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#76 Re: Interplanetary transportation » A revival of interest in Nuclear Thermal Propulsion? » 2021-04-14 06:59:59
The nuclear light bulb designs had exhaust streams mostly free of radioactivity, and a more limited specific impulse. It was said to be about 1300 s Isp. Startup was fairly quick, but shutdown slow (faster than solid core). These were somewhat heavier, but still far lighter than solid core. You did not lose your core overboard at shutdown like open cycle. But it does take time and a lot of energy to recreate the fireball.
GW
NASA reference design has a predicted Isp of 1875 s
https://ntrs.nasa.gov/api/citations/196 … 012569.pdf
It has a chamber pressure of 500 atmospheres: what perplexed me is how to maintain the perfect equilibrium between the pressure of the propellant inside the chamber and the fuel inside the nuclear light-bulbs during the start-up, the thrust phase and the cool-down, when it takes only one atmosphere of difference to have the bulbs exploded or imploded.
#77 Re: Human missions » Starship is Go... » 2021-04-13 16:37:39
Quaoar:
I suppose the answer to your "windscreen necessity" question is "it depends".
It depends on what kind of craft and how you expect to fly it. If it is a manned spaceplane like space shuttle or Dreamchaser, the pilot really needs a way to see outside to land the thing like an airplane.
Automated landings are possible (see X-37B), but you'll notice the airlines do NOT choose to use that technology. Most of their passengers would not fly, if they ever thought the pilot couldn't see to fly the plane, or wasn't flying the plane because some idiot computer was flying it.
In the case of Spacex's passenger-carrying Starship, the landing is automated. The windows are for the entertainment and sanity of the passengers. Sightseeing out the windows really is psychologically important for passengers. Quite different from trained crew.
GW
But it would be possible to study some kind of solution like false windscreens that are plasma monitors connected to external cameras, or glass windscreen that are covered by PICA-X protected panels during the atmospheric entry, or a glass panoramic module for observation that is jettisoned before the entry.
#78 Re: Human missions » Going Solar...the best solution for Mars. » 2021-04-13 14:49:13
Louis,
I may understand your aversion for nuclear on Earth, where an accident can contaminate a region and make a lot of damage like in Fukushima, but on Mars, where there is no air to pollute there is no problem: you just need to put your reactor far from the habitat and the buried glacier you use as a source of water.
If you want to colonize the space, we cannot go too far with chemical rockets: we need NTRs and we need to evolve them from the solid core to the more advanced gas core, so in a future we can go to the asteroid belt and the moons of Jupiter.
#79 Re: Not So Free Chat » Dinosaurs » 2021-04-13 14:36:27
That is something which we can do with as you noted the next best match and cloning it is possible to bring the extinct back from the dead.
It is called deextinction:
The foal in the picture is a young quagga, a South African equine who is back after become extinct 100 years ago.
#80 Re: Exploration to Settlement Creation » Settlement design » 2021-04-13 14:26:57
So again, I'm suggesting a science/exploration mission using Mars Direct. That means just 4 crew for the first missions. Construction of the first permanent base would have 12 crew. Once they finish their own base, they would build a larger facility for 100 crew. That would prepare for the first SpaceX Starship. Those first 100 would then build even larger accommodations for next 1,000 settlers.
4 crew for an almost two year stay on Mars means a lot of cross training capabilities: you need 2 pilot engineers for flying the MAV/ERV and two doctors able to perform basic surgery, orthopedic, because you cannot risk to lose an astronaut for a stupid appendicitis or a broken leg, and possibly even dentistry. So the doctors must be cross-trained in basic research in biology to study possible martian microbial biota and engineers must be cross trained in geology and geochemistry. You can do it but a six crew has far better chance to survive.
#81 Re: Human missions » Starship concrete Mars landing pad » 2021-04-13 13:33:19
They won't be landing on buried glaciers. They've already identified potential sites that have hard rock landing areas with water ice buried quite close to the surface within a few kms.
That's quite better, but they need at least two reliable rovers with a big tank and all the stuff to dig, melt the ice and pump the water, which means years of R&D. So why not to start to develop the rover in parallel with the Starship?
If they don't address all these secondary issues, after completing the Starship they risk to wait another 10-20 years to develop all the right stuff to get the return propellant.
