You are not logged in.
OK, I revisited the calculations and did a bit of convergence here and there with a spreadsheet to help me out. I'll post that stuff over at "exrocketman" in the next day or two. Lots of figures and spreadsheet images. It'll take me a while just to get it done. Everything is built out of multiple examples of common modules for hab, supply storage, propellant and engine. The landers are just a mass allowance. I haven't added up the final launch costs, but it's 6 men, and two separate landings 3 men at at time.
I built up two vehicles in LEO, one manned, one not. The manned vehicle returns to LEO using propellant modules sent to LMO by the unmanned vehicle. Everything else gets left behind in LMO. The manned vehicle spins head-over-heels at 3 rpm for 1 full gee artificial gravity both ways. My Hohmann transfer delta-Vees are pretty crude overestimates, intended to cover plane changes and propellant boiloff: I made the vehicles capable of 8 km/sec (1-way) LEO to LMO. I used 8.5 months 1-way, and a 27 month round trip to estimate supplies. I was using about 15 kg food+water+makeup O2 per person per day, but that's just a wild guess.
I used the old NERVA data because that's the only design that ever got any significant testing, to the best of my knowledge. It was essentially flight-qualified when they shut it all down. There were other better ideas, but none got the massive testing that NERVA got, to the best I know. Any better nuke just makes things look better. My finished vehicles are closer to 800-something tons than the 580-ish I originally thought. The NERVA modules are smaller and lighter, too. I sized for 0.05 gee ignition acceleration or better, same engine all three configurations at the same thrust.
Can't test nuke engines in orbit, must test on a celestial body somewhere. Rocket development testing cannot be done hanging weightless in space where every test is a flight test. You must have a stable thrust stand that does not move. Fact of life. If we can't do it here at home, then the moon is close enough to reach without nuke power.
GW
Most of the aerodynamics data for forces was correlated to flat planar areas. I'm guessing you are talking about dead-broadside return of these stages. If so, length x diameter is probably the reference area you want, and the broadside drag coefficients for circular cylinders apply. That stuff is a strong function of Mach between about 0.75M to about M3.
Nice to see such low numbers. That'll decelerate more, higher up in the thinner air, for sure. Ride might be rougher, just like a Piper Cub vs a jumbo jet. I'd worry about air pressure crush as it decelerates through about M1 about 20,000 feet. Structures that light are very flimsy. Air pressure crush after entry was over is what broke up both Skylab and shuttle Columbia's cabin section.
GW
Hey, I provoked a good run of conversation with that modular Mars mission post! (#226 above)! That's what it's all about.
I do think I need to re-run those calculations. I think I messed up reconciling available volume with contained mass for the LH2 propellant tanks. The concept is still "in the ballpark", though.
The inflatable habitat idea is just the thing. Bigelow comes to mind. I think it needs to be combined with two other things: (1) meteoroid "armor" made of alternating layers of foam and foil about 15 inches thick, and (2) the design constraint that access to the pressure shell be unobstructed for fast puncture repair.
Aerocapture at Mars is fine, once we've done it a time or two. Suspenders-and-belt thing. First time up, it might not work right. So, that should not be the manned trip. I'd prefer to stage the first exploration trip from orbit, anyway, so I could visit multiple sites on the one trip.
Still thinking about practical lander designs. I'm pretty sure retro thrust during the entry hypersonics will be needed. Not so sure a supersonic chute is worthwhile.
Testing nuke rockets down here requires a great deal of public education to undo all those decades of hysterical, nonsensical misinformation. I think a plume capture facility could be built (although hardly necessary), but it might be cheaper in the long run to test them on the moon. Good reason to go back there, actually.
I sure would like to see some real work done toward inflatable heat shields. Intriguing idea. Some sort of flexible elastomeric ablative is needed for the outer layer, I guess. Nifty idea for one-shot use.
GW
Here's an odd thought, applicable to mammals, anyway. Not so sure about birds and reptiles or fish.
