You are not logged in.
How to test gas core?
Short form: same as the solid core program. Put it on a stable thrust somewhere and and fire it. Same as all rockets. The first tests simply cannot be flight tests, that's way too much to bite off all at once.
Yep, the exhaust is radioactive, similar to the Phoebus and Kiwi predecessors to the final NERVA. True enough. A properly-working open-cycle gas core machine will be running at low concentration radioactivity: around 1000:1 hydrogen to uranium fission products by mass. But the total uranium mass fed to the burn will be expelled as fission product mass. The early ones will be much worse, in a concentration sense, until the containment flow scheme works right.
Closed cycle ("nuclear light bulb" designs) will have a clean exhaust, unless the physical containment fails. It will in early testing, occasionally. The problem with closed cycle designs is that the core fission products get retained. I like open cycle better. On shutdown, it's "an empty steel can". Thermally and radiologically, it's "cool" in minutes to hours. Retained cores are dangerous for decades to centuries.
Problem: our rules no longer allow us to free exhaust radioactive plumes. It is possible (in a very expensive facility that we do not currently have) to capture the plume and separate the hydrogen from the radioactive "dirt", and sequester the dirt for disposal.
Wild idea: do it on the moon instead, as free exhaust. Exhaust speeds far exceed lunar escape, and there are no air and water to pollute, or neighbors to bother. It might (!!!) actually be cheaper to do it that way on the moon, instead of plume capture here on Earth.
Even a resurrected NERVA tested here on Earth will have to be tested plume capture.
It's far faster, more effective testing as open plume. The program proceeds much faster and effectively. Resources get concentrated more on the rocket and less on the facility.
Can't do it here? Then do it there! On the moon. It's close enough to reach quickly and with relatively low-performing rocketry. Emergency help is but 3 days away.
BTW, the Th-232 to U-233 breeder cycle, once bootstrapped into operation, yields fission fuel with shorter-lived daughter products. It'll work as a reactor fuel, probably even in nuke rockets, but is not "concentrated" enough to be a bomb. No plutonium in the cycle, either.
I kinda like the concept of a U-233-fed open-cycle gas core engine. It's something easily abortable on launch, and not very dangerous in a crash. At lower power, Isp is near 1500-2500 sec with engine T/W's 10 to 30, or perhaps higher. No waste heat radiator, regenerative cooling is adequate. Higher power, you need the big, heavy radiator: They were going for 6000 sec Isp at engine T/W maybe .05 to 0.1.
I sure wish we'd already done it. The bench tests 40 years ago looked very promising for both gas core reaction controllability, and for open cycle containment by that 1000:1 ratio. That was the target for "perfect containment" at their residence time and burnup rates back then. But it was just some academic-institution bench tests.
GW
Myself, I would get started going to the moon and Mars with the chemical launchers we have, or soon will have, like Falcon-Heavy. I'd (in parallel) work on resurrecting the old NERVA, since it did everything but actually fly on Saturn-5, and use that type of engine to build single-stage landing craft capable of tail-sitter landings on Mars, and returning to Mars orbit, all in one propellant loadout. Any boat that can do that, can ferry very heavy payloads to and from lunar orbit, down to the lunar surface.
In parallel with all of that, I'd also be working the gas core nuclear thermal rocket ideas, as those could potentially solve the radioactive core problems at performances as far beyond NERVA and Timberwind as they are beyond kerosene-oxygen. I'd probably do the nuclear rocket work on the moon, as doing down here requires no free exhaust, these days. Good reason to go back to the moon, in my opinion. Safe place to do dangerous work of high payoff potential, yet close enough to reach easily with stuff we have right now.
T/W > 10, maybe > 30 at Isp 1500-2500 sec? Good single stage launcher. T/W near 0.1 at Isp near 6000 sec? Good orbit-to-orbit "hot-rod" engine. The bench tests ca. 1969 indicated these things are indeed feasible.
Of course, for really gigantic orbit-to-orbit colony boats, there's good old nuclear pulse propulsion. The bigger the ship, the higher the Isp, and the easier the shock absorber design is. That comes quite a bit later, though.
GW
Bob Clark:
What I had posted for the two-stage airplane was not the final form, just what I had done at that time, and where it pointed. I tried to climb really high before pulling over and accelerating the ramjet first stage airplane, and the air was too thin for thrust-minus-drag to accelerate the mass at a practical rate. That's why the first stage went too far downrange to fly back.
Numbers down closer to 60,000 feet for the staging altitude look better. I need to re-run that same study with 60,000 feet staging, and see if the booster flyback becomes practical. I simply haven't done it yet. But I'm pretty sure it would work.
From what I'm told, there are 3 variables of importance to selecting the staging of a HTOL 2-stage vehicle. They are, in order of importance, speed, path angle, and altitude. The most important is speed. That's why I picked ramjet: with external or mixed-compression inlets, it's capable of useful thrusts to M5.5 to 6, and can take over as low as M1.6-ish. I went with separate rocket and ramjet engines that can be burned in parallel, that's how I achieve about 45-degree path angle at staging in a sudden pull-up transient without deceleration (combined cycle probably won't be able to do that).