#82 Re: Human missions » Starship is Go... » 2021-04-13 09:58:08
Starship crew/passenger versions have windows planned for the lee-side dorsal surface near the nose. These are at risk for the same phenomena that limited angle of attack to a very narrow range on Shuttle. I dunno; we'll see.
X-37b does not suffer from this risk: there is no windscreen to be vulnerable. Dreamchaser would be vulnerable; it has a windscreen.
.
Just a question about windscreens: are they really necessary? Why not to put some little cameras connected to monitors?
P.S.
I remember my travel to Milan with the Covid Team with a military plane: the fuselage had not windscreens. Initially it seemed to me quite claustrophobic, but when the plain went up and down I didn't notice any discomfort to my ears, unlike the civil planes which have plenty of windscreens. I deduced that windscreens are not perfectly sealed and there is always some air leakage.
#83 Re: Civilization and Culture » Pro-natalism - what can we learn from Israel? » 2021-04-13 08:46:06
Why wouldn't a Mars community be able to expand life support? Makes no sense to me, though it might be an interesting plot device!
You can expand the life support but it will always have a limit. If the birth rate exceeds the rate of expansion of the life support, the legislators will be forced to intervene.
If you add to this the sad fact that people in command are usually very willing to control and regulate the life of the other people, it would be unlikely they would not overuse the new formidable power tools that a closed ecosystem give them, easily turning a libertarian dream in an orwellian nightmare.
P.S. I wrote a novel about a 3 million people closed cycle building where only the firstborns inherit the right to marry and procreate from their parents. It was not set on Mars but in a post atomic Italy (if you go outside the building you will be eat by cannibals). When I have time I'll translate it in English.
#84 Re: Interplanetary transportation » A revival of interest in Nuclear Thermal Propulsion? » 2021-04-13 08:06:43
There is an oft-forgotten characteristic of solid-core nuclear thermal devices that influences very strongly when, and when not, to use them. The startup transient is not as fast as chemical propulsion, but the real problem is the long slow shutdown to preserve core cooling after you cut the nuclear power. That makes this type of propulsion infeasibly inappropriate for short, sharp burn requirements , like course corrections.
GW
In fact, the never built NTR Copernicus used the RCS rockets for course correction and is forced to store tons of NTO-MMH even if its main NTR rockets have an Isp of 920 s.
#85 Re: Civilization and Culture » Pro-natalism - what can we learn from Israel? » 2021-04-12 11:37:16
I'm sorry to have noted this very interesting topic a bit later.
I've some question to discuss about birth rate in a martian colony where the life support system can sustain a limited number of people.
We like to imagine our colonies as brave free people settlements like the Galt's Gulch of Atlas Shrugged. But if our Mars Town life support can keep alive 1000 people and no one more, the birth rate will be very likely regulated by law: every time an old colonist die, a couple of young colonist will get the permission to procreate, to maintain constant population. If new domes are in construction, the colonists will get the permission to generate the right number of child to populate them, but no one more. And probably it will be also implemented some mechanism to equate males and females (i.e. artificial insemination with X or Y spermatozoa).
I know it seems very orwellian and dystopic, but what if air and food are not enough for all?
#86 Re: Interplanetary transportation » A revival of interest in Nuclear Thermal Propulsion? » 2021-04-12 10:58:43
A nuclear powered lower stage for a super booster taking off from and landing in the ocean, would be interesting idea. The booster could use water as propellant. Propellant temperature and exhaust velocity are less important in this application, as propulsive efficiency is poor at submach speeds and expansion ratio is poor at sea level. Of more importance is thrust and acceleration. The booster needs to push the upper stage into the stratosphere with enough residual energy to counteract gravity losses as the upper stage accelerates to orbital velocity.
So we need a cheap thrust producer that ascends vertically, reaching an altitude of perhaps 60,000' before separating the upper stage at a velocity of about 1km/s. It then descends and lands roughly where it started from. If the landing is uncontrolled and it breaks up, then the nuclear components remains intact and sink into the ocean. The ocean provides a heat sink for decay heat removal and and radiation shield between flights.
The use of a heat exchanger may in fact be possible in this more limited application. The reactor would most likely be a lead cooled fast reactor using triso fuel. Water could be heated in steam generators to ~1000°C before entering the expansion nozzle. Total delta-V - about 2km/s, some of which would be reserved for controlled landing.
