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mrmorris

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<font color="yellow">So we should be saying <br />pf = propellant fraction</font><br /><br />Ayup. My mind was thinking 'propellant fraction' but my fingers (traitorous &%$#@!s that they are) were typing 'payload fraction'.<br />
 
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bobvanx

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Propellant fraction<br /><br />propellant fraction<br /><br />propellant fraction<br /><br />Am I retrained yet?<br /><br />From those slides, "Drag Lossess ~400m/sec" or nearly a quarter of the deltaV needed to achieve LEO is a really darn interesting number. I wonder how John Carmack *is planning* to overcome this?<br /><br />Oh, wait, his vehicle is about altitude, so it's just "Gravity losses ~1400 m/sec" that he's trying to solve. pf =.88 will be more than enough for him.<br /><br />OMG<br /><br />If John succeeds in building a vehicle that generates a 1400m/sec deltaV, and then he pops that onto a steam cannon, or electric rail gun, he'd have an orbital vehicle. The short (100 m) steam catapults aircraft carriers use accelerate airplanes by roughly 75m/sec.<br /><br />A powered launch platform is one way to leave the "first stage" on the ground, and it could certainly be robust enough to be easily reusable. Heck, put the Disney folks on it: have you ridden their electromagnetic catapult roller coaster, AKA "California Dreamin'?" It pops you up to 65mph in less than 150 feet.
 
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bobvanx

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>possibilities of Burt getting rights to the X-33's ... Titanium boxes... they bolted right onto the aluminum skin of the vehicle.<br /><br />Burt + Aluminum = not in our lifetimes
 
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spacester

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<img src="/images/icons/laugh.gif" /> <img src="/images/icons/laugh.gif" /> <img src="/images/icons/laugh.gif" /> Did I say aluminum? I could have sworn I said composite, "bolted right onto the studs set into the composite", yeah that was it, that's the ticket, yeah . . . <br /><br />I don't see how a steam catapult would get you to orbit. What I was hinting at is that he has said he is going into the elevator business or something akin to that phrase. And not just to go up and come back down, but that the most effective flight path to orbit for RLVs was to go straight up into space and then start picking up the (massive amount of) orbital speed from there.<br /> <div class="Discussion_UserSignature"> </div>
 
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bobvanx

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Do you remember the rowboat crossing the flowing river optimization problem they made us do in the first semester of vectors?<br /><br />|~~~~|<br />*~~~|<br />||~~~|<br />||~~~|<br />|~~~|<br />|~~~|<br />|~~~|<br />|~~~#<br />|~~~~|<br /><br />Goes like this, for those who were never exposed to it:<br /><br />You have to get across the river and down the bank to the *. What's the fastest route? You have a rowboat # and you can run |.<br /><br />Row perpendicularly across the river, then jump out and run along the riverbank?<br /><br />Row at an angle, straight towards the *?<br /><br />Or a mixture? Row slightly upstream, then jump out and run?<br /><br />Getting to an orbital (or even a great-circle sub-orbital, something useful like Mojave to Woomera) state is like that problem. What White Knight does for SS1 (or any other air-launch) is it gets you partway into the river.<br /><br />On another thread, the discussion of a low-altitude hotel is like bringing the far shore closer.<br /><br />Using an electromagnetic gun or steam catupult is like moving this shore out into the river.<br /><br />No matter how you solve it, at some point you've got to be going 17,000 mph. The engineering is to find a way to do it with a pf of less than unity (duh. Apologies to those to whom that's obvious). So any little "tricks" we come up with (balloon launch, powered launch sled, air-drop or my favorite, a rotating tether capture at the apex of a Mach10 semi-ballistic flight) serve to get that pf down to .9 or less.<br /><br />All of which has next to nothing to do with XCOR news. Sorry 'bout that.
 