I just couldn't make it work right at 100,000 feet. Frontal thrust densities are an order of magnitude higher at 60,000 feet. Altitude is the least important of the three variables, so I feel pretty good about the HTOL 2-stage approach with a rocket+ramjet airplane 1st stage. The 2nd stage can be a rocket ballistic pod, or a rocket airplane, whatever the mission needs.
Configuration design for drag reduction and for impinging-shock avoidance are the truly critical issues. Shock impingement heating can cut through structures in a second or two at M6+. We saw that on the X-15 flight that carried the scramjet test article.
GW
I suppose you take your time before dismounting, and then move away quickly, once you clamber down. I envisioned a crane arm that makes a nice personnel and cargo elevator. Proximity is bad (inverse square), but really it accumulates over time. What you don't want to do is stand near the thing any longer than necessary. (One of the reasons I like gas core concepts better is faster thermal and radiational cooldown: essentially an empty chamber.)
A few dozen meters away is a pretty good distance temporarily for unload trips. Pitch camp maybe a km or two away. You're pretty safe up in the lander cabin with the propellant and structure for a shield. Nice shelter for solar flares if it's built tough as an old boot, and you have a couple of water and/or wastewater tanks to hide beneath.
The Mars Society archives on-line have my original paper. A version of it is posted on "exrocketman" dated 7-25-11. Some second thoughts about using NERVA vs electric as the backup scheme is posted 9-6-11. That site is http://exrocketman.blogpot.com If you click on the identifier "space program", then it shows only those articles with that identifier.
GW
What RobertDyck said is exactly correct. Decide what has to go to the moon and land there, and what has to return, first. Then get it there from LEO. Then launch it. That is the correct design sequence. And that's why what you intend to do on the moon so entirely drives the design.
Designing the launch vehicle first (as with NASA SLS, designed by Congressional politics, not engineers) is a wasted exercise.
Getting what you need to LEO need not take one launcher. We now know how to dock things together and assemble very large items in LEO. It's not so much the number of launchers that drives cost, it's cost/mass delivered, and what payload sizes are already flying. Why build a bigger rocket and have to amortize its development costs, if you have a smaller rocket that is "big enough" and already "cheap enough".
Spacex Falcon-Heavy is 53 metric tons to LEO from Canaveral, at roughly $600-1000 /pound (same as roughly $1200-2000/kg). Closest rival is Atlas-5-Heavy at 20-25 tons and more than twice the price/mass.
SLS will be built from retreaded shuttle components by the same entities that built the shuttle, working in the same ways they always did for shuttle. It will never be as cheap as Falcon-Heavy, or even Atlas-5. Shuttle was $1.5B for each 25 ton payload.
GW
The mass ratio and the thrust/weight has to be there for surface launch. Adding engines usually drives the mass ratio down, unless you scale up the tankage a bit. That's why most first stages are so large.
First stage engines are usually of different design than upper stage engines: shorter bells. It really needs to operate perfectly expanded at launch level (usually sea level). She'll be underexpanded as you climb, which costs performance, but then so does over-expansion, and it's far worse. Usually first stages leave the sensible atmosphere, so it's in vacuum by burnout. Upper stage engines can be designed to be optimal in vacuum-only, with very long bells.
So, it's not exactly the same engines. Even for the same propellants, first stage Isp is a lot lower than "typical" vacuum Isp designs. Be careful trying to scale from one application to another.
GW
Josh:
What I had in the paper at last August's Dallas convention in part called for a single stage NERVA-propelled lander, a big rover with a drill rig on it, a swarm of small robots to assist 3 persons on the surface, and an inflatable Quonset hut to live in. You erect the hut and base your activities at a safe distance away from the NERVA, not in the lander. But the lander makes a better shelter if a solar flare occur. Surface time was a week or two. You leave a transponder at each site, to enable future precision landings.
My paper said send 3 down while 3 monitored and did science from orbit. Then alternate crews and landers. I put enough delta-vee into the 20% inert 10% payload landing boat to land 25 degrees out of plane and return, with rocket braking all the way down. That's no aerobraking credit, but a gravity and drag penalty on the way up, so its a very conservative size-out. Having at least one lander ready in orbit at all times for a rescue trip was one of my mission requirements.
You could do that with any mission making multiple landings. My design was 6 persons, and 3 landers so that the loss of one did not terminate the mission. The three landers pushed all the landing propellant supply unmanned to Mars orbit. I sent the manned vehicle with enough propellant to return, in case rendezvous failed for any reason. Suspenders-and-belt.
