A Better Approach to Space?
Virgin Galactic is testing their new rocket and it is pretty spectacular. I have said before but I will repeat it, what could be cooler than living in an age where private billionaires are competing at space travel (Musk, Branson, Bezos, Allen, etc.)? I have always thought that Virgin's approach of using fixed-wing aircraft to get as much height as possible followed by a rocket launch from that altitude made the most sense from an efficiency standpoint. Unfortunately, I am not sure you can get really large payloads or escape Earth's gravity with this approach. Anyway, enjoy:
John Moore:
Orbital ATK (which has a large operation here in the valley) was doing that 28 years ago with their Pegasus rocket. I'm not sure why more don't do it, but perhaps, as you suggest, it's a scalability issue.
June 2, 2018, 7:07 pmSam P:
The main problem is that you don't save all that much by using an airplane as a "zeroth" stage. Delta V to get to 100 km altitude is about 1.5 km/sec (e.g. X-Prize). You need greater than 6 km/sec more to turn that bunny hop into an orbit. A high flying aircraft might be 15 km altitude, so you might save 15% on delta V (though much more on fuel due to exponential growth in the rocket equation) to get to altitude, but you still need to gain a lot of velocity
Now, if you have a rocket than can almost make orbit or make orbit with no to small payload, mounting it on an airplane might make sense to get just a little more. Also, you suffer less atmospheric drag and can optimize your engine for lower pressure than sea level. These are some reasons why people think about doing airplane launch. On the other land, you could just make your first stage somewhat bigger and not have to build and test a special purpose airplane.
June 2, 2018, 7:23 pmjdgalt:
I've wondered for a while now why he hasn't tried the cheaper "rockoon" concept.
A "rockoon" is a combination of a rocket with a lighter-than-air, probably hydrogen, balloon. The idea is that you take some existing rocket which could launch from earth surface (which could be almost any design, whether it's an Ariane, a Saturn 5, or even a Shuttle in launch configuration) and hang it by the nose from a large balloon. (You also need to build in a parachute that can deploy if the balloon pops prematurely.) Float the whole thing up to 10,000 feet or so (as high as it'll go) on the balloon, THEN light off your rocket. This would function as a cheap "zeroth stage." Granted it isn't reusable, but the parts thrown away are trivial and easily paid for by the fuel savings.
June 2, 2018, 8:47 pmLoneSnark:
You could gain more. But an airplane will usually be subsonic and limited in altitude, so unless fuel is extremely extensive, the tiny efficiency of using a 0th aircraft stage is dwarved by the other design costs. Designing an entire stage that only gets you to 60k feet and mach 1 is certainly not worth the engineering effort.
June 3, 2018, 12:32 amirandom419:
This is merely a stepping stone. They get you just to the edge of space, so you can say you actually did it. This is similar to the Pegasus that launched stuff into low earth orbit (sometimes) from a plane. I'm hopeful that the SABRE hybrid jet engine might get us further.
https://en.wikipedia.org/wiki/Pegasus_(rocket)
June 3, 2018, 4:16 amhttps://en.wikipedia.org/wiki/SABRE_(rocket_engine)
NeoX Zheng:
I think they use this mainly because the only thing they want is suborbital trip.
For such a system to make sense for orbital launches, we need either
1) an engine optimized (very well) for lower pressure
2) a launch aircraft capable of reaching very high velocity (probably ~10 mach)
No idea which one is more likely though. Really hope a hybrid engine like SABRE can work.
June 3, 2018, 7:44 pmJohn Smith:
Many people have no idea about how big rockets are. A 747 weights about 400,000 pounds. A Falcon Heavy weighs over 3 million pounds. Most of that is fuel because of the 2nd problem: Getting into orbit is much more about the energy needed to go fast enough, around 17,000 MPH, not the energy needed to be high enough. If a 747 can get you up to 500 MPH, then it's short by a factor of 34 and the energy difference is 34 squared or 1156 times.
June 3, 2018, 11:37 pmsean2829:
I think this is what Stratolaunch.com is about, envisioned by Paul Allen. They have built the largest aircraft in the world that can ferry and launch a very heavy rocket to the upper atmosphere. I've heard that while it's easy to scale the aircraft, it's hard to scale the rocket. The problem is that the rocket is launched mostly horizontal and then has to accelerate, make a turn toward the upper atmosphere before heading horizontal again in orbit. Apparently that is a 6G turn while the rocket is accelerating . The rocket the stratolauncher is capable of carrying can be as big at 800,000 lbs before ignition. That is a lot o weight to turn and if its a solid rocket motor, these are huge propellant grains of oxidizer filled rubber which are prone to crack when stressed too much. I remember reading about the big rocket in the early press releases but it's not on the website any more.
June 4, 2018, 4:28 amJohn_Schilling:
As others have alluded to, an airplane flying at 600 knots and 30,000 feet is less than 7% of the way to Low Earth Orbit in terms of raw delta-V requirement. There are some secondary benefits, like not having to design your rocket engine for sea-level operation and/or takeoff thrust, but those only bring you up to effectively 8-9% of the way to LEO. Maybe 10-12% if you can use a supersonic aircraft as your launch platform.
It's almost certainly not a winning proposition unless you get the carrier aircraft essentially free, either because you can use a surplus airliner, or because you hired Burt Rutan to design your rocket ship and it's in his nature to design weird airplanes whenever possible. And surplus airliners aren't big enough to launch e.g. communications satellites.
Still not sure what the Stratolaunch team is up to, except that hiring Burt Rutan's company to build a really big weird airplane just about the time Burt Rutan retired probably added some cost and schedule risk.
June 4, 2018, 7:20 am