Why not external tank on X-33?

[mlorrey]

<blockquote><font class="small">In reply to:</font><hr /><p>"The Falcon1 actually has the highest mass fraction of all the launchers out there, about .9, as I recall, which is well within SSTO range for the Isp they are operating at. I believe,...... . "<br /><br />The first stage needs to reach orbit in order to claim SSTO. Stying in orbit after reaching it is even tougher, however. <br /><br /><br />"Making SSTOs is easy, making them reusable is not. "<br /><br />True enough. But making SSTO is plenty tough enough. <p><hr /></p></p></blockquote><br /><br />That Elon decides to never try launching the 1st stage alone does not mean it isn't SSTO capable. The numbers do not lie, as, in a rare agreement with Jeff Bell, the 'cold equations' of spaceflight don't let you pull a rabbit out of the hat, or to hide one there in the first place. The numbers say that for x average Isp there is a mass fraction for which a vehicle is SSTO capable, no matter whether its launchers find doing so to be economically useful or not.

 

[tomnackid]

LH2 in the space program for environmental reasons is absolutely idiotic.<br />------------------------------<br />LH2 is not in the US space program for environmental reasons. You are the only one who is saying that. LH2 is in the US space program for the same reason it is in the Russian space program and European space program--for performance reasons. As I pointed out before LOX/LH2 rockets have been seen as desirable since Tsilokovsky invented the concept of the liquid fueled rockets--long before "environmentalism" became fashionable! It is relatively cheap and easy to handle--even taking into account the fact that it is a cryogenic gas. It non-toxic so it can be handled by workers without the expensive and time consuming precautions needed for fuels like hydrazine (I hope that keeping your workforce alive and healthy is not seen as being too "tree hugger-ish").<br /><br />The problem with particulate forming in your exhaust stream has nothing to do with environmentalism (Ok, maybe some people will stay awake at night worrying about things like this). The problem is that particulate matter reduces your exhaust velocity and thus your specific impulse. Boranes and boron spiked fuels are prone to producing oxides that result in particulate matter, thus reducing some of the theoretical gains in efficiency. These certainly aren't unsolvable problems, but my impression is that most of the research into borane fuels faded away because the ACTUAL gains in ISP weren't worth the added expense and trouble they brought. Maybe there is some new developments in the field. <br /><br />The conventional LOX/kerosene first stage with LOX/LH2 upper stages is close to an ideal compromise for most purposes. Due to the lack of developmental funds the Shuttle had to go with LOX/LH2 + SRBs from pad to orbit--NASA would rather NOT have done it this way, but it was the only option left to them. I can assure you it had nothing to do with "tree huggers" controlling congress. Interestingly Max H

 

[dobbins]

"Ah, the tree huggers come out."<br /><br />I'm not a "tree hugger", I'm a realist. Once you achieve a high flight rate the effects of solid fuel rockets or anything that produces pollution will come under political pressure. Pretending it won't or using snide names isn't going to make that fact vanish.<br /><br />You can't ignore economic or political realities any more than you can ignore technical ones.<br /><br />

 

[najab]

><i>Once you achieve a high flight rate the effects of solid fuel rockets or anything that produces pollution will come under political pressure.</i><p>Commercial airliners make over 50,000 flights a day in the US alone, and I don't see <b>significant</b> political pressure - despite the fact that they aren't pollution free.</p>

 

[nacnud]

but if they produced a lot more soot...

 

[mlorrey]

Okay, fair criticism. Here are some questions:<br /><br />a) what would be necessary to dissolve the boron in the kerosene?<br />b) how about piston pumps? Particulates in piston pumps shouldn't be a problem.<br />c) powering turbopumps with ram air from the boundary diverter between the ramscoop and fuselage, rather than fuel/lox combustion and/or bled from the compressor stage? In the USAF we used to use this air for the ECS, however, 3/4+ of the ECS thermal load was the heat generated by the radar set. Without it, you wouldn't need most of this power for that purpose.

 

[mlorrey]

Additional questions:<br /><br />leaving aside boron for the moment, can you please make up some good excuses why NASA would use LH2 over RP-1, or, better yet, Methylacetylene or cyclopropane? These two are the tops of the non-slurried fuels when you properly calculate your performance for both btu/kg as well as kg/liter energy density, yet NASA doesn't touch them.<br /><br />You can't say MA is rare, it is the primary component of common welders MAPP gas.