The prime design called for fast trip zero-gee manned ship, but the backup was a "slowboat" NERVA, which would be spun end-over -end for artificial gravity. No more than one year at microgravity, and set right at 1 gee for artificial, again suspenders-and-belt, based on what has already been done. That's probably what we really want to do.
My paper's design made 16 widely-separated landings in the one trip, plus a visit or two to Phobos. A real planetary ground truth survey. There were 3 unmanned vehicles sent one-way to Mars plus one manned vehicle that goes two-ways and is recovered in LEO to be used again. All 4 were in the 600 metric ton class as assembled by docking 34-ton modules in LEO. (That was based on Spacex's projections for Falcon-Heavy before they settled on 53 tons.) It true exploration based from orbit (what all is there? where exactly is it?)
The return from that trip would be the critical ground truth information for selecting one or two experimental base sites. These would be the places where you set up the real ISRU that you tried out on the first mission. Some of those first examples will have worked, most won't. But that experience takes "theoretical" designs and turns them into machinery you can rely on, for the second trip. Again, suspenders-and-belt. This second trip is surface-based work, and "looks" more like what people are proposing in these forums and most of the papers I see. But the chances of success are much smaller, if you don't do what I suggest for a first survey mission.
The experience and supplies generated by those couple of well-sited experimental bases is what enables a more permanent settlement, that could blossom into a real colony if some trade commodity could be identified. Done right, this could happen fairly fast. Done wrong, the inevitable failures and fatalities might well kill the process.
Suspenders-and-belt. And an armored codpiece!
GW
I did exactly what I suggested in my previous posting, and created the launch costs plot. I did it for the 3 Spacex Falcon birds, 3 of the Atlas-5 family, and for Delta-4 heavy. I plotted the data in metric units as $/kg vs metric tons delivered to LEO from Canaveral. I also replotted in US customary, for folks who know those units better: $/lb vs US tons delivered to LEO from Canaveral. Those data are public view over at http://exrocketman.blogspot.com
GW
I didn't make the meeting where Paul Webb spoke. I would like to have. My dad knew him decades ago as a crew escape expert, before all the pressure suit stuff was known as well as it is today. Dad and I were both aeronautical/aerospace engineers.
I did make the Dallas convention this last summer - gave a paper in the "advanced technology" session on doing a whole slew of landings in one trip to Mars. I got to meet 3 of the original NERVA guys at that meeting, and I bought the book that one of them was selling. It matches my memories of NERVA pretty well.
I quite agree that we really don't need to build another Saturn-5. But if we did, one could resurrect the old NERVA upper stage design for it, and do exactly what I wrote in my previous posting, making a bunch of landings in one trip. The new government heavy-lift launcher design is a new Saturn-5-like vehicle based on retreading shuttle technology. But I seriously doubt any government design will ever be more cost effective than they ever were, which is ineffective.
On the other hand, the new Spacex Falcon-Heavy that is supposed to fly next year is priced at around $1000/lb ($2000/kg) of payload, for 53 metric tons deliverable to LEO. There is not one single reason in the world why a nuclear transfer stage, a couple of big capsules, and a whole slew of modern LEM equivalents cannot be assembled in orbit from 2-5 Falcon-9 launches. One trip, a bunch of landings. Same as Mars.
There is also not one reason in the world why a smaller nuclear upper stage could not be fitted directly to Falcon-Heavy itself. The options are wide open.
GW
i
Why not use the bigger NERVA 3rd stage design, which is restartable (and was back then flight-ready), and push more than one C/SM and a whole slew of landers to the moon, and make several landings at different sites, all in one trip? Isn't that a better return for the launch cost and all the trouble of going there?
Trouble with reviving NERVA or Timberwind or Dumbo or any of them is the lost engineering art as just about all of those guys died or retired. Rocket science ain't all science. It's about 50% art that was never written, just carried in the minds of the practitioners. It's about 40% science, all written down somewhere. And it's about 10% blind dumb luck. And that's in production work. It's worse in development - the art factor is a lot higher. If you lack it, you will think think the blind dumb luck factor is huge, and mostly bad luck.
Been there and done it....
GW
Try graphing Spacex's costs as $/kg payload to LEO vs kg payload to LEO. You get a decreasing curve of costs, not linear. Then spot Atlas-5 20-25 tons at about same per-launch cost as Falcon-9. Bigger tends to be lower cost but Atlas-5 falls well above the Spacex curve. This in spite of Atlas flying in one form or another since 1956. That will illustrate best what I have been saying about the importance of small logistical tail and a not-gigantic (bloated) company. If Spacex were to close up shop today, Atlas's prices would at least double tomorrow.
GW
Thinking like suspenders-and-belt is how you survive in space. I would definitely try some fuel production on the first landing. I would not count on it in any way for the return from that first landing.
Question: why is everybody still focused on one trip-one landing? Why not make one trip and several landings? It's a lot of trouble to go there. Why not make it really worthwhile?
