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Post by ka9q on Jan 2, 2011 23:39:38 GMT -4
The R-7 and Atlas were developed as weapons, not space launch vehicles. So was the Titan. The Titan II launch vehicle gave the Gemini astronauts a quicker and more exciting ride than the later launch vehicles (Saturn and Shuttle) designed specifically for human spaceflight. On Gemini VII, the peak accelerations were 5.5g and 7.25g just before first and second stage cutoff, respectively. They also got to orbit in less than 6 minutes! Compare that to the Saturn V, which peaked at 4g at the end of first stage flight, shutting down an engine to limit the acceleration to that value. As you'd expect, it took longer to reach orbit; injection occurred at T+0:11:39 for Apollo 11. The shuttle is even gentler, with a peak acceleration of only 3g just before main engine shutdown.
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Post by Obviousman on Jan 3, 2011 0:30:29 GMT -4
Is it worth mentioning the Space Task Group, or is that going into too much detail?
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jan 3, 2011 1:17:23 GMT -4
The R-7 and Atlas were developed as weapons, not space launch vehicles. So was the Titan. The Titan II launch vehicle gave the Gemini astronauts a quicker and more exciting ride than the later launch vehicles (Saturn and Shuttle) designed specifically for human spaceflight. On Gemini VII, the peak accelerations were 5.5g and 7.25g just before first and second stage cutoff, respectively. They also got to orbit in less than 6 minutes! I believe it was Andrew Chaikin who talked about that in his book. As I recall, the Gemini astronauts said it was a pretty nauseating ride as well. The guidance system was such that the nose of the rocket keep bouncing up and down as it frequently made small pitch adjustments. And I also seem to remember something about the capsule being oriented such that horizon appeared vertical to the astronauts, which caused some disorientation (though I may not be remembering that part correctly).
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Bob B.
Bob the Excel Guru?
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Post by Bob B. on Jan 3, 2011 10:13:03 GMT -4
Is it worth mentioning the Space Task Group, or is that going into too much detail? Do you know of a good place where I can mention that in what I've already written with making major edits or inserting of a new slide? I think I'm also going to add a slide at the end of the first part about ASTP. That's probably a good place to wrap up discussion about the Space Race. Plus it was the last use of Saturn and Apollo hardware.
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Post by ka9q on Jan 3, 2011 10:45:18 GMT -4
The guidance system was such that the nose of the rocket keep bouncing up and down as it frequently made small pitch adjustments. That would be true, to at least some degree, for all launchers. At launch, the c.g. of the overall stack is toward the rear so any pitch or yaw maneuver will swing the nose around quite a bit. This made the Saturn V's tower-avoidance yaw maneuver quite noticeable. I hadn't heard that, but it could well be true. I can think of a couple of reasons. Launchers usually have to maintain a certain roll angle so that their antennas can see the proper ground stations. It may also be dictated by the guidance platform, especially if it only has 3 gimbals.
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jan 3, 2011 12:05:37 GMT -4
The guidance system was such that the nose of the rocket keep bouncing up and down as it frequently made small pitch adjustments. That would be true, to at least some degree, for all launchers. My impression from reading the astronaut's stories was that it was excessive for Titan compared to other manned launch vehicles. I don't suppose the Titan engineers considered a man riding the rocket when they designed it.
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Post by ka9q on Jan 3, 2011 19:04:33 GMT -4
I don't suppose the Titan engineers considered a man riding the rocket when they designed it. Exactly. Both Titan I and II were designed (and used) as ICBMs before Gemini adopted the Gemini II as its launch vehicle. They probably had little choice at the time since the Saturn wouldn't be ready yet, and Gemini had to get going as a gap-filler to work out all the techniques that Apollo would later need: rendezvous, docking, space walks, 1-2 week flights, etc.
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Post by Glom on Jan 3, 2011 21:29:19 GMT -4
Another thing I recommend is using "server side includes" when working with webpages that use identical code. For example, if all your pages use the same header and footer, you can create the header and footer as separate files and then tell the other pages to include them when viewed. That way changes to the header and footer only involve modifying two files, not a hundred. Not even that. CSS wouldn't go amiss since the style markup being used is actually long since deprecated as a web standard. It was deprecated as part of the HTML4 web standard which came out in 1997! The only reason it works at all is that browsers still build in backwards compatibility for outdated markup although that may not last. But everyone in the web community would like for such things to disappear ASAP to allow for a cleaner and more efficient web. Bob, I really urge you learn proper web standards for all our sakes.
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Post by Glom on Jan 3, 2011 21:30:40 GMT -4
That would be true, to at least some degree, for all launchers. My impression from reading the astronaut's stories was that it was excessive for Titan compared to other manned launch vehicles. I don't suppose the Titan engineers considered a man riding the rocket when they designed it. It also had much more of a kick to it, didn't it? 8G at the peak.
