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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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  • $\begingroup$ An alternative to what? $\endgroup$
    – GdD
    Sep 6, 2016 at 7:28
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    $\begingroup$ 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...) $\endgroup$
    – Andy
    Sep 6, 2016 at 11:31
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    $\begingroup$ 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. $\endgroup$
    – GdD
    Sep 6, 2016 at 12:08
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    $\begingroup$ You keep using this word, "glider". I do not think it means what you think it means. $\endgroup$
    – Russell Borogove
    Oct 6, 2016 at 14:42
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    $\begingroup$ 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.) $\endgroup$
    – Russell Borogove
    Oct 6, 2016 at 15:05

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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.

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  • $\begingroup$ 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.) $\endgroup$
    – Andy
    Sep 6, 2016 at 12:47
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    $\begingroup$ @Andy The third image in the original question (from video) says it stabilizes at 70 degrees. sine(70 degrees) = 0.94 $\endgroup$
    – uhoh
    Sep 6, 2016 at 12:55

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