GW
The thing the crew will spend the most time inside will be the habitation module of the orbit-to-orbit vehicle. That's the one that needs to be voluminous, and it does not ever need to land. If it's at one end of a long ship, just spin it end-over-end nd you have artificial gravity. That solves a whole host of life support problems if you have artificial gravity. That approach does require de-spin for maneuvers, but that's no real problem.
I like the submarine example, too. The old diesel electric boats, particularly German U-boats and the 1920-vintage US S-class, were very cramped, not intended for more than a month or so at sea. They were called pig boats for a very good reason. The larger fleet subs were still cramped, but livable for a nominal 3 month war patrol. 60-70 men inside a 300 foot ship that was chug full of machinery. Crawl through one sometime. There's several on display as memorials. The new nuclear attack boats are better still (see Nautilus, on display), and the missile boats are very spacious by submarine standards. 6-9 months possible in them.
Here is something to consider: it's not the gross volume but net, after subtracting off for machinery and equipment that occupies spaces. When you do that, the various space stations don't look so very spacious inside, excepting the old Skylab. You can see this effect in photos. It is very significant to psychology.
What you take to the surface of Mars, you really don't have to live inside-of for so long, plus you can go outside! An inflatable pitched near the descent vehicle makes a lot of sense.
I'd be very careful comparing a Mars lander design to the Apollo LEM design. The velocity requirements are very much higher at Mars, plus you have entry heating and ascent drag to deal with. Likely 3-4 stages if chemical. I'd go nuclear - single stage is possible with NERVA-type technology, which we could still resurrect (not quite all those guys are dead yet).
GW
At 2-6 mbar pressures, it is energetically very, very unfavorable to utilize the Martian atmosphere as a source of CO2 carbon, although it certainly CAN be done. Compressing gas from low density to high density is very, very, very, very energetically inefficient, and we are talking final processing pressures here measured in tens of atmospheres, or tens of thousands of mbar. That is one whale of a compression ratio. We've never built machines like that before. The ones here going from 1 atm to 10's of atm are AT MOST 70% efficient. Efficiency decreases SHARPLY as required pressure ratio rises.
Sources of solid-CO2 dry ice near or at the poles would be a whole lot easier to utilize: you just mine it. You do have to be careful of sublimation in uncovered deposits, that's all. Once indoors and warmed up, it's a gas at atmospheres to tens of atmospheres. Very little inefficient gas compression is required, IF you plan your process correctly. This sort of thing WILL NOT be possible at all the interesting landing sites, only those at or near the poles, where the dry ice is hidden amongst the water ice (the OTHER very thing we are looking for).
This is exactly what I keep writing about. No two sites are alike in resources. The point of "exploration" is to go and find out for sure (1) what all is there, and (2) where exactly is it? This is to support any possible future activities utilizing those very same ill-distributed resources. What if there once was life on Mars? There might be coal, oil, and gas deposits. Who knows? Nobody, yet!
It would be just about as easy and expensive to tote all your propellants for the first mission(s) from Earth, as it would be to try to make carbon-based chemical propellants (methane, etc) in-situ from such a thin atmosphere, local ice deposits notwithstanding. That's because of the 10^4 compression ratios required. Ridiculous prospect, technologically.
Now, hydrogen and oxygen from water only, that's a different picture. Ease and feasibility depends on ice deposit THICKNESS and PURITY. If favorable, then how easy and efficient that process might be! Even if restricted to solar PV efficiencies under 10%. You do not learn about thickness and purity from orbit, or by scratching the surface 10 cm deep.
GW
Now, now, guys! Civility!
I just posted on another thread some of the cost figures per pound of payload to LEO from Spacex's page a few months ago. We can do a lot better than $5000/kg I saw posted here a conversation or two back. That corresponds roughly to $2400-2500/pound. That's Atlas-5 at max capability 20-25 metric tons to LEO, and the same for Falcon-9 at 10 metric tons to LEO.
Spacex's projections just a couple of months ago for Falcon-Heavy are 53 metric tons to LEO, which works out to $800-1000/pound (roughly $1600-2000/kg). I doubt very seriously the new government design could ever even possibly approach that cost figure, in spite of being 100+ tons, and there is a scale effect. It's shuttle derived hardware and ways of operating, which derive from shuttle at $1.5 billion per launch of 25 tons max. Not carrying an orbiter will help, but not all that much. You work it out. Ridiculously expensive.
I'm for using Falcon-Heavy, not screwing around with some ridiculously expensive government design.
GW
Many months ago I looked over Spacex's page and investigated Falcon-Heavy. Back then, it was projected at 34 metric tons to LEO. More recently, they're projecting 53 metric tons, with an uprated Merlin engine variant. Propellant cross-feed helps squeeze out all the performance they can get. The price per launch they quote works out at $800-1000 per pound for delivering 50-53 tons to LEO, by far the cheapest in the industry.