First stage needs a strong thrust rather than high exhaust velocity, so chemical propellants like LOX-RP1 or LOX-CH4 are perfect, considering that a hydrocarbon chemical rocket has a thrust/weight ratio far greater than a NTR.
A NTR makes sense for the second stage, which needs acceleration lower than 1 gee (typically 4-5 m/s2), where an exhaust velocity of 9-10 km/s consents a lower mass ratio, even with a heavier NTR and a bigger hydrogen tank, increasing the payload fraction.
And there are also some problems in using a nuclear first stage:
1) in case of incident all the launch facility would be contaminate
2) neutron scattering due to the dense atmosphere would hit the astronauts even if the rockets has a shadow shield
#87 Re: Human missions » Going Solar...the best solution for Mars. » 2021-04-12 07:35:39
I stopped reading that nonsense as soon as it stated its calculations were based on 10% efficient solar panels. 10%? What century was that written?
Bog standard panels are 15-20% efficient these days. And clearly, on Mars, cost is of little concern, so Space X are going to go higher. I think 25% is a reasonable working figure, although efficiency, certainly in the early stages, could be as high as 30%.
Another factor to bear in mind is that because of Mars's wobble, the optimal zone for PV production extends up to about 30 degrees north. So don't assume being at that latitude is the same as being at that latitude on Earth.
The rest of your analysis is totally flawed. Living standards across the world have risen hugely since conventional oil production past its peak. The cost of oil in fact is about the same it was 50 years ago. The societies with the highest green energy production are among the most prosperous on Earth, not the least.
With multi-junction technology solar panel can even reach 45% of efficiency, but what happens if a sandstorm darken the sun two months?
You need a backup nuclear reactor anyway.
#88 Re: Interplanetary transportation » A revival of interest in Nuclear Thermal Propulsion? » 2021-04-12 04:22:15
I agree to GW: if we want to fly now, the only way is to build the old NERVA. But if the problem is to develop a more advanced evolution without contaminate the environment, I suggest the vapor core option,
https://www.google.com/url?sa=t&rct=j&q … LYs2231OZ0
where uranium tetrafluoride vapors are sealed inside carbon composite cells, which are cooled by hydrogen propellant. In this way the propellant never touch the fuel, like in the solid-core and in gas-core NTR designs, and the exhaust is completely free of radioactivity. This hybrid approach can reach an Isp up to 1200 s.
Probably it will take less years and dollars to develop a vapor core on Earth than to build a moon base (where on the other hand we can develop a more advanced and performing gas-core).
#89 Re: Human missions » Starship is Go... » 2021-04-09 16:05:32
Thanks
#90 Re: Unmanned probes » Official MSL / Curiosity Rover Thread | Aug 5, 2012 10:31 p.m. PT » 2021-04-09 07:27:32
Carbon composite would be even worse than aluminum. The failure stresses are even lower, and the elongation-to-failure very far lower.
Driving in the rocks with non-inflatable wheels requires high tensile strength and large elongation-to-failure. Period. End-of-issue. You also need strength when soaked out to extreme cold.
As I said, there is no point sending a rover to Mars with a power system lifetime of multiple decades, and wheels that are useless in a couple of years. Yet NASA has now done this mistake twice.
The material of choice for this is a 300-series stainless steel. There is no other choice available. I would recommend 304L, myself. A real materials guy might have something even better, but odds are, it'll be a 300-series stainless.
Stupid is, as stupid does.
GW
Which is the weight difference between an aluminium wheel and a steel wheel of the same size?
#91 Re: Interplanetary transportation » Ballistic Delivery of Supplies to Mars: Lithobraking » 2021-04-09 06:05:21
TH ref 179 above, "sail" the atmosphere question: we have long done exactly that, since Gemini in the mid 1960's.
The amount of turning you can get depends upon the max side force to drag force ratio you can achieve, and that depends upon the shape of the entering body. It is only available as an effective item during the hypersonics, where the aerodynamics forces far exceed the weight force.
Blunt capsules have a peak force ratio on the order of 0.1 or less. The more it looks like an airplane (space shuttle or X-37b), the higher that number can be, perhaps near or slightly above 0.5. In no case is it sufficient to turn a steeply-entering trajectory into a survivable shallowly-entering trajectory.
