Air Launching Rockets

[Kryten]

55 minutes ago, wumpus said:

I'm sure there was a herculean effort by the Orbital people to get that idea in space, and NASA seems only willing to write a white paper.  

Orbital had extensive help from NASA for the Pegasus project. It even launched from a NASA aircraft for the first five flights.

[magnemoe]

On 7.12.2016 at 7:50 PM, cantab said:

"Both designs utilized a Boeing 747-400F as the carrier aircraft...All concepts, when fully loaded, exceeded the allowable Gross Takeoff Weight (GTOW) of the aircraft platform."

That's not very encouraging. You're taking one of the largest and highest-capacity aircraft available, and it's still not enough.

As I see it the big advantage of subsonic air launch is operational and infrastructure simplicity. No big specialist launchpad, just any decent airport. No worries about weather at the rocket launch location (though obviously bad weather could ground the plane). Operate out of the temperature latitudes but actually launch the rocket much closer to the equator. But if you're proposing to use cryogenic fuels and aerial refuelling, that's adding a ton of complexity, hazards, and consequent cost. There are good reasons aerial refuelling isn't used in civil aviation, and that's even before we consider the extra problems of handling cryogenics.

Pegasus is well-established, and actually one of the most-launched orbital rockets. 23 ton rocket putting around 500 kilos into LEO. Pegasus II was much more ambitious, but so much so that it was to require a brand new aircraft. That's a major extra development cost. Let's say we instead use the highest-payload "off the shelf" aircraft we can have, a 747-8F with a maximum payload of around 140 tons, then scaling up the Pegasus design we're looking at a 2-3 tons to LEO payload. Meanwhile in that same original paper, the "PD-2" design without using aerial refuelling but using RP-1 and LOX rather than solids gets about 6 tons to LEO.

That I think is about as big as is worth going with air-launch.

Problem is that reuseablity don't scale down well. It add a lot of weight, complexity and development cost. 
Only worthwhile for an rocket you launch often 
an scaled down version of the mars colonial transporter/ falcon heavy with reusable upper stage would be perfect, that is with an upper stage who return after an orbit. 
This also lets you do heavy lift by if you use an light single use upper stage. 

 

[wumpus]

15 minutes ago, Kryten said:

Orbital had extensive help from NASA for the Pegasus project. It even launched from a NASA aircraft for the first five flights.

I don't think any US company gets into orbit without considerable NASA assistance (I suspect only Spaceship 1 has gotten into space without such assistance, and only because of the X-prize rules demanded such).  And they can certainly quash any US program they don't want to succeed (the SLS is a different story.  And I'm sure NASA is divided on supporting it).

3 minutes ago, magnemoe said:

Problem is that reuseablity don't scale down well. It add a lot of weight, complexity and development cost. 
Only worthwhile for an rocket you launch often 
an scaled down version of the mars colonial transporter/ falcon heavy with reusable upper stage would be perfect, that is with an upper stage who return after an orbit. 
This also lets you do heavy lift by if you use an light single use upper stage. 

I'm assuming the plane in the paper is the same one that carried the shuttle (orbiter), so no extensive modifications (until you have to refuel the LOX, or top up LH2 in the rocket).  It does have wings (on the first stage), so expect the possibility of a shuttle-style landing (have fun managing the CoM/CoP/CoT* issues) of the first stage from Falcon 9 [mach 6?] speeds for relatively easy recovery (assuming you can solve the CoM/CoP/CoT issues).

It isn't all that bad, it certainly justified the time it took to write it (basically to wave in the face of any congressman/staffer/NASA ex-Air Force general appointee who asks why you can't use air launch).  It just certainly isn't worth the billion-dollar investment it would require NASA to take this from paper to space (unfortunately this might not be sufficiently obvious when waved in the face of those who want to believe).

* Center of Mass/Pressure/Thrust.

[AeroGav]

1 hour ago, wumpus said:

In any air launch system I've seen, the means to get in the air is extremely reusable (balloon lift may be an exception).  The entire system would likely be more reusable than the shuttle system.  One thing to remember is that NASA has flown at least one space ship (shuttle orbiter) on top of a 747 and presumably still has it (possibly mothballed).  Converting the hard point to decouplers would be an issue, but I've claimed the real issue would be separation (possible if you idle the 747's engines).

I doubt that filling the RP-1 (in the rocket and the plane) is much of an issue.  It seems to be standard procedure for both the Air Force and Navy.  Filling the LOX is another story (presumably it leaks out too fast), and I really have to wonder if they need to refuel the LH2 from inside the plane.

While it might not have been interns, I'm pretty sure it was a junior bunch.  Certainly it helps for NASA to have studied all the issues, just in case they miss something pretty important.  Although I wouldn't be too surprised if some of the bits (like having to match Delta-Heavy) were sent down from on high to make sure that it was effectively impossible.  I'd have liked a project manager to tell them to go back and compute the size they would get without in flight LOX refueling and this type of thing makes me wonder if it wasn't to show how impossible air launch really is (it might work for smaller, but current tech seems to be doing ok there as well).

It isn't that far from Orbital's original recipe.  They have a plane with hardpoints (just ignore the tail in the way).  They have the means to lift (just ignore the LOX refueling).  I'm sure there was a herculean effort by the Orbital people to get that idea in space, and NASA seems only willing to write a white paper.  Considering the "details" above, that's probably for the best (don't forget that Pegasus is solid rocket, at least stage 1).

I like your theory about why they felt they had to match a Delta.      Hopefully there'd be no need to deal with LH2 fuelling in flight, because the mass of the upper stage's hydrogen  won't be that high of  a proportion of the complete stack.

I mean, the air-to-air refuelling is being used because the takeoff weight of the 747 is limited by minimum climb gradient with a failed engine, or brake energy limits (in a rejected takeoff, can it stop before the brake disks melt).  Once in flight, you bypass these limits, but ultimately run into what the structure can withstand   (  Force = mass x acceleration,   the 4.3g ultimate design load of the wing spars assumes a certain max weight).     So you make the rocket so big the 747 can only legally take off with the first stage RP-1 tanks empty,  once in the air you can fill them up with a tanker plane.

But if you made the rocket even bigger, so that takeoff is only possible with cryogenics offloaded, then you'd surely hit the structural limits of the plane when you tried to fill the rocket, even assuming the technical hurdles could be overcome.