Atlas-5 at its max capability (20-25 metric tons) looks an awful lot like Falcon-9 at 10 tons, both near $2400-2500/pound delivered. There is a scale effect here: larger rockets deliver more, but don't cost that much more to launch. That means Spacex is already doing better down in the 10-ton class by around a factor of 2.
They did it by logistics, not reusability. Smaller, leaner company, and a design that requires a village, not a major city, to support each launch. (Reusability would help lower costs further, except I seriously doubt it can ever be achieved at the 4-5% inert mass fractions in Falcon-9 stages.) That can be taken further still, if the launch folks would talk more with the missile folks about simplifying and designing for very small support crews.
My crude investigations say it would be easier to achieve reusability in 3-stage to orbit than 2-stage to orbit. With 3 stages there is a lot more room for higher inert fractions. That's what has to "cover" the structural survivability "beef", and all the added recovery gear (whatever it is).
GW
Why not use the experience we have from Mir, Salyut, and Skylab, as well as ISS? The unique one was Skylab. Alone of all of them, there was a huge open space in which to live. That would be an upper design bound on volume per person. I'd use Salyut and/or Mir as a lower design bound on volume per person. ISS falls in-between, and seems a tad crowded sometimes, with 6 on board.
What that says is any crew module for the long ride to/from Mars is going to be a big one. Not necessarily really heavy, but voluminous. There needs to be some real elbow room inside, like Skylab, and some spaces where individuals can go to get away from everyone else. I's suggest a few Bigelow-type inflatables docked together would be the most practical way to launch it, using Falcon-Heavy at $800-1000/pound. I'd also suggest recovering it and using it on subsequent missions to Mars and elsewhere.
GW
If these paper airplane things really were released, I suspect most of any survivors went into the ocean. I never heard about any results though. Fascinating experiment!
As for Falcon tanks "exploding" during entry, I am not surprised. Heat shield cork or not, with inert mass fractions in the 4-5% range, these items are quite fragile. A tumbling cylinder is going to get crushed from the side by stagnation pressures as it tumbles broadside. High internal pressure could stave that off a bit, but stopping the tumble with a drogue to take the loads end-on is actually more effective, and the toughest, heaviest part of your heat shield can be smaller.
Spacex's cost reductions come from a smaller logistical support tail, not from reusability. Falcon-9 at 10 metric ton payloads has the same price per unit mass delivered as Atlas-5's max 20 ton payloads (both near $2400/pound, if memory serves). Falcon-Heavy will beat Atlas 5 by about a factor of 3 on unit price, and deliver more than twice the mass at 53 metric tons. And that's without effective reusability. Small logistical tail is the real driver for cheaper access to LEO, not reusability.
Reusability might help, though. (But, I really doubt it would be more dramatic than what Spacex achieved with its smaller logistical tail.) Maybe the first step is detach and save the engines only. Use the tankage sacrificially to protect the engines during reentry, then detach and parachute the engines to the sea. They'll have to be tough engines, sea water does bad things to hot metal. They'll need a float, too.
GW
Shuttle leading edges went to 3000 F and required carbon-carbon composite precisely because the wing loading (vehicle weight divided by wing planform area) was up around 1 or 2 hundred pounds per square foot, just like a fighter jet, and all the space capsules. The white tiles on the sides and upper wing surfaces, and the black ones on the belly and lower wing surface, were low-density alumino-silicate, with a solid phase change that causes cracking at 2300 F. Those were restricted to peak 2000 F skin temperatures on the shuttle. Carbon-carbon is weak enough structurally, to be sure. Those tiles were far more fragile yet.
If the vehicle has a much larger aerosurface for its weight (low wing loading, say 10-20 pounds per square foot), peak skin temperatures reduce to under 2000 F, although total heat to be absorbed and disposed of actually increases. Skin temperature drives the material selection problem, the other is handled fairly easily. You will be decelerating at higher gees to make this happen. Plus, it's all a transient.
Myself, I rather like the idea of enduring a bit rougher ride in order to make my re-entry vehicle out of simple aluminosilicates, perhaps even plain old fire curtain cloth on a steel tube frame. I think it is very funny that the better, less fragile reentry vehicle might actually be built similar to the venerable old Piper Cub of the 1930's.
And, low density ceramics should be fiber reinforced as ceramic-ceramic composites, not that fragile stuff on the shuttle. I have done this, a quarter century ago. They are still extremely low density, yet fairly tough structurally. Really tough compared to that fragile nonsense they flew on shuttle. I used mine as a ramjet liner, which survived hours of burn and hundreds of excursions into very violent rich-blowout instability. The only reason I quit then was the project was done. Could have gone on for many more hours.
As it turns out, the materials I used then are still available. I checked just the other day.
GW
Paper airplanes from ISS. Interesting. The ultimate in low wing loading. Did they ever run this experiment? Especially since the ignition point for paper in air is about 451 F or 233 C? (not as exciting in metric, thanks to Ray Bradbury).