I would point out that this effect is inherent to the entry designs for Spacex's Starship at Earth, and at Mars. It's in the simulations they have shown at times to the public. Early in the entry, where speeds are comparable to escape, the 60 degree angle of attack (between body axis and wind vector) is oriented to lift downward. The downlift helps prevent bouncing off the atmosphere. Later in the hypersonics, the lift vector is upward, once velocity is comparable to, or under, orbit speed. The uplift keeps the trajectory from bending downward too quickly. This flattens and extends the ride. Only once out of hypersonics do the two simulations (Earth and Mars) differ, and quite strongly.
That same lift control was used on Gemini and Apollo, and by Space Shuttle, and now X-37b. I think Dragon uses it, based on the asymmetries of the burn patterns along the lateral sides. No doubt CST and Orion will use it.
GW
If SpaceX Starship is used for an Earth orbital fly like the old Space Shuttle, would it reenter with the AOA lifting upward?
#92 Re: Exploration to Settlement Creation » Settlement design » 2021-04-09 03:10:24
Micrometeoroids do not survive Mars atmosphere. On Earth they burn up 100km above the surface, on Mars 30km. It doesn't matter how many kilometres, they still burn up. The Moon has to worry about micrometeoroids, Mars has to worry about dust storms.
May be I'm wrong in calling them "micro", but some kind of meteoroid big enough to survive the atmosphere heating can reach Mars surface and a regolith bag on a steel frame might be useful to protect the habitat, like the sand bags used by soldiers.
For the dust storms, which may obscure the sun for more than a month, the only solution is a backup nuclear reactor.
#93 Re: Exploration to Settlement Creation » Settlement design » 2021-04-08 17:12:22
I would agree that Bigelow-style inflatables won't become a permanent dominating feature of the Mars settlement but they will be v useful on Mission One I suspect. As for additional radiation shielding that can be provided by having a separate steel frame that can be assembled over the inflatable and filled with regolith or water (including along the sides) so that the inflatable has good protection from radiation events.
Nice idea: a steel frame holding regolith bags. It seems good for radiation shielding but even for micro-meteoroid impact
#94 Re: Human missions » Starship concrete Mars landing pad » 2021-04-04 08:12:08
The landing pad is only one of the two thing SpaceX needs to be successful: the other is a buried glacier to make the retun propellat.
Is a buried glacier sufficiently stable for landing?
If the answer is "yes", what will happen to the starship when the astronauts melt the ice under her to make the return propellant?
If the answer is "no", it is possible to make a concrete landing pad on a buried glacier, then melt the ice to get water without cracking the concrete?
Why not for SpaceX, once the landing issues are solved, to test the prototypes stability on terrains similar to Mars surface?
#95 Unmanned probes » Venus sample return mission » 2021-04-02 02:30:33
- Quaoar
- Replies: 2
Venus is quite a forbidden planet: it has a surface atmospheric pressure of 9.3 MPa (93 bar), an atmospheric density of 67 kg/m3 and a surface temperature of 740 K.
If in a near future we will be able to build a rover for this prohibitive environment, we will probably also want to retrieve some sample.
Given we have found a way to store propellants at the right temperature, I suppose rockets would have a very poor performance at 9.3 MPa of ambient pressure (It's right, GW?).
So a good strategy might be to reach the upper atmosphere with a helium balloon, then fire the rocket.
#96 Re: Planetary transportation » Alternative fuel aircraft » 2021-04-01 16:34:12
In a future dominated by green economy we can imagine hybrid electro-turboprop airplanes: with superconductive electric motors driven by hydrogen fuel cells. LH2 cools the superconductive coils of the stator, then is burned in the combustion cells generating electricity and the exhaust is recuperated by a turbine mechanically connected to the propeller, like a conventional turboprop (or by a turbo-alternator) giving extra power, with an overall efficiency near to 80%.
#97 Re: Interplanetary transportation » Interplanetary Sailing Using Laser Beams Luna Phobos » 2021-04-01 16:19:05
For Quaoar re #15
Thank you for reporting your research on laser ablative propulsion.
I am most interested in practical applications of advanced ideas, while at the same time appreciating and applauding theoretical contemplations.
In the present instance, with a constant focus on the title of this topic, I am inspired by your post to imagine a configuration of ice in a cylindrical housing, mounted in a rail on the Moon. In this forum, Calliban in particular has written about magnetic rail launch from the Moon. I like this idea, and hope that it comes to pass some day. However, on Earth, no one (in any Nation to the best of my knowledge) has successfully accelerated ** anything ** using magnetic acceleration over a distance greater than the length of an aircraft carrier.
