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Post by ka9q on Jul 14, 2010 18:53:05 GMT -4
Thanks, Jay, for the writeup on film cooling. Very informative.
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Post by ka9q on Jul 14, 2010 5:53:43 GMT -4
That's a really good idea, I'm glad I thought of it! :-)
You could erect a tent or tarp of aluminized Mylar or Kapton over the LM so that it couldn't see black sky. It would still see the surface, but the top layer of regolith has a pretty low heat capacity and conductivity so it probably wouldn't soak much heat away into the ground. This would all have to be calculated, of course.
But now that I think about it, would all this really be necessary? The LM already has excellent insulation in both stages. The insulating blankets are obvious on the descent stage, but I think the ascent stage has a similar thermal design; the blankets are merely hidden by the thin metal panels that form the outer micrometeoroid shield for the crew cabin.
As I recall, some of the projections (particularly the RCS engines) do get quite cold and were supplied with electrical heaters. During translunar coast they were powered by the CM through an umbilical to conserve the LM batteries.
I think surviving the lunar night will be entirely about electrical power. Batteries aren't big enough even with solar panels to charge them during the day. Even fuel cells might be pushing things unless they can be reversed and operated as electrolyzers from solar panels during the day to store H2 and O2. That means you need either very large gaseous storage tanks or some way to liquify the O2 and H2.
It sure would be a lot easier if you could just carry a small nuclear reactor...
In-situ O2 generation should be a very high priority in any return to the moon. Half the moon's crust is oxygen! Practically every chemical component of the soil is an oxide, silicate or titanate, including ilmenite, iron/magnesium titanate, that is especially easy to process into free oxygen.
The advantages of using what's already on the moon instead of hauling it up with you from the earth seem so overwhelming that I can't understand why it wasn't a major part of Constellation.
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Post by ka9q on Jul 13, 2010 12:12:58 GMT -4
Since electrical power was so tight I too would put a deployable solar panel near the top of my list. I often wonder why one wasn't included on the later J-class missions.
I suspect that a revamping of the LMs avionics would save so much power that this alone would greatly extend the water and power supply.
Of course, a major part of any such study would have to start with numbers: the actual amounts of consumables and their consumption rates, and how they could be changed with new technology. For example, what did the LM consume in its powered-down lunar stay state? During the EVAs it consumed no O2 at all, though it had to later refill the PLSSes and repressurize the cabin. And so on.
If you want some *real* fun, figure out what would be required to keep the LM and its crew alive during a 2-week lunar night. Here I don't see any alternative to nuclear power; the plutonium fuel for the SNAP-27 RTG dissipated about 1.5 kW, on the order of the waste heat produced by the powered-up LM, so if you're really careful you could use one to stay warm. You'd still need electricity and other consumables to get you through what is likely to be a non-productive period, though.
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Post by ka9q on Jul 13, 2010 7:01:13 GMT -4
Does anyone here actually teach classes based on Apollo as a case study? Jay seemed pretty knowledgeable about it, so I figured maybe he's done it.
Here's an idea that occurred to me that might make a fun project for a space systems engineering class: do a preliminary study on the modifications you'd make to a stock Apollo LM to increase its lunar stay time as much as possible. You're encouraged to update the subsystems to modern technology, but the overall structure, function and mass have to remain essentially the same. I.e., it should still be able to fly in a Saturn V (if we had one flight-ready).
I know how I'd personally approach a project like that, but I bet some engineering students would come up with some brilliant ideas that wouldn't even occur to me.
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Post by ka9q on Jul 13, 2010 6:53:22 GMT -4
"Manual control" is better described as "semi-automatic" control. Instead of letting the computer land automatically, Armstrong put it into a mode where he could fly it essentially like a helicopter through the computer.
The LM flies by vastly different principles than a helicopter. It reacted to 16 binary inputs - the control valves for each of the RCS thrusters - plus the throttle and gimbal for the descent engine (three analog inputs). True "manual control" would have implied that Armstrong generated all these signals directly somehow, which he didn't. He pointed the stick in the direction he wanted to go and the computer figured out which RCS thrusters to fire and how to gimbal and throttle the main engine to get it there.
In the mode Armstrong used, I believe he'd tell the computer to maintain a certain rate of descent, increasing or decreasing it by toggling a switch much as you adjust the speed of an engaged cruise control. This too required some fairly complex calculations to account for the rapidly decreasing mass and shifting c.g. of the LM and the fact that the thrust vector wasn't always pointing down (as when Armstrong pitched over to speed past the boulder field).