Ship the livestock as frozen embryos in LN2 cold storage. Thaw and raise in a lab on Mars. Once the herds are started, shipment of live animals on Mars is no more time-consuming than truck shipment here.
They already do it with humans in the in-vitro fertilization thing.
GW
So am I (confident of high-grade steel in small batches by more-or-less conventional methods). Maybe not by 3-D printing. But then, who knows what they'll come up with fairly soon?
GW
I just spent about 30 minutes roughing out a first-cut Mars mission for a crew of 4-6. It's not an optimal mission. It's crude, just rocket equation stuff. All docked modules of 25 metric tons each, 5 m dia and 15 m long, just like the biggest existing payload dimensions today. Two of these form a habitat module with about 50 cu.m free internal volume per person (spacious!). Two more are a storage place for life support consumables. There is a NERVA engine module that I assumed was 20 tons, for a big engine or pair of engines. Isp 900 at T/W 3.6 as demonstrated in 1973.
I assumed two landers made of two 25 ton docked modules, for 50 tons each. These are just allowances, no design. Hab plus storage plus landers is 200 tons dead-head payload. The propellant modules are also 5 m dia x 15 m long, and need cryo-coolers plus solar PV panels for LH2. I used 10% inerts. At LH2 densities, I get 9 tons propellant and 1 ton hardware per module.
Two ships, one manned, one unmanned, both Hohmann transfer from LEO to LMO (about 8 months one way). The manned ship returns to LEO, where it can be used again. The one-way delta vee total with plane changes is pretty close to 8 km/sec.
Outbound /manned: 36 propellant modules in a 4-parallel stack 9 modules long. NERVA at one end, hab /storage/landers at the other. My stack is about 195 m long, and masses 580 tons at departure. Essentially-dry tanks at Mars in LMO. There's about 95 m from the cg to the hab. That's 1 full gee artificial gravity at only 3 rpm, spun head-over-heels.
Outbound /unmanned: same 36 propellant module stack done 4 x 9, plus a NERVA module, and for the 200 ton dead-head payload, 20 propellant modules stacked 2 x parallel 10 modules long. That's the return propellant for the manned ship. That one is 295 m overall, same 580 tons at departure, and does not spin.
You will leave behind in LMO the two landers, one of the NERVA's, and the two 36-module propellant stacks that are empties. The hab+storage+the 20-module propellant stack, plus one of the NERVA's, comes all the way back to LEO for use again, if desired. That cluster is about 220 m long, and about 320 ton at departure for Earth. Hohmann again. It's about 100 m from cg to hab, so 3 rpm will do for one full gee artificial gravity, spun head-over-heels.
We could have done this, given a lander, in the 1980's except that they did not understand the zero-gee risks and radiation risks well enough back then (they probably would have neglected the artificial gravity and the shielding, guaranteed death/disability for a 2.7-year mission). By the 1990's, those risks were understood well enough to know that artificial gravity and some solar flare shielding was required. Period. No questions about either requirement. Again, given a lander, we could have done this in the 1990's.
And given a lander, we could have done it anytime since.
My other two points:
1. Artificial gravity is quite easy to do (!!!!!!) in any practical ship design. So there is no excuse not to do it. Provides safety, and makes all the life support jobs much easier (water, wastewater, cooking, ventilation, etc).
2. You can do this trip chemical, but the masses are very much larger. You'll have to stage off empty tanks between burns. NERVA (or a better nuke) makes a whale of a lot of sense.
Last point: the critical missing item all these decades is the lander. All the other tinkertoys have existed since the 1970's, and we have know how to design with them to those critical life support requirements since the 1990's.
I'm counting 4 Falcon-Heavies to send up the hab and storage modules, plus the two landers. There are 92 propellant modules , each of which could be launched by an Atlas-V 552 or by a Falcon-9. There are two NERVA modules, each of which could be launched by an Atlas-V 552, or both at once on a Falcon-Heavy. That's around 98 launches, most of which are $60-70M each retail. That's roughly $7 B in launch costs. Assuming launch costs are around 20% of program outlay, we're looking at near $35 B to send 4-6 men to Mars and make two landings while there. That's 2012 $.