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Post by Mr Gorsky on Jan 4, 2011 9:18:36 GMT -4
Bob, I really urge you learn proper web standards for all our sakes. Having come across that problem with all my own websites (i.e. I was still using HTML3 standards when everyone else was moving on and finding myself lacking the time to relearn everything) I moved all of my websites onto WordPress (database and PHP based), which does all the hard work for you and makes tweaking, upkeep and updating significantly easier than maintaining your own HTML code. Plus, whilst you can dive into the code and put your own stuff together, there are so many free themes and plug-ins available that you really don't need to do any of that if you don't want to (as I don't at this point). WordPress itself has moved on significantly from its blogging roots, to the extent that a couple of my WordPress sites don't even use a blog.
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Post by ka9q on Jan 4, 2011 12:07:12 GMT -4
It also had much more of a kick to it, didn't it? 8G at the peak. 7.25 g's just before second stage shutdown on Gemini VII. The Saturns were a picnic by comparison, in fact the acceleration during much of S-II flight and the first S-IVB firing was below 1 g - less than they felt sitting on the pad waiting for launch! I would think that the Titan II was probably more efficient because of its greater acceleration. They would be hard to compare, though; different number of stages, different propellants, different engine designs, etc.
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Bob B.
Bob the Excel Guru?
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Post by Bob B. on Jan 4, 2011 12:48:00 GMT -4
I would think that the Titan II was probably more efficient because of its greater acceleration. They would be hard to compare, though; different number of stages, different propellants, different engine designs, etc. High thrust-to-weight ratio boosters experience a low gravity loss but a high drag loss. On the other hand, low thrust-to-weight ratio boosters experience a high gravity loss but a low drag loss. For instance, let's consider a rocket in the first few seconds as it lifts off the launch pad. If the thrust-to-weight ratio is one, the rocket will simply balance its own weight and won't go anywhere until it burns off propellant. Most launch vehicles have a liftoff thrust-to-weight ratio of at least about 1.2 (the Saturn V was actually a little less than this). In that case, about 83% of the rocket's thrust is simply counteracting gravity, while only 17% is lifting and accelerating the rocket. If you increase the thrust-to-weight ratio to, say 1.5, now 67% of the thrust is counteracting gravity and 33% is lifting the rocket. The high thrust rocket loses less of its potential in overcoming gravity. However, since the high thrust rocket accelerates faster, it is traveling at higher speed through the lower atmosphere. This means it is subjected to more drag and, therefore, must expend more of its propellant to overcome this drag than a low thrust rocket would have to do. I haven't studied the variables enough to know if there's an optimum and where it might be. From what studies I have done, it seems clear that gravity losses are generally much more than drag losses, at least in the thrust ranges of most space launch vehicles. My gut feeling then is that higher thrust is going to be more efficient. I have to believe that thrust is more likely to be limited by acceleration loads on the payload rather than by a need to lessen drag.
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Post by ka9q on Jan 6, 2011 9:36:00 GMT -4
Yes, of course the drag would be worse for a high acceleration. I don't know why I forgot that...
Another factor is the pitch profile. You'd like to climb above the thickest parts of the atmosphere as quickly as possible to reduce drag, but flying a lofted trajectory is inefficient because of the increased gravity loss.
I'm certain that the only way to optimize these things is to burn a lot of computer cycles. Easy for us now, but I can imagine what it was like in the 1960s to get enough computer time to do serious mission planning.
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Post by Glom on Jan 6, 2011 14:19:28 GMT -4
Time for us to see what language can get this done the quickest.
The scenario is this.
A single stage rocket with a specific impulse of 1,000s (it's unobtanium oxide fuel that breaks down exothermically). The payload is 5,000 kg and it is to be inserted in a single burn into a 400km altitude circular equatorial orbit. The launch site is bang on the equator. The rocket can be as powerful as you like providing net acceleration does no exceed 8G. The LV itself has a base OEW of 100,000kg and can take on 2,000,000kg of fuel should you want that much. You can grow the LV if you need more but for every 25,000kg of fuel you add, you'll need to add 1,000kg to the OEW of the LV.
Gentlemen, start your compilers or interpreters.
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Bob B.
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Posts: 3,072
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Post by Bob B. on Jan 6, 2011 15:50:32 GMT -4
Time for us to see what language can get this done the quickest. The scenario is this. A single stage rocket with a specific impulse of 1,000s (it's unobtanium oxide fuel that breaks down exothermically). The payload is 5,000 kg and it is to be inserted in a single burn into a 400km altitude circular equatorial orbit. The launch site is bang on the equator. The rocket can be as powerful as you like providing net acceleration does no exceed 8G. The LV itself has a base OEW of 100,000kg and can take on 2,000,000kg of fuel should you want that much. You can grow the LV if you need more but for every 25,000kg of fuel you add, you'll need to add 1,000kg to the OEW of the LV. Gentlemen, start your compilers or interpreters. Generally speaking, about 9,000 m/s is needed to achieve LEO, taking into account gravity and drag losses. If the ISP is 1,000 s, then a mass ratio of only about 2.5 is needed. Since the total dry weight of LV and payload is 105,000 kg, then the launch mass would be 262,500 kg. Therefore, only 157,500 kg of propellant is required. Let's say we want to limit the acceleration to 3.5 g at engine shut down, then, assuming the engine is not throttleable, our thrust would be 367,500 kgf (3,604 kN). The thrust-to-weight ratio at liftoff would be 1.4.
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