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Post by ka9q on Jul 13, 2010 6:40:09 GMT -4
I've seen numerous rocketcam videos of Delta-II launches showing how the RP-1 fueled first stage engine plume behaves as the launcher rises through the atmosphere. (Obviously until SRB staging you don't see much of the RP-1 plume alone.) It dims and blossoms out until all you see are irregular radial black streaks (which must be a perspective illusion).
But the camera on Falcon 9 showed no visible plume during second stage flight at all. I attribute this difference to the difference in atmospheric pressure between where the two stages operate.
Most launchers drop their payload fairings shortly after staging, typically corresponding somewhat arbitrarily to the point where aerodynamic heating on the payloads is equal to or less than solar heating (1.3 kW/m2). So at least by this definition of "atmosphere", Delta-II's first stage never burns outside it while the Falcon 9's second stage certainly did.
(BTW, when the fairing is dropped, aerodynamic pressure is already nil. Peak heating and peak pressure occur at different times in both ascent and descent because of that crucial difference in the exponent on velocity in the equations for momentum - mv - and kinetic energy - 1/2 mv2.)
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Post by ka9q on Jul 12, 2010 8:36:58 GMT -4
I assume at least some of you saw the recent successful launch of the Falcon 9, right?
This launcher is highly unusual in burning kerosene/LOX in an upper stage. Outside of some Russian designs, from which I have yet to see an onboard launch video, I can't think of another launcher that does. They all burn either LH2/LOX or hypergols, which we know produce nearly invisible plumes.
What surprised me about the Falcon 9 video was that, in a vacuum, even its kerosene-fueled upper stage had an invisible plume. This confirms what we've thought all along: atmospheric oxygen burning residual carbon is what makes the plume of a kerosene-burning rocket glow brightly.
In a rational world this would put yet another nail into the silly Apollo denier claim that plumes should be seen from the LM ascending from the moon.
Aside from the brief flashes we see at ignition, is there any visual record of any kind of liquid rocket engine producing a steady, bright plume while burning in a vacuum?
I leave out solid propellants because they produce solid combustion products (e.g., aluminum oxide) that catch the sun. They make for the most spectacular dawn and dusk rocket launches.
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Post by ka9q on Jul 12, 2010 5:20:19 GMT -4
The Wikipedia JPEG page says in a caption, "The DCT transforms an 8×8 block of input values to a linear combination of these 64 patterns...". Is that the two-dimensional equivalent of saying that a complex audio waveform can be broken down into individual sine waves? Exactly. The DCT is almost the same thing as the FFT (which is commonly used to decompose an audio signal into a collection of sine waves) except that an image is 2-dimensional. Sine waves form one possible set of "basis functions" from which you can construct an arbitrary waveform by combining them at specified relative levels. The same is true with 2D images, except that your basis functions also have to be two-dimensional. The DCT basis functions look like checkerboards, though they only have square spaces when the horizontal and vertical frequencies are the same; the rest are rectangular. The DC component is a special case, as it represents the average brightness of the entire macroblock. Unlike the Fourier transform, which uses both sines and cosines, the DCT - discrete COSINE transform, uses only cosines. But the set is still big enough to synthesize any picture pattern you want.
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Post by ka9q on Jul 7, 2010 22:44:50 GMT -4
It's not the DCT per se that creates these artifacts, but the quantization of its output.
The DCT is a fully reversible function; you can take its output and get back exactly what you put into it. But that wouldn't help compress the image. You get compression only when you reduce the accuracy of each number used to represent the output. (The output of any lossy compression scheme won't be the same as the input. It can't be.) The DCT was chosen because you can quantize its output a great deal without the eye noticing too much.
The DCT is closely related to the Fourier transform. Each "macroblock" in the image (typically 8x8 pixels) is run through the DCT, and the outputs represent the amplitudes (or "strengths") of each 2-dimensional "frequency" in that part of the image. The higher frequencies correspond to the smaller details, so those numbers can be more heavily quantized without you noticing. The lowest frequency is DC, i.e., the average brightness of the whole macroblock, and this is sent with the greatest precision.
Note that the compression doesn't just get rid of fine detail (though in principle it could, if you cranked the compression way up). It just represents its "intensity" less accurately. So when you look closely at the reconstructed image, you'll still see small details but they won't quite match the original. They won't even match the nearby pixels, especially at macroblock boundaries. This is how you get those artifacts, especially around sharp edges.
