa.gov/archive/nasa/casi ... 001880.pdf
You're much better off devoting the same nuclear materials to cracking chemical propellants. Think in RoI terms again: you can have those materials working full time producing propellant for a fleet of vehicles, instead of spending most of their time coasting through deep space while hauling big blocks of radiation shielding around along with their payloads.
Re: The "Real Aerospace" Thread
Unfortunately, the Beresheet lunar probe crashed. It had some kind of fault that shut down the engine during descent. They were apparently able to restart it, but it was unable to recover at that point and impacted at around 1 km/s. They did at least get some good images.
http://www.planetary.org/blogs/jason-da ... ashes.html
In better news, SpaceX did their first operational launch of Falcon Heavy, putting the payload in its target orbit and recovering all three first stage cores.
http://www.planetary.org/blogs/jason-da ... ashes.html
In better news, SpaceX did their first operational launch of Falcon Heavy, putting the payload in its target orbit and recovering all three first stage cores.
Re: The "Real Aerospace" Thread
A reminder that spaceflight is still hard, despite SpaceX making it look easy.
Re: The "Real Aerospace" Thread
Watch their blooper reel and remember when everyone was suggesting parachutes and airbags, big nets, giant robot arms, physically improbable cable arrangements...Arioch wrote:A reminder that spaceflight is still hard, despite SpaceX making it look easy.
It's interesting how quickly they went from not being able to land a rocket to being able to reliably plant them in the middle of a drone ship at sea (though they're still not confident of doing so at the edges of the envelope...the next Heavy launch was scheduled to use a different center core, in case they lost this one). Which illustrates what you said...there were a lot of little things they had to get just right for it to work.
Re: The "Real Aerospace" Thread
I remember. And it says something about SpaceX that they have the failure reel on their own YouTube channel.Mjolnir wrote:Watch their blooper reel and remember when everyone was suggesting parachutes and airbags, big nets, giant robot arms, physically improbable cable arrangements...
It's interesting how quickly they went from not being able to land a rocket to being able to reliably plant them in the middle of a drone ship at sea (though they're still not confident of doing so at the edges of the envelope...the next Heavy launch was scheduled to use a different center core, in case they lost this one). Which illustrates what you said...there were a lot of little things they had to get just right for it to work.
I expect their booster recoveries will always have a certain failure rate. There's a lot that can go wrong with a pitching unmanned platform in the middle of the storm-prone Atlantic. There's probably a point past which trying to raise the success rate above a certain percentage will cost more than the boosters they lose.
But I never get tired of watching the boosters land. And seeing the two Falcon Heavy boosters land side by side is just... poetry.
- Mithramuse
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Re: The "Real Aerospace" Thread
This. Just so absolutely this.Arioch wrote:But I never get tired of watching the boosters land. And seeing the two Falcon Heavy boosters land side by side is just... poetry.
Also, why the #%&^ weren't we doing this a whole lot earlier... though yeah, I know, the Big Gov't Contractors just don't have any financial incentive to innovate in this way.
Re: The "Real Aerospace" Thread
hi hi
It's a shame we don't have hard numbers on how much it is costing Space X to inspect and refit their reused boosters because as a privately owned company they're completely opaque, but I think it is a safe bet that they're not launching rockets for 10% of the cost like some of Musk's earlier predictions. It's probably closer to more conservative estimates of 70-90% the costs. But it's anyone's guess how much of their current pricing system is profit margin, subsidy, and reduced cost.
We've done reusable space vehicles before, and the cost savings have never been great.Mithramuse wrote:Also, why the #%&^ weren't we doing this a whole lot earlier
It's a shame we don't have hard numbers on how much it is costing Space X to inspect and refit their reused boosters because as a privately owned company they're completely opaque, but I think it is a safe bet that they're not launching rockets for 10% of the cost like some of Musk's earlier predictions. It's probably closer to more conservative estimates of 70-90% the costs. But it's anyone's guess how much of their current pricing system is profit margin, subsidy, and reduced cost.