That 70 mile magnetic acceleration track will surely happen some day.
In the mean time, your thinking about laser propulsion in the context of interstellar travel has inspired creation of ** this ** topic, which is about laser propulsion of vessels of various sizes in the Solar System.
Launch from the Moon is a particularly interesting application of the idea.
Water has many advantages as a propellant, so I am focused upon that material for this application.
I hope you will continue developing your ideas for using regolith as a propellant from the surface of the Moon.
The impact of particles of regolith upon the terrain should be fascinating to observe from a safe distance.
The impact of water particles (molecules seem the most likely size) will also be significant, so it would seem best to make sure the output of the propulsion exercise is directed away from the ground.
And ** that ** thought leads to (for me at least) awareness of a fundamental issue with aft laser propulsion ....
The output of the energizing activity will inevitably flow back to the laser at considerable velocity.
For that reason, I find development of this idea somewhat impeded, and hope that other forum members may be able to think of a work-around.
(th)
For launching from the moon we can imagine an ablative pusher plate made with very cheap materials taken from the moon surface, like basaltic fibers and pressed regolith as adsorption layers, alternate with transparent silica layers. Moon surface-to-LLO deltaV is only 1.87 km/s, so exhaust velocity doesn't need to be as high as from earth surface to LEO
#98 Re: Terraformation » Is terraforming Mars impossible? Maybe not... » 2021-04-01 11:16:47
------
But I agree, Orbital Habitats are going to be a big part of the future. I think it is insane to not think about having them for Mars.
But all of this is only one persons opinion.
Done.
A Mars habitat has to be built underground for radiation protection and to avoid meteoroid strikes
#99 Re: Interplanetary transportation » Interplanetary Sailing Using Laser Beams Luna Phobos » 2021-04-01 10:31:58
New model:
I have read many articles about laser ablative propulsion: they said that when a laser pulse strikes a plate the ejection plasma jet is almost perpendicular to the plate, even without a nozzle.
https://www.researchgate.net/publicatio … s_in_Space
So I've eliminated the magnetic coils and I have imagined some kind of Orion-like spaceship, with a pusher plate made of multiple ablative layers which are ablated by high frequency laser pulses, generating thrust (probably even the ablative plate needs some kind of shock adsorbers to attenuate vibrations).
What we have is a clean Orion-drive, powered by an external laser array instead of the pulse units, that can reach the low orbit without radioactive fallout.
The multi-layer pusher plate may be very interesting, because we can use high molecular weight materials for the layers ablated in the first phase of the orbital flight, to maximize thrust, and low molecular weight materials for the second phase layers to maximize specific impulse.
#100 Re: Interplanetary transportation » Sodramjet: the Mach 16 chinese scramjet » 2021-04-01 09:41:12
Let's just say I am skeptical. The conditions they tested at allow the compressible flow models they used. Conditions in real flight at their Mach 6-16 conditions do not allow the use of their model.
That being said, the notion of a standing detonation may in the end prove useful. Just bear in mind that is is a play-toy tested in a shock tunnel, which is an extreme transient. I remember when scramjets were similar play-toys tested in similar shock tunnels. It took 50 years to get from there to the X-43A.
And bear in mind that ANY airbreathing engine of ANY type will operate at a chamber pressure somewhere about 3 to 15 times local ambient. At super high altitudes where the air is so thin that it is kissing cousin to the vacuum of space, then 3 to 15 times nothing is still nothing, for chamber pressure. Nothing for chamber pressure is nothing for thrust.
Which is why ALL airbreathers have no frontal thrust density when flown too high. It's very fundamental physics, and it is the very definition of "service ceiling". Often ignored by marketeers (as it was for X-30) but there it is! No one is ever really going to power a spacecraft to orbit with an airbreather.
I say that with great confidence, despite the possible future ascent success of the UK Skylon to be powered by SABRE engines. Those engines breathe air to Mach 5. Then they operate as rockets. Period.
BTW, Skylon as envisioned will not survive descent from orbit. The bow shocks off the engine nacelle compression spikes will cut the wings off. There is a very good (and totally compelling) reason that no spacecraft ever flown has had nacelles on pylons. It's called shock-impingement heating.
GW
Thanks GW.