I think I need to amplify for Bob Clark on airbreathers. Ramjet and gas turbine are quite different, even though they may share the same kinds of inlet components. Gas turbine air massflow is set by the engine speed and the subsonic air density at the compressor face. In essence, it's almost a constant volume flow rate at any particular rotor speed, and that volume flow is more-or-less proportional to the rotor speed (which is your throttle setting). The inlet has to be operated subcritically in order to match the massflow it scoops up with the massflow demand of the engine. The usual specifications give the maximum static thrust of the installed engine, and a minimum TSFC figure, which is not obtained at that thrust, but at around 2/3 thrust or thereabouts. Afterburner complicates things further. In-flight figures are different yet.
Ramjet does not have a massflow demand inside to match. The inlet airflow maximizes at whatever the cowl lip can sweep out, or can be less due to spillage, but never greater. Ramjets can (and were) designed to operate subcritically for maximum pressure recovery, but most modern all-supersonic systems are supercritical inlet for maximum massflow. If the internal flame stabilizer is not too lossy, then max massflow is more important than max pressure recovery. Systems that fly subsonically or transonically still design and operate the old way.
For ramjet, the internal fluid mechanical conditions from forward and aft must match up at the nozzle entrance: the amount it can flow must match at recovered pressures to the amounts scooped up and injected as fuel. If the nozzle can flow more, then you reduce pressure recoveries by going further supercritical. If the nozzle cannot flow enough, then you reduce massflow by going subcritical and spilling air. These calculations can get quite complicated, especially if the nozzle unchokes, or a C-D nozzle separates due to overly-high backpressures. All of them are best done with compressible flow models.
GW
To answer a question Josh asked earlier, I spent some time at what was then LTV Aerospace in Dallas working on the old "Scout" launcher. I used a combination of jiggered rocket equation stuff and motor manufacturer catalogue data to set up the real trajectory code stuff. The "gold standard" was (of course) the trajectory code. My job was to determine feasible advanced configurations for "Scout", and feasibility of some really unusual missions for it to do. "Scout" was a 4 stage solid propellant vehicle. They lost 1 of 4 in flight test, then never another one in 30-some years.
For Bob Clark: airbreather thrust, particularly ramjet, is very strongly (dominantly) dependent upon flight speed and altitude air density. The nozzle thrust is calculated same way as a rocket (chamber total pressure, gas properties, pressure ratio across the nozzle, and nozzle geometry), the pressure is just lower and the expansion ratio a lot less. You do need to worry about the difference between static and total chamber pressure, unlike most rockets.
The ram drag is the drag of decelerating the ingested stream of air into the vehicle. Its massflow multiplied by its freestream velocity (in appropriate units of measure) is the way that is done. But, nozzle force minus ram drag is only "net jet" thrust. There are several more propulsion-related drag items to account.
There is spillage drag for subcritical inlet operation (which also means reduced inlet massflow!), additive or pre-entry drag for ingested stream tubes in contact with the vehicle forebody, and the drag of boundary layer diverters or bleed slots, quite common with supersonic inlets. None of those are simple to calculate "from scratch" (we use wind tunnel test data to correlate empirically a coefficient for each as a function of Mach and vehicle attitude angles), and taken together they are often quite a significant force.
If you subtract that sum of drags from net jet thrust, you have the "local" or "installed" thrust, corresponding with just plain airframe drag. Most airframers work in that definition. If you don't, then you have to add that sum of propulsive drags to the airframe drag to get the corresponding proper drag for "net jet" thrust-drag accounting (not very popular outside the propulsion community).
GW
I am not sure which thread to post this in, as elements of it are being discussed in several of these conversations. But, the screen names are the same in all those threads, so I guess it doesn’t really matter.
Most of what I have to say is more strategic thinking than tactical. Bear with me. The questions being debated in these threads bear more on what to do on Mars than on how to get there, and on what order we try to do these things on Mars. But some of the necessary strategy is being left out.
ISRU is one of those things. Knowing what is there to use for ISRU is another. Life support, etc, gets into this, too.
Betting the farm on remote sensing results:
There always has been, and still is, a difference between remote sensing and ground truth. We’re better at it than we used to be, but still often wrong, at one level or another.
For one thing, the level of resolution is still generally pretty coarse, usually closer to km than m. For another, there is still great uncertainty exactly what hydrogen compound you are detecting, water or something else. I’m sure that’s still true for some of the other species, too. But I’m no expert in it.
So, you may land on a site with your equipment to convert ice and CO2 to methane and O2, and find out the real ice vein is 3 km away, after you set up your camp, which is outside the logistical capability you brought with you. And, it’s more icy dirt than dirty ice, unlike what the sensing from orbit said. This puts your fuel-making schedule out of reach of the launch window home, and you have already bet your life on making fuel for the return trip, equipped the way you are. So, you die there.