GW
PS, if you were really smart, you would make the landers powered by NERVA's, and use them to push the return propellant supply to Mars. All 3 vehicle configurations get smaller when you do that. You get to pay less and make more landings while there.
Go look at this news article, and you will understand what I said in other posts about bloated contractors and bloated government agencies. You have to read between the lines, though. Not even the reporters who wrote the article truly understand why this happened. Or what it really means.
http://www.msnbc.msn.com/id/3033063/ns/ … nce-space/
It’s very sad these ex-shuttle folks are caught in the middle. But, working in defense contracting, in a second/third tier vendor, I never had a $100K/yr job, either. Not like those NASA folks. Although I did make a comfortable salary, not seen by me again since the defense plant where I worked got closed by corporate politics-for-profit, and in egregious error at that.
Teaching college is far less than half of that salary. Teaching high school is less than a quarter. I’ve done both, since the defense jobs ended in these parts. But, we’re still alive. We might even get to retire in 5 years or so, no thanks to the defense industry, but due to our own efforts, and no small measure of good luck.
There has to be a coordinated plan for space exploration, and the necessary oversight to prevent contractor and agency bloat. The US Congress provided neither, not in 50 years. What a bunch of incompetents and malfeasants!
THAT is why the NASA we have today is not the NASA of 1959.
THAT is why the NASA of today cannot send men to Mars, even though the knowledge and most of the technology to do so, existed as far back as about 1990.
THAT is why it appears a private outfit like Spacex seems more likely to go to Mars, if anybody ever goes at all. Very sad, that outcome. Especially since it is the role of government to explore, and business to exploit, not the other way round.
THAT is why I say the things I say.
GW
Hey, that's pretty neat. I haven't kept up with this technology, so I don't know a whole lot about it. I do know it started out as plastics-only. It's very encouraging to see powdered-metal parts being made. Strength and stiffness are far superior that way.
I bet this technology continues to evolve. It would be really slick to see metal parts made with properties approaching those of a conventional cast or forged part. The properties of powdered-metal parts still fall short of that goal, I believe. Could be wrong, but I don't think so, not yet.
With powdered-metal printed parts and tools, there's some real promise for replacing massive infrastructure with relatively small equipment and software. That's the very thing we need to set up permanent bases and colonies on Mars and elsewhere.
I like it.
GW
Louis:
"Regarding the therapeutic nature of 0.38 G, don't you think with properly distributed lead weights we can certainly reverse bone loss?"
Nope. I don't. There are no credible data to support that notion, widespread as it seems to be.
GW
I just posted the atmospheric entry model I am going to use, all updated and checked out, over at "exrocketman". Anybody with access to a spreadsheet program can do this. The dynamics portion is OK as it is, and was since 1956.
For the heating effects, I updated this to an integrated total instead of the crude closed-form estimate, and I corrected the convective stagnation-point heat flux correlation from something published recently that was in error. I still have no radiation heating model, but if you triple the convective values, you should be "in the ballpark".
GW
http://exrocketman.blogspot.com see article dated 7-14-12
I think my point is that we don’t need to “practice” for Mars (or anywhere else). We already know what we need to know.
We could have gone to Mars successfully about 20 years ago. Everything we needed to know about, was known then. Including radiation and microgravity effects. The only real limitation persisting to this very day is an effective Mars lander in the 10-100 ton dead-head payload class.
That’s about a 5 year technology development and demonstration. We already know what to do, we just haven’t done it. Mostly because of “not invented here” problems at NASA.
Microgravity is a simple problem to solve. Beyond roughly about 14 months, it becomes increasingly and essentially permanently irreversible to expose yourself to zero gee. It’s demonstrably bad enough at 6 months, that’s why ISS crews are changed out at 6-7 month intervals.