 

[tomnackid]

A lot of the decision apparently has to do with the fuels use in regeneratively cooled engines. Hydrogen has high ISP, is an outstanding coolant, but has poor bulk density. Its a case of "Two out of three ain't bad." LOX can theoretically be used to cool and engine but is not ideal because of its corrosiveness. In fact I don't think any operational rocket has used LOX for regenerative cooling--correct me if I am wrong. So, LH2 may result in bigger, heavier, more un-aerodynamic structures, but besides having a high ISP you can run your engines hotter and at higher pressures to make up for the negative aspects. You also open the possibility of reusable engines--saving money and accelerating engine deelopment in the long run.

 

[mlorrey]

All XCOR engines regeneratively cool on LOX

 

[tomnackid]

Leaving aside boron for the moment, can you please make up some good excuses...<br />----------------------------------<br />And I'll thank you to not be so nasty and obnoxious. Sorry to burst your bubble about some imaginary giant "tree hugger" conspiracy to force NASA (and Russian and ESA apparently as well) to use LH2! <br /><br />

 

[gunsandrockets]

"I'm not a 'tree hugger', I'm a realist. Once you achieve a high flight rate the effects of solid fuel rockets or anything that produces pollution will come under political pressure."<br /><br />As for realism, it would take one heck of a flight rate increase before a liquid fueled rocket spiked with boron would equal the pollution from the current flight rate of the Shuttle's giant SRB.<br /><br />Or do you think the EPA is going to suddenly crack down and veto the ESAS plan which is so reliant on the SRB? The peak flight rate under the ESAS plan calls for the launch of 11 SRB per year; that would be 6 SRB for two lunar missions per year, 2 SRB per year for ISS crew rotation missions and 3 SRB per year for ISS cargo resupply missions.

 

[gunsandrockets]

"[this linked graph] Describes boron-kerosene slurry performance in ramjets. Scaling these curves to rocket engine Isp ranges indicates slurry performance in the 400-450 secs range."<br /><br />That's all very interesting as far as ramjets. But do you have any links to rocket engine use of boron hydrates or boron slurries? Has a rocket engine ever been built and tested which uses either of these fuels? What were the test results? <br /><br />

 

[gunsandrockets]

"All XCOR engines regeneratively cool on LOX "<br /><br />The only XCOR rocket engine I ever heard of that used LOX for regenerative cooling is the one they proposed that would burn LOX and molten liquid aluminum (oxygen and aluminum derived from lunar materials)!

 

[gunsandrockets]

[...In fact I don't think any operational rocket has used LOX for regenerative cooling--correct me if I am wrong.] "V-2 onwards ?"<br /><br /><br />Uh, NO!<br /><br />Think about it for a moment. What do you suppose happens when red hot metal is brought into contact with concentrated oxygen?<br /><br />I suppose it's possible to make it work, but difficult. Keeping the heat below the point of combustion would be a tricky balance. Odds are some special anti-oxidizing coating on the metal surfaces would be neccessary too (silicon?).

 

[mlorrey]

Actually, G&R, the engines on the XCOR EZ-Rocket are LOX regeneratively cooled (the fuel is alcohol, btw). <br /><br />It's not that hard, really. Most of the heat of combustion exits the engine, some heats up the nozzle metal. Before ignition even starts, LOX has run through the nozzle cooling, so it is precooled, and given that the engine doesn't operate unless x amount of LOX runs through the cooling tubes, there is no reason why the temp of the LOX can't be kept below the ignition temp of the metal of the nozzle.

 

[gunsandrockets]

"The Falcon1 actually has the highest mass fraction of all the launchers out there, about .9, as I recall, which is well within SSTO range for the Isp they are operating at. I believe, though, that they would not be able to launch more than 200-500 lbs into orbit, ... "<br /><br />The first stage of the Falcon 1 rocket uses the Merlin rocket engine. And according to SpaceX, the vacuum ISP of the Merlin rocket engine is 304s. That would make the exhaust velocity of the Merlin 2982 m/s; I'll be generous and call it 3 km/s. Then I'll take down my handy copy of Arthur C. Clarke's "Interplanetary flight" and have a look at the mass ratio vs exhaust velocity table on page 24.<br /><br />Hmm...If the Falcon one rocket 1st stage has a mass ratio of ten and an exhaust velocity of 3 km/s, then the final burnout speed is about 7 km/s. Even if the mass ratio is increased to 15 the final speed is about 8 km/s. And that doesn't take into account gravity losses, air resistance, or reduced engine ISP under atmospheric pressure. And since orbital velocity is 8 km/s...<br /><br />Then the first stage of the Falcon 1 rocket could not make it to orbit even carrying zero payload.

 

[scottb50]

I think the biggest problem with using LOX for cooling is corrosion, the higher the temperatures the more reactive the metal and Oxygen gets. Also the density of the Oxygen needs to be higher for more efficiency. The opposite applies to the propellant, the less dense, or hotter the better. <div class="Discussion_UserSignature"> </div>