X-106 "Christa", the Hyper Dart

[gunsandrockets]

"I suspect its orbital payload won't reach what we're planning." <br /><br />If the 10 tonne rocket airlaunched from the Foxhound is equally efficient to the Pegasus 18.5 tonne airlaunched rocket then the Foxhound system can put at least 445 pounds into orbit. And since the Foxhound is so much faster and higher flying then the L1011, the Foxhound system might even do considerably better than 445 pounds.

 

[gunsandrockets]

Tap_Sa says, "For the record, I don't doubt boron slurry's ability to produce 450+ second Isp. Boron does have exceptionally high heat of combustion. What I do doubt is Merlin's ability to cope with burning that stuff without melting in a split second." Tap_Sa then goes into extreme detail describing why the Merlin would melt.<br /><br />To which mlorrey replies, "Okay, so we'll be able to return to a normal chamber pressure, depending on what the exit pressure is vs. ambient pressure. Given that we're lighting it off at altitude, rather than ground level, the standard nozzle should easily have positive exit pressure even at a lower feed rate and bring chamber pressure well within design limits (which we don't know yet, so this is all hand waving)."<br /><br />I don't think you get it. For any rocket engine at a constant operating temperature, you can't increase ISP without also reducing thrust-to-weight. Likewise you can't increase thrust-to-weight without reducing ISP. Attempting to increase one performance variable without reducing the other variable melts the engine.<br /><br />In other words, pick any two factors...<br /><br />1)very high ISP<br />2)very high thrust to weight ratio<br />3)engine does not melt<br /><br />...because you can't have all three.<br />

 

[mlorrey]

This isn't the trade off. Isp is pounds of thrust-seconds per lb of propellant. By throttling down, you use less fuel for the same level of thrust, because while the heat of combustion is higher, by throttling down you reduce chamber pressure, thus expansion from the heat cools the combustion products while attaining the same exit velocity. There may be thermal issues, but with an ablating nozzle, as the Merlin has, the singular issue is ensuring the ablator is thick enough for the engines performance for one flight. Ablating nozzle will only be used for one flight, so the only concern is to ensure it doesn't burn through.<br /><br />Particularly if, as my research indicates, boron carbonates form a layer on the nozzle surface, this is an effective insulator that will protect the ablator from at least some of the increased heat.<br /><br />I am sure that we won't be able to use boron slurried kerosene on an engine out of the box. I fully intend to start the test program out with plain kerosene and work into testing nozzles with increasing concentrations of boron to examine the results and make necessary changes.<br /><br />As it is, boron is only needed to put the vehicle into orbit. Without it, a small upper stage can place a significant microsatellite into LEO from a high speed suborbital trajectory.

 

[rocketman5000]

the combustion chamber of the engine is also ablative. I agree that the nozzle exit temperature could remain the same by more expansion, but it still burns at a higher temperature meaning that the combustion chamber and throat still see higher temperatures.

 

[mlorrey]

The 31 sec isp improvement is only an improvment over using pure gelled hydrogen as a monopropellant (i.e. not with LOX, just letting the atomic gelled hydrogen expand), it has nothing to do with kerosene/lox combustion. The HEDM program has not published any figures for improvements of boron slurried kerosene combusted with LOX. <br /><br />The scale of improved performance is not claimed by me, but by USAF Spacecast 2020 and Spacecast 2025, as well as the Aerospatiale Onera boron slurried fueled ramjet research, which I've posted charts from. These sources corroborate each other.<br /><br />http://www.onera.fr/conferences/ramjet-scramjet-pde/<br /><br />specifically: <br />http://www.onera.fr/conferences/ramjet-scramjet-pde/images/slurry-lfrj.gif<br /><br />http://csat.au.af.mil/2025/volume2/vol2ch05.pdf<br />"For example, a 5 percent boron additive to solid hydrogen is projected to produce a 107-second Isp<br />improvement in efficiency, and other additives such as titanium and boron/titanium composites show<br />promising results.8"<br /><br />Now, I understand that you, shuttle_guy, and the other industry insiders may have an interest in bottling up this genie, either to keep a black program black, or because you pray to the LH2 gods. It doesn't matter to me. There is too much evidence supporting my arguments.<br /><br />However, I'd already answered your objections previously in this post: http://uplink.space.com/showthreaded.php?Cat=&Board=businesstech&Number=411328&page=0&view=collapsed&sb=5&o=0<br /><br />But you can't seem to let it go.