Exercise regimens make no real difference at that 12-14 month length of exposure. We already know that. There is no credible data whatsoever (!!!!) to suggest that 0.38 gee on Mars is any way therapeutic. The round-trip Mars mission is about 24 to 28 months long. So, there is NO solution space for traveling to Mars in zero-gee.
You can completely forget that concept. That way lies a dead crew.
So, spin the damned ship for artificial gee. Simple as that.
I’d suggest a 1 gee design, simply because there is no credible data whatsoever (again !!!!) to suggest that anything less is therapeutic. That’s no more than 4 rpm for no less than a 56 m radius. Basic physics. Radius times square of angular velocity equals radial (inward) acceleration.
But no “Battlestar Galactica” is required, nor is any Rube Goldberg cable tether system required. Build your ship as a slender baton from simple docked modules in LEO, and spin it end over end. Nothing hard about that at all. Something 5 m wide at 20:1 L/D is 100 m long. Stack it up in parallel to achieve 10 m wide at L/D = 20:1. 200 m long, that’s a 100 m radius at about 2 rpm on the outbound leg.
5 m wide fits or replaces a lot of the existing payload fairings we already have. Modules can ride up “naked” on existing launchers in the 20-25 ton class, or two at once on a Falcon-Heavy, very soon.
What is so hard about that?
For the lander, you just need to use low levels of retro thrust during entry to end the hypersonics at an acceptably high altitude, and perhaps low levels of retro thrust with a chute (if you choose to use one), then finally high levels of retro thrust for the final touchdown.
Take a good look at Spacex’s Dragon with the Super Draco’s installed. That’s a small 2-3 ton dead-head payload one-way (only !!) lander right there. Retro thrust is exactly what they’re doing.
Retro plume instability is solved by cant angle. Theirs (Spacex) is 45 degrees, but you don’t need that much. They did that to avoid firing a rocket plume through a hole in a heat shield. But, if you seal the engine compartment behind the heatshield to prevent throughflow, you actually can have a hole in a heatshield. Throughflow is death (Columbia 2003). No throughflow is a static gas column, the best insulator we know. We already did this ca. 1970 with Gemini-B for the MOL test flight.
Radiation is not a problem if you use a “real” habitat module for the interplanetary transits. 20 cm of water (or wastewater) stops solar flares. End of problem. You have to have water and wastewater tanks anyway. So position them as radiation shielding. At least around the flight deck as your radiation shelter.
The cosmic ray stuff at 60 REM/yr max is very close to our current “acceptable limit” for astronauts of 50 REM/yr, even in a peak radiation year. The min exposure is 24 REM/yr. Varies sinusoidally with time during the sunspot cycle. There is a career limit, which is violated in a second flight to Mars, if conducted in a peak radiation year.
All these things have been known since about 1990 or so. As I said, we could have gone to Mars about 20 years ago.
GW
Louis:
As far as I remember, the only weights the lunar astronauts lifted on the moon were those 100 kg suits.
Lower body negative pressure temporarily relieves the upper body and face swelling that is typical of zero-gee body fluid redistribution. It's no real solution to anything.
The only real "secrecy" is that no one at NASA (or ESA or any of the rest) wants to admit that there is no solution to extended living in zero gee. Artificial gravity is absolutely required beyond about a year or so for sure, and beyond about 6 months, with some serious but recoverable effects. That's why ISS crews are changed out at no more than about 6-7 months. Didn't you notice that?
The other "secret" is that "they" completely failed to find out how much gee is therapeutic, after all these decades doing stuff in LEO. In other words, "they" didn't do their damned jobs for all those tax dollars we spent. "They" (all the government space agencies) do not want you to know that. The favorite-contractor corporations get tarred with the same brush: they also did nothing, choosing instead just to profit from the status quo.