That is one possible example of why I say betting your life on doing effective ISRU on the first trip is a stupid thing to do. Try some ISRU, yes, on the first trip! But, don’t bet on it really working until you’ve done some ground truth exploration. And that means digging and drilling, deep. Maybe a km or two.
That brings up what the point of “exploration” really is:
Politically, and to the general public, which includes space travel opponents, you cannot sell colonies up front. That comes later: you sneak it by them with explorations and experimental bases, gradually.
But, we all know the only point of going there is ultimately colonies that become self-supporting independent nations, that become future trading partners. So, how do they support themselves? What might they have to offer in trade? You cannot answer questions like that until you know what is there, and I do mean really what is there, and where it is. Actual ground truth.
“Exploration” is not flag-and-footprints, it is not sightseeing for photos, it’s not samples within 10 cm of the surface. “Exploration” is really obtaining the answer to the two very deceptively-simple questions:
(1) What all is there?
(2) Where exactly is it?
Those wordings I mean exactly as written, word for word. They are not just Texas slang. Think about it a bit, and see the power of writing it exactly that way.
You cannot do that kind of exploration from orbit with remote sensing because of resolution and ground truth troubles, although that’s a good start. You certainly cannot do it with the one launch-one landing, or one trip-one landing approach, precisely because no two sites are alike. Not here. Not there. Not anywhere.
And you cannot do it with surface probes that just barely scratch that surface. We now know that 5-10 cm soil cover is enough to stabilize ice against sublimation on Mars. There could be ice everywhere. But is there? Where there really is ice, how much? How pure? How deep does it go?
I submit to you that, under this definition, we did not explore the moon during Apollo. (Although, politically, lunar exploration is now an impossibility to sell as a reason for going back to the moon, because “we’ve already been there”.) What we did with Apollo was magnificent, but it wasn’t exploration. Half the landings were 99+% engineering flight tests (no rover car). Only the last one brought a real geologist along.
President Nixon killed Apollo after Apollo 17, when we had built all the hardware to fly through Apollo 22. Exploration wasn’t on his mind. Only “flag-and-footprints” was, and we’d already done that. Essentially, we built a transportation system that could take men to the moon, and then abandoned it, before actually exploring on the moon. (More about governmental stupidity below.)
Another point: the men vs robots argument is a false zero-sum game for accountants to mis-use.
It takes both. At this time in history, robots only see what they are programmed to see. Men can see what is actually there, if it hasn’t been over-trained out of them. The men complement and complete the work of the robots, toward answering those two questions that are the essence of exploration. Apparently, few people actually see that relationship. Lack of strategic thinking, I suppose.
We’ve been sending robots to Mars since 1965. It’s time to finish the job and send the men, for that oh-so-essential ground truth. We already have the tinkertoys to do this. We could have done this at any time since the old Mir space station gave us the answer to zero-gee exposure: roughly 1 year max. So spin the crewed vehicle for artificial gravity. Why is this still an issue?
I suggest that the first trip make 1 to 3 dozen landings, scattered all over the planet, at the most interesting places the robots have found. If you resurrect NERVA for reusable single-stage “landing boats”, you can really reduce the LEO assembly job required to mount that kind of a mission. They can even push all the landing propellant to Mars unmanned for you.
You’d better plan on spinning the manned vehicle portion of what flies to Mars, for artificial gravity, at pretty close to the only thing we know for sure works for a 2 year mission: 1 gee. That solves an enormous list of life support issues, if there is artificial gravity. Including cooking, eating, bathroom plumbing, etc; a whole host of difficult design problems radically simplifies. Just spin the damned ship. End over end. Build it long, not wide or complicated. Easy.
As for radiation, around 2 years is likely OK unshielded for cosmic rays. But, you’d better provide a shelter for solar flare events, and if you’re smart, you design it to be the flight deck, too. That way you can conduct critical flight maneuvers, no matter the solar “weather”. On the surface, your exposure to cosmic rays is halved, because the planet beneath your feet is a half-sky shield. But on Mars, you still have to worry about sheltering from solar flare events. A meter of water or dirt is more than enough.
Given an operating Falcon-Heavy at 50+ metric tons per launch (supposed to fly the first time this year from Vandenburg), we really could build that trip’s assets in orbit almost right now, twice as cheaply as with Atlas-5 at 25 tons per launch. Actually, the lander is the pacing item. It does take a bit a time and effort to salvage the art of NERVA rocketry. There’s only a tiny handful left of the men who actually did it. They’re 80+ age now.
Remember, “rocket science” ain’t just science. It was (and still is) about 40% science (written down for all to read), 50% art (residing only in the minds of those who did it), and 10% blind dumb luck (subject to Murphy’s Law in the extreme). Company CEO’s and government lab heads really hate it when you tell them that little inconvenient truth. But they need to be often reminded of it.
You could even use NERVA as the crewed vehicle propulsion to further reduce the sizes of the vehicles sent to Mars. Makes LEO assembly even easier and cheaper yet. Think: 5 years, $30 B. But only if done by the right folks.