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spacester

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Good analogy, I like that. I am a big skeptic on analysis by analogy, but I like this one.<br /><br />I think I figgered out how to claim this is on-topic. What are those rocket engineers at XCOR up to, anyway? Well I don't know, so I'll distract them by talking about their fellow rocketeers while you go peek under their tent. <br /><br />Besides, I went to the latest Armadillo thread and it was all about Rutan lately. <img src="/images/icons/laugh.gif" /><br /><br />I'm just saying I see Armadillo as being in competition with a steam catapault; they both move the near shore out a bit. In Carmack's case, he's trying to move it all the way out to the deep channel where there's little resistance to speeding up. You'll never steam catapult out to the deep channel of space. The whole idea of trying to outfox the rocket equation, that's good stuff, but steam catapult? <br /><br />By going up to 101.6 kilometers, you have invested 1.400 km/sec in potential energy. You have also incurred gravity and drag losses. Well, you weren't accelerating to orbit, so drag losses will be not that high, I'll SWAG a number like, oh, say, um, 0.10 km/s and then we have to figure gravity losses.<br /><br />dVgrav = g,eff * t,b * sin(phi)<br />g,eff = local gravity on the way up<br />t,b = time of burn in seconds<br />phi = flight angle<br />sin(phi) = 1.00 for straight up flight<br />g = GM / (R)^2<br />Rb = Re + altitude<br />Earth: Re = 6371 km<br />Earth's GM = 398600.44 km^3 / s^2<br />altitude = 101.6 km<br />Rb = 6371 + 101.6 = 6473 km<br />g, end of flight = 3.986E5 / 6473^2 = .009514 km/s^2<br />g,eff = ( 9.807 + 9.513 ) / 2 = 9.6605 m / s^2<br /><br />y = ½ a * t^2<br />t = sqrt( (2 * y) / a)<br />y = 101600 m<br />a = 1.43 "gees" = 14.11 m / s^2<br />t = 120 seconds<br /><br />So a two minute flight would have<br />9.6605 * 120 = 1159 m / s <br />1.16 km/s in gravity losses<br /><br />All together, he's going to need<br />1.400 + 1.16 + 0.1 <br />= 2.66 km/s deltaV performance from his rocket powered elevator to spac <div class="Discussion_UserSignature"> </div>
 
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bobvanx

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Whew! Your numbers baffled *me!*<br /><br />Seriously though, you've proved quite well that Carmack's space elevator is not a pipe-dream. <br /><br />I don't know when I'll next get to invent a reason to talk with the engineers at XCOR, but trust me, I am super willing to share whatever I'm allowed to. <br /><br />Like the fact that XCOR and Scaled have employees in common.<br /><br />Here's why I keep mentioning some sort of launch-assist. From a quick Google that got me to a Navy future tech website:<br /><br />"Electromagnetic rail guns are the only systems that have demonstrated a capability to launch projectiles to 4.4 km/sec... fielded rail gun systems are expected to achieve muzzle velocity of from 2.5 to 6 km/sec."<br /><br />The G forces of reaching even Mach 5 (2.5 km/sec) in the length of a gun muzzle are of course lethal to anyone who would be along for the ride. But, if the goal is deltaV of about 9m/sec, and you can get even 1.5m/sec of that from something you leave on the ground...<br /><br />And here's why Carmack's a good one to fiddle with this. His test bed is mechanically robust (can I use the word robust-ish? That video of the crash and the pressure tank spiraling away still makes me laugh in horror) while his ability to rewrite the flight software is clearly outstanding. So while it's non-trivial to write the control code for a rocket that would be launched from a rail gun/powered launch pad, I think he's the guy that could figure it out.
 
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bobvanx

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On the Discovery Channel show about SS1, when Burt's talking about Tier Two and we see a pic of a 7-person Virgin Galactic type of craft, does it have an over and under rocket nozzle configuration just like the EZ Rocket?<br /><br />I ddin't notice while I was watching, but as I play the scene back in my head, I think that's what I saw.
 
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Swampcat

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<font color="yellow">"does it have an over and under rocket nozzle configuration just like the EZ Rocket?"</font><br /><br />I saw the same thing. It was also mentioned in another thread about the Discovery Channel's special "Black Sky." <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>
 
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bobvanx

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I asked an engineer over at XCOR, and was told that Burt wasn't interested in their engines.<br /><br />Looks like the over and under is merely convergent evolution.
 
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space_dreamer

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Hopefully the publicity of the X Prize being won means, that it will be easer for small space companies like XCOR to get investment.<br /><br />No new news on there web site though (I’m hoping it’s a case of; no news is good news,).<br />
 
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