Does this help?
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Post by ka9q on Jul 2, 2010 20:44:45 GMT -4
Then maybe what looks like a roll control thruster is actually the turbine exhaust. If you want thrust, you might as well shape it like a classical convergent/divergent nozzle, right? That could be, but it doesn't look like the picture in this webpage (about half way down the page). That's a static firing, and it doesn't look like the turbopump exhaust is steerable. Maybe that's a first stage engine being tested, and the second stage model has some modifications, including a steerable nozzle on the turbopump exhaust?
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Post by ka9q on Jul 2, 2010 20:37:49 GMT -4
Arrived 28 JUN 10. Interesting to note that it shows footage of the Saturn I, which is pretty rare in my own experience. As opposed to the Saturn IB?
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Post by ka9q on Jul 2, 2010 20:31:03 GMT -4
Come to think of it, I don't think a cold fire test was performed on the Phase III-D satellite launched as Oscar-40 in 2000. Had one been done, it would have detected the "remove before flight" cap left on a vent port in the helium valves.
These pressure-fed engines, though vastly simpler than large launcher engines with turbopumps, are still rather complicated, with various "gotchas" like this one. The solenoid that controls the engine does not directly drive the propellant valves. Instead it controls a helium valve that in turn drives pressure-operated valves that control the propellants.
The cap in question was on a vent that relieved helium pressure in the operating line to the oxidizer valve when the control valve was in the non-firing position. So when the engine was commanded to fire, it fired normally. But when it was commanded to stop, the oxidizer propellant valve remained open; the helium holding it open could not vent to space. Because the engine was hot from having been fired, the pure oxidizer rapidly corroded and burned through the combustion chamber, streaming into the spacecraft. This was followed by pressurized helium when the oxidizer tank emptied. It blew off the bottom panel of the spacecraft, tearing some cabling.
The amazing part is that one transponder on the spacecraft continued to function for several years after this incident.
The point of all this? Take the claim that hypergolic engines are "simple" with a large grain of salt. And never forget that there's simply no alternative to testing...
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Post by ka9q on Jul 2, 2010 20:18:11 GMT -4
Jay, what's a "cold fire"? I know it's practice to test at least some hypergolic engines (e.g., the 400N MBB thrusters in the amateur Phase III satellite series) by putting isopropyl alcohol in the tanks, loading the helium tank, and "firing" the engine. I'm not sure if they expended a pyro valve, or simply bypassed it for the test after checking that the firing circuit was working.
Although the engine does not actually fire, this tests the major system components - the control circuits, the engine propellant valves and the helium subsystem, especially the pressure regulator. And it provides an opportunity to check for leaks.
Isopropyl alcohol is used because its density is similar to hydrazine and it's non corrosive (though it is hygroscopic, so you have to be careful not to allow water into the system.)
Is this what you mean by a "cold fire"?
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Post by ka9q on Jul 2, 2010 20:08:44 GMT -4
What is the difference between a sump tank and a storage tank? The Apollo SM had four SPS propellant storage tanks, two each for fuel (Aerozine-50) and oxidizer (nitrogen tetroxide). Each propellant had a "storage tank" and a "sump tank" arranged in series so that the sump tanks directly fed the engine and the storage tanks fed the respective sump tanks. In the drawings that I've seen, the bottom of each storage tank fed the top of the sump tank through a long tube in the latter tank. Helium pressurized the top of the storage tank through a long internal tube so it would drain first. The sump tank stayed full until the storage tank emptied. Then the storage tank would feed helium to the sump tank, and it too would drain. I wondered why the two tanks weren't simply interconnected at the bottom, but then I realized why. When the storage tank emptied, helium began to flow through the interconnection. If not for that long tube, the helium would vigorously bubble up from the bottom of the sump tank and possibly be ingested by the engine. The tube ensured that the helium came out in the ullage space at the top of the sump tank and the engine drew solid liquid.
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Post by ka9q on Jun 29, 2010 7:09:58 GMT -4
Ya know, I always thought that had sufficient ambiguity to be conveniently ignored by the HB crowd The more I hear that Kaysing quote, the less ambiguous and the more damning it becomes. I.e., Kaysing never really believed that the Apollo missions were fake, at least not at first. He said it only because he knew it was an outrageous thing to say. Maybe he did convince himself later, but that wasn't his reason at first.
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