The original NASA Mars mission was on the books for the 1980's. We could have sent them back then, but in 20-20 hindsight, that crew would most likely have died from microgravity disease or solar flare radiation. We actually knew how to handle all of those things by the mid 1990's, roughly.
The only remaining problem is a practical Mars lander - as evidenced by the technological troubles the JPL guys are encountering putting ever more massive rovers on the surface. You have to do something different from Viking 1/2 by the time you exceed around a landed ton or so.
GW
I think the two cable-connected bodies spinning about a common center of gravity will indeed work as artificial gravity. With a long tether, it is easy to provide up to (or exceeding) one full gee at very low rpm, and that's a good thing.
On the other hand, I really do not believe you can do course correction burns on a tether system, no matter how carefully you might time them. The system is far too structurally compliant. Any disturbance whatsoever (even people moving around inside) induces oscillations, which can very easily amplify, as I see no possibility of intrinsic damping in such a system. At the very least, you have to de-spin and disassemble/re-dock for every maneuver.
The spinning "stiff" object has the advantage hands-down over any tether system, on a safety basis alone. No assembly/disassembly required. Just thruster burns. And it doesn't have to be a 3:1 L/D (for stability) object spinning about its long axis (the usual assumption leading to "battlestar galactica" designs). It can be a simple slender baton spinning end-over-end. That's stable, too. And very easy to spin-up or de-spin. For any maneuvers. Any time you want.
I would rather see us deliver close to one full gee so we know for sure the astronauts will be physically fit on arrival. We have no data (none!!) to suggest they will be fit spending the trip at 0.38 gee. One can always put a lab space at the 0.38 gee level of a spinning design to practice skills for Mars, while spending off-hours and scheduled rest periods at one full gee for health.
As for habitat volume required, that issue is actually related to gee levels and health, too. The only partial-gravity data we have is surrogate bed rest data, and it is a stretch beyond any credibility to infer from that what level of gee is therapeutic. What is certain (the ONLY thing that is certain!!) from those bed rest studies is that enforced immobility will cause disease, even in a 1 gee gravity field. That is a supremely-important result.
Therefore, there must be room inside the spacecraft to move around all day long, and to exercise. Two guys riding in a Dragon cannot do that, 1-gee tether rig or no. You just gotta have a gym. Zubrin is just plain wrong about going to Mars in a small capsule. They won't survive the whole trip, not unless you fly very, very fast (like 1-2 months one-way).
Most of our knowledge about handling water and wastewater, plus the effects of free convection effects in ventilation and fresh-air maintenance, was derived down here at 1 gee. The smart thing to do would be to take advantage of that, and simply design for pretty close to 1 gee. We already know how to handle zero-gee for "short" periods, when maneuvering, or docking, etc. But there's no need to endure that (or try to compensate for it) for long periods. Just spin the ship head-over-heels, with the habitat at one end.
It's a very fuzzy limit, but most of the aeromedical experiences we have gained over about the last half century would suggest most folks can withstand 2 to 4 rpm spin rates pretty much indefinitely. At 4 rpm, the 1 gee radius is but 56 meters. If we were to assemble the transit vehicle in LEO from docked modules, it's pretty hard to imagine such a ship under 100 m long, don't you think? Even if only two guys go.
That's not 2 guys in a Dragon on a Falcon-Heavy, by any means. But it's still something we could build, and right now, too. I think most of these direct-launch/direct-landing minimalist designs are just not realistic, because they're just not survivable scenarios for the people.
GW
Adaptation:
Boy, I hope so! MCP is definitely the way to go for EVA.
GW
Their competitiveness depends on massive cultural change. I know the crowd at ATK Utah, I used to work with folks from there, being for a while part of the same corporation. That was some time ago, but large corporations change slowly. Back then, I didn't see the capacity for the massive cultural change needed to reduce prices dramatically. Big fat bloated defense contractor. Very typical.