That brings up the proper roles of government vs industry (and the stupidity problem):
To see through to my point here, one has to ditch the political ideologies and belief systems of both the “right” and the “left”. Both are false. History teaches a different lesson.
History teaches that most businesses simply do not willingly invest in new technologies. Instead they apply something already known to work, for their new products. That way is cheaper, pure and simple. Sorry, but that’s been true for millennia. Even the modern “market” in the aggregate works this way, being nothing more than the sum of all the businesses.
Governments, on the other hand, are notorious wasters of resources, being more consumed by internal politics than outside threats or opportunities. But, they are historically the ones who worked the technology and exploration problems. Longer-range armed ships were developed for government navies, built by businesses, but paid for by government. It was Queen Isabella who financed Columbus’s voyages, all four of them.
That same model worked pretty well for the Dutch and English East India Companies. The political objective was a revenue-producing colony. Business got to make near-monopoly profits from it. To carry it out required government funded technology (better ships) and government-funded exploration (where to put the colonies). This has worked well in one form or another for centuries now.
In the second half of the 20th century this model got developed into the independent research and development (or IR&D) process, here in the US and in Europe, mostly for military work. The divisions between government and industry are a bit blurrier in Europe, but it’s still the same basic process. Government decides technology and exploration objectives, and then funds businesses to develop and build the stuff. The real “smarts” is in the contracting-business staffs, not in the government labs. (Not many folks see it that way, though. I suppose, mostly, because few really looked closely.)
That model took us from propeller aircraft in 1939 to landings on the moon and probes to a few of the planets by 1969. Pretty effective way of doing this job, with one vulnerability. If government gets too eaten up in its own politics, then it does a bad job setting objectives, and a bad job funding the ones it does set. That is fundamentally why the US space program went nowhere but LEO the last 4 decades, and why our airliners are but incremental improvements on what we had flying by about 1970.
Remedy that vulnerability (fundamentally a political effort curing massive governmental stupidity), and the IR&D model gets revitalized. In its heyday (1939-1969) there was the fastest, most intense period of technology development and exploration humanity has ever seen. There was nothing like it before, or since.
Otherwise, it will take those very rare visionaries in business, like Musk and Branson and a small handful of others, to do anything at all worthwhile, and the pace will be very much slower. They, not government-as-it-is-all-balled-up currently, have done things toward a space tourist industry, and toward successfully reducing launch costs to LEO.
Imagine what we could do with a revitalized IR&D model, and folks like Musk and the rest to carry it out! Santa, here’s my list ……….
GW
I personally never found in 30 years any analysis worth a plugged nickel between the extremes of the jigger-factored rocket equation, and full 2-D (or 3-D) computerized trajectory analysis.
GW
Mars 500 had little to do with anything but the psychology of confinement for long periods in tight spaces. But that itself is valuable, so I don't really consider it a joke.
However, on the other hand, the mission model behind it looks like 60's Apollo: 99+% flag-and-footprints, 1% (or less) real science. One landing for one trip. What nonsense!
It is clear that no one associated with Mars 500 learned the lesson of what we did wrong in Apollo. So few have 20-20 hindsight vision? Disappointing.
GW
Louis:
Hey, we're both looking for Musk to lead! He'll have to, NASA won't. Too petrified with gigantic-bureaucracy disease. The NASA we have is not the NASA we need, and hasn't been since the 70's. Excepting upstarts like Musk and a small handful of others, the contractor base we have is not the contractor base we need, either. Same stultification problem as NASA, just commercial instead of governmental. Same disease. A committee is a life form with 6 or more legs and no brain.
I wouldn't trust NASA to develop the supple space suit we need right now for everything we want to do in space, not just going to Mars. They have (or at least did recently have) MIT under contract to work on mechanical counterpressure suits, but then saddled them with an unnecessary design requirement (1/3 atm equivalent pressure), so that no material existing can do the job.
Yet NASA paid (in part) for the late 60's demonstration of the very kind of suit we need (which worked just fine at 20-25% atm pressure equivalent). Go see my posting for Jan 21, 2011 on "exrocketman" (http://exrocketman.blogspot.com) for an analysis of the proper design requirements for a mechanical counterpressure space suit. Then check out http:www.elasticspacesuit.com to see what Paul Webb did so long ago, with nothing more than pantyhose materials.
You can also check out the paper I gave at the Mars Society convention this last August in Dallas, Texas, USA. You can find it in the Mars Society electronic archives, or go look on "exrocketman" for a version of it posted and dated 7-25-2011, and some second thoughts on the back-up propulsion dated 9-6-11. If you like, just scroll down to find the first article with "space program" as a keyword, and click on that keyword. Then it shows only space-program-related postings. The order is newest-first. That's one kind of the different style of thinking that might lead to dozens of Mars landings, all in one trip, for well under $50 billion.
GW