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In an answer to a previous question, I learned about the game-changing nature of Towed Glider Air-Launch System technology.

While the glider in the Towed Glider Air-Launch System (here, here, and here) started off as a true glider, in this YouTube video it now has what looks to me like rocket propulsion. The animation calls it a "sustainer motor", which I assume means that it can sustain all of the the altitude and airspeed gained during the tow while also reorienting the rocket to near-vertical before ignition.

Is this just an alternative, or is this actually what Towed Glider Air-Launch System has actually needed all along to be as competitive as advertised? It suddenly seems like a much more complex, expensive, and risky technology, but that's just my impression.

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uhoh
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  • An alternative to what? – GdD Sep 06 '16 at 07:28
  • @GdD an alternative to the glider as originally described - sans rocket. – uhoh Sep 06 '16 at 08:22
  • Is this question about the "sustainer" motor? Because I think that is there purely to maintain airspeed for the several seconds it takes to get the rocket pointing up for release. (You can't pitch up while being towed - that would cause serious imbalance problems for the tug.) – Andy Sep 06 '16 at 10:52
  • @Andy what something is supposed to be for and what it really is can be significantly different. It sure looks like a rocket to me now. Considering structural modifications to accommodate thrust like that must be non-trivial as well as attitude control - does the "sustainer" motor have vectored thrust and it's own attitude control system? How does this glider handle the sudden thrust and vertical climb? It gotta be a substantially fortified glider now to maintain velocity while climbing against gravity. – uhoh Sep 06 '16 at 11:17
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    The sustainer motor is probably not unlike a (longer burning) JATO bottle, just a way of maintaining airspeed while the glider pitches. So the glider is doing all of the steering/control. I agree it's a complex-looking system; it looks possible, but is it practical and cheap? (In 1981 I believed the space shuttle was going to be practical and cheap too...) – Andy Sep 06 '16 at 11:31
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    I don't think this question has an answer at this point. The project is at the concept stage, trying to get budget to answer these questions. – GdD Sep 06 '16 at 12:08
  • @Andy "just a way of maintaining airspeed while the glider pitches" understates the situation. This is going from horizontal momentum to vertical momentum, JATO is a "little bit" of extra horizontal thrust which gives an airplane lots of lift. This is vertical flight without lift, the thrust must work directly against gravity now. It's gotta be quite powerful to have any effect against gravity’s decelerating effects. Let's try to find something quantitative, rather than phraseology. – uhoh Sep 06 '16 at 12:09
  • @GdD Let's see if there is an answer - I think there indeed may be one. I have a feeling that the concept could never have worked without the glider being rocket propelled from the beginning, and it was added to the animation when it became impossible to avoid quantitative analysis. I think the glider could never actually pitch vertically - carrying all that weight - without stalling, loosing control, and tumbling towards Earth. – uhoh Sep 06 '16 at 12:11
  • @uhoh "Let's try to find something quantitative, rather than phraseology" - I wasn't understating the situation I think. With the mass of the glider + rocket you could roughly estimate the thrust required to maintain airspeed in, say, a 60 degree climb. Similarly for a JATO takeoff where you need to gain quite a lot of momentum in a short time - that can be estimated too, it's fairly simply physics. I'm just trying to explain why they need power for pitching, which I think is what you asked in the question... – Andy Sep 06 '16 at 12:24
  • @Andy I am asking if this was always going to be necessary - if this was always going to have to be a rocket, not just a glider. – uhoh Sep 06 '16 at 12:27
  • I see - I think the extra rocket isn't completely necessary - you could fire the rocket's own engine while the glider pitches up. The little booster rocket might save on main engine propellant though. It all depends on how much inertia the rocket has and how quickly the glider can pitch it up. As @GdD says, it's all concept at this stage. – Andy Sep 06 '16 at 12:38
  • @Andy that's just back to horizontal launch with disposable wings. And unless you can deeply throttle this powerful boost-to-orbit engine, these wings will be far too large for that. – uhoh Sep 06 '16 at 12:41
  • I did not say disposable wings... please read comments before replying. – Andy Sep 06 '16 at 12:48
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    You keep using this word, "glider". I do not think it means what you think it means. – Russell Borogove Oct 06 '16 at 14:42
  • @RussellBorogove who are you talking to? If it's me, I've ridden in gliders myself, I know what they are - very quiet and peaceful and fun, and sometimes you suddenly hit a pocket of lift from the hills below and then whoa!!.The system is called a "Towed Glider" by the people designing it, so I use the term here because that's what they call it. It has enough lift to raise the rocket's altitude while it is being towed by another plane, and it glides for a short time after release without going into free-fall, but they absolutely must start the sustainer motor before it rotate to 70° for launch – uhoh Oct 06 '16 at 15:04
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    I just think it's hilarious that the word "glider" is being used to describe a system that is jet and rocket propelled for the entire ascent phase. The space shuttle was every bit as much a glider as this is. (It's a reference to "The Princess Bride", not directed at anyone in particular.) – Russell Borogove Oct 06 '16 at 15:05
  • @RussellBorogove Ya OK I see what you mean. I suppose it could be towed by a propeller airplane - if it was big enough. You are right - usually gliders are not towed by jets. I guess it really becomes a glider only on the return. – uhoh Oct 06 '16 at 15:13
  • Stratolaunch is like this – Muze Oct 09 '18 at 03:52
  • @Muze your edit is destructive. No, it's not "like the Space Shuttle" at all! The space shuttle is an orbital spacecraft. – uhoh Mar 27 '19 at 21:48
  • @uhoh an orbital spacecraft that is behaving as a glider on reentry. During that time it is technically a glider. – Muze Mar 27 '19 at 21:53