Actually, I didn't see such capacity for cultural change in any of the giant defense contractors, but ULA has proven me wrong. Their Atlas-V is pretty competitive with Spacex's Falcon-9, in spite of being defense conglomerates, which Spacex is not. So it can be done. (One can always argue about how it was done, because things are easy to hide in a giant conglomerate.)
As for the Frankenstein-design of Liberty: there's nothing wrong with using older technologies to do a job, as long as they serve well. There's nothing wrong with bastardizing-together a mish-mosh of things to do a job, as long as it works well. You're corresponding with a guy who would have no qualms at all about pulling a steam locomotive out of mothballs to pull long freights up steep grades faster, or to put sails on ocean-going tankers and freighters to save fuel.
A big solid right off the pad makes a lot of sense, because thrust, not Isp, is the real issue in the first several km going up. Solids do that in spades, and much better than liquids, in part because of the high molecular weight in the exhaust, and in part because the typical throat to casing diameter ratio is just far higher (frontal thrust density achievable). You do have to address extreme reliability if men are to ride, because they cannot be turned off once lit. In that respect, I think the hybrid might be a better deal.
Putting a liquid upper stage on a solid lower stage also makes a lot of sense. This is particularly true if you can generate Isp in the higher range of liquid capabilities, combined with precision thrust control and termination, something solids completely lack. The Ariane upper stage on Liberty has all of these qualities. Restart capability is the most important, personally I don't know if the Ariane stage has that. For Liberty to be really viable, upper stage restart is a critical capability.
That is an awfully long and slender stack, though. There are many possible modes of structural oscillation, most of them dangerous. The most slender vehicles I remember were Scout and Vanguard. They did OK structurally (Vanguard had other problems). Liberty looks worse to me. I think I would have proposed it as a side-by-side cluster of 3 or 4 smaller solids from the strap-on stable, not stretched shuttle SRB technology.
GW
Here's a question for those of you who know more about microgravity disease than I do.
Consider: a 6 month tour on the space station is followed by a 3-4 gee re-entry (very few minutes) and permanent 1-gee upon landing. They have to be carried. Cannot even sit up. And that's with daily vigorous exercise in zero gee on the station.
It takes 6-8.5 months to fly to Mars one way. Say we design for no more than 3-4 gees during entry. Again, a very few minutes' exposure. What makes anyone believe these astronauts will be able to sit up or stand in 0.38 gee any better than they do in 1 gee? Especially when we have zero real experience on which to base this judgement?
Would it not be far more prudent to just plan on providing spin artificial gravity at near 1 gee during the trip, and have them arrive physically fit?
Am I missing something here?
GW
OK, I've got the old density/scale-height model for dynamics of deceleration in working order in a spreadsheet. Those results look quite realistic for Earth entry at lunar return speeds. The convective heating rate model is sort-of realistic, but rather crude. The closed-form total heat absorbed relation is just plain wrong, I only got believable results from numerically integrating estimated heating rate with time, as computed down the simplified straight-line slant trajectory. This is pretty close to all they could do with the original warhead re-entry problem in the 1956 time frame. It looks pretty good to me.
I have no idea if the same stagnation-point empirical heat rate equation works in Mars's "air". It ought to be ballpark, though. The dynamics look pretty believable. Heat rates are predicted lower there than here, so that's in the right direction. I have the same model set up for Titan, but the escape velocity there is not even hypersonic.
This stuff needs to "soak in" for a few more days, then I'll post the basic models over at "exrocketman". Meanwhile, I need to try some entry variations in all three locations, looking for practical deceleration gee limits vs penetration too low at Mars. I also need to look at correlating some trends of scaling up ballistic coefficient vs vehicle mass. I'm using m/CD*A = 200 kg/sq.m right now.
This is off-hours/late evenings stuff, so it'll take a while. Have patience.