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Is this just an alternative, or is this actually what Towed Glider Air-Launch System has actually needed all along to be as competitive as advertised?

According to this update:

The project’s next step will be to demonstrate the horizontal launch of a small launch vehicle from the onethird-scale glider. That will be followed by the build up and demonstration of a one-third-scale “sustainer motor” top-mounted to the glider. This variable thrust motor will provide the energy to change the glider and launch vehicle orientation from horizontal to a highly inclined flight path angle to maximize launch efficiency.

So it seems that the "sustainer motor" is needed to change the orientation from horizontal to vertical. Vertical air launch of the rocket is only one of the key benefits of this technique, but it's an important one.

At a high altitude (necessary to make this an advantages technique) the pitch up maneuver requires a significant amount of time - most of the horizontal momentum must be changed to vertical momentum. As an un-propelled glider, all of that momentum must be transferred to the rarified atmosphere, and that takes time. The rate of descent will accelerate as soon as the pitch results in loss of lift, and more altitude will be lost.

Without a simulation, I can only speculate what might happen, but without significant lift, but plenty of kinetic energy, the whole thing may become unstable and begin to tumble. In short, at high attack angles, a glider with no propulsion will just stall and then loose it's ability to control its orientation. In order to maintain orientation in near-vertical flight, thrust is required.

It is likely that rocket propulsion of the glider was always needed, but not included until recently in discussion.

Assuming ballpark figures of a 10,000 lbm "glider" carrying a 20,000 lbm launch vehicle* from here, the "sustainer" motor would need more than 30,000 lbf of thrust to maintain a vertical velocity, ignoring drag. That is something like a pair of SuperDraco engines, but with really really deep throttling since the thrust must be much lower while the flight is horizontal and the drag is only about 1/15 of the weight.

*although a glider that can cary an 80,000 lbm launch vehicle is mentioned here.

uhoh
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  • By the way they say a "highly inclined" flight path and you say "vertical". Fair enough, it's your answer, but do they state an actual angle anywhere? (I mention this because going up close to 90 degrees could waste quite a bit of time before the launch can be started.) – Andy Sep 06 '16 at 12:47
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    @Andy The third image in the original question (from video) says it stabilizes at 70 degrees. sine(70 degrees) = 0.94 – uhoh Sep 06 '16 at 12:55