GW
Actually, I'm not too worried about the reuse of old casings. Those are just pieces of steel. I think they're T-250, but it might be something else, similar. As long as they meet dimensional requirements, haven't been overheated, or reached fatigue life, they should be fine. The dimensional requirements are pretty stringent at the sealing surfaces, where the O-rings ride. Those segments will become non-reusable from "dings" hitting the ocean, more than anything else.
Re-insulating the case is not all that big a deal, although in that size, it isn't trivial, either. The insulation is sheets of fiber-reinforced rubber compound wrapped around a bladder inserted into the segment. You pressurize the bladder, which compresses the sheeting against the primed case segment, and you cook it while pressurized to cure the rubber, preferably in vacuum, for best quality control.
Casting propellant isn't all that big a deal, although not trivial in that size. For one thing, it requires a huge mix of propellant. That's a giant facility. OK, there's cast tooling emplaced in the primed/insulated motor case segment to form the finished surface of the propellant. Depending upon the effective viscosity of the propellant (this stuff is thixotropic/non-Newtonian, like concrete, but worse), you gravity or pressure cast this stuff into the segment, under high vacuum. Then you cook it with tooling in place for a few days to cure the propellant. Typical AP-Al-composite, HTPB binder, etc. Just huge.
What baffles me is how they expect to support all of this with a population about the size of a small town instead of the major city they used during shuttle. You have to count all the vendors and subcontractors in that population. Spacex does it mostly in-house, and with a very small population, that's how they do liquid-propellant launch so cheaply. How you do that with giant solids, I'm not so sure, but I am sure that it can be done: because that's what we do making small solid motors for tactical weapons.
GW
"It's all in the cultural change"
RobS:
I haven't tried that model yet, but I suspect you are quite correct. The various data indicate aerobraking altitudes in the 800-900 km altitude class at Titan.
Surface pressure is about 1.5 that of Earth, and the densities look high quite a long way up because of the intense cold. Much colder, and the ideal gas model wouldn't apply, it's quite cryogenic, about 90 K at the surface. I've got the temperature, pressure, density, and sound-speed profiles vs altitude posted over at "exrocketman" in a recent article there (for Titan, Mars, and Earth). The Titan data are based on what the Huygens probe actually saw on the way down.
As soon as I get a bit more of this simplified entry-model stuff worked out and verified as reasonably correct, I'll post some of it at "exrocketman". I pretty much believe the dynamics right now. I think the entry convective heating is still crap, though.
GW
"exrocketman" is http://exrocketman.blogspot.com
from Spacenut's post no. 4 above:
"The thrust Oscillation was talked about quite a bit when the Ares was still in developement mode under Constellation and was resolved by isolation of the upper stage by shock obsorbers in the interchange section."
Using shock absorbers to attenuate thrust oscillations means the oscillations are still there in the motor, where they really are dangerous, because they are evidence of combustion instability. As I said in some earlier post, the 5-segment design is known to have those problems in horizontal ground tests. It has never flown. This kind of thing can be solved, but my guess is they need to keyhole-slot the propellant grain in the segment closest to the nozzle. That would reduce the peak bore velocity (while preserving burn surface neutrality), which should reduce or possibly even eliminate the tendency to oscillate.
No forward-segment slots, though. That's how they (ATK, formerly Hercules) blew up AFRPL's test stand about 20 years ago. Cost them a billion dollar write-off, it did. They need to talk to their tactical guys about doing a combined thermo-structural / fluid dynamical interior ballistic analysis. Tactical guys do that all the time, the big motor guys do not. But they should. That same kind of inattention to adequate design analysis is what sunk the Titanic.
They don't need that crappy 3-O-ring joint either. That's still a disaster waiting to happen, just like the original 2-O-ring joint was before it (Challenger, 1986). One O-ring will do just fine. If it leak checks OK at 5 psi, it'll hold all the way to case burst. You just need one, and you DO NOT put goo in the insulation joints "upstream" of it: that guarantees an O-ring burn-through (no matter how many you have). The static gas column in the bent space approaching the O-ring is better than any insulator we can build. The only thing you don't want is a straight line-of-sight path for radiant heating from the fire in the motor.
Yeah, it can be solved and made to fly just fine. The tougher problem is corporate cultural change. How do you build it and operate it with a small logistical tail instead of a large one (like the one supporting shuttle SRB's)? That's the real secret to lower launch costs. Spacex is the example that proves it.
GW
Rune:
I used the 1956-vintage entry model in one of those papers by Justus that you pointed out for me, to set up and verify an entry dynamics model (nonlifting). I've got it worked out for a sample case of Earth entry at escape speed. My profiles down the slant path look like what Justus reported for deceleration gees. His dynamical models look pretty good. But I'm also sure he is no entry heat transfer guy.
He didn't understand the hypersonic entry correlation for entry heating rate very well, and mistakenly used two different k-factors for his peak heating rate and total integrated heat closed-form estimates. As a result the actual time integral of his heating rate is factor 1.6 off from his closed form estimate equation for that same integral. Since he clearly didn't understand the heating correlation was a dimensionally inconsistent correlation equation, and that the same correlation coefficient should be used in both estimates, I do not trust the heating rates and integrals he reported in his paper.
Typical scientist, didn't pay close attention to stuff outside his narrow little specialty area of expertise. I'll correct the convective heating rate correlation later, and see if I can find a correlation for the missing radiation heating. (Which is larger!)
Meanwhile, the deceleration model looks pretty good. The peak in the gee profile is just about the right magnitude and speed as predicted closed form in the scale-height model. And it looks good enough to use for Mars and Titan. End of entry hypersonics for a blunt object is just about local Mach 3. (For a pointed object, that end is nearer Mach 5.) Below that, drag coefficient is a strong function of Mach until you are substantially subsonic. That's "ordinary" supersonic aerodynamics.
My goals here are crude first-order paper "designs" for a "universal" one-way chemical lander, a two-way chemical lander, and a reusable nuclear two-way lander. "Universal" means the very same machines work on Mars, Titan, our moon, Mercury, and all the airless moons of Jupiter and Saturn. I'm designing about 10 tons delivered "dead-head" payload to the surface of Mars from a low orbit. (My paper at the last convention in Dallas, TX, was 6 tons delivered.)
Once there's a practical lander design, there's no more excuses not to go, with big rovers or with men. To any of those locations, not just Mars.
GW
There are perils to a space elevator concept. These have already shown up in fiction written about such things. There's human sabotage and there's collisions with space debris or small bodies. Either brings down the elevator, which is a pretty harsh and widespread crash event. That's a ;lot of tonnage to fall.
I understand very little about tethers, but I do understand there's a huge point mass involved and it's a sort of centrifugal-force slingshot thing. A failure in a system like that releases both the point mass and all the sling material. Same sort of massive, widespread crash potential as the elevator.
I rather suspect we will eventually build and use these things, but we also will be very selective in where we site them, because of those dangers.
GW
Louis:
I would guess that by the time there are people staying more-or-less "permanently" on Mars, we'll have added GPS capability to the satellites in orbit around Mars, the same way we did here. You could use pretty much the same equipment.
If not, the first "truck" transports will have to be driven over graded roads by humans. But once Mars GPS is in place, you could do a simple set of pre-programmed GPS waypoints along the road, and have very simply-automated "trucks" drive point-to-point.
Later, as the local colony grows, you could reduce the energy consumed in "truck" transport by around a factor of 10, by going to rail transport. This makes sense when the shipments are bigger, and enough infrastructure is in place to make the steel. I suspect ties there will have to be steel as well, at least at first, since no practical concrete we know sets in that cold. I also suspect that a good concrete-equivalent will be developed.
It's a bootstrap process, just like it was here.
GW
Hi Louis:
I think your assessment in the last post is just about right.
GW
Actually, dumping mass before touchdown is a very old and time-honored tradition. Lots of airplanes have dumped fuel before landing, for about half a century now.
GW