Why so few HALE UAVs?

[Voltzz]

To be honest i am not that knowledgeable on large UAVs so it could very well be that i am just missing something obvious, but i have asked myself this question for some time: Why are there so few HALE UAVs? There is ofc the Global Hawk/Triton and two Chinese Projects (Soaring Dragon, Divine Eagle), but if you take the most often quoted criterion of operating altitude >15km, there is nothing else.
There are some MALE drones that are sometimes called HALE (Sokol Altius, Bayraktar Akıncı) and there are some drones that approach the size of a HALE (EUROMALE), but none of them breach the 15km. Pseudo-satellites like the Airbus Zephyr or the BAE PHASA-35 otoh reach even higher altitude and endurance, but have dramatically reduced payload compared to the Global Hawk.
For armed UAVs this situation makes sense to me, higher altitude would be of little advantage while significantly reducing weapons payload. But sensor payload like SAR/MTI radars or SIGINT would clearly profit from the altitude while needing far more payload than pseudo-satellites can offer.
HALE UAVs might not be very survivable against modern air-defenses, but MALEs arent either. HALEs configured as sensor platform could at least have the safety of a large stand-off distance. And considering the current obsession with long-range precision-strike weapons, the sensor output could be just as lethal as an armed UAV.
The point of this post is not to find the one MALE UAV that can fly at 15,001m altitude, im just wondering why there are not more Global Hawk competitors.

 

[red admiral]

Operating for long durations above 15,000m / 50,000ft is really difficult for either gas turbine or IC aircraft. Your propulsion system produces much less thrust and so your design is much more sensitive to mass and drag. So effectively you end up with a smaller payload fraction than if you were operating at lower altitude. This makes these aircraft expensive, and niche because they're optimised for this one thing.

For using a SAR/GMTI from high altitude then although the horizon is further away, you still need sufficient power and antenna gain (size) * to actually get useful performance out to the horizon. This is actually a really really big radar if you want to do this, which isn't that practical unless you have a really really big aircraft.


* And a bunch of other things

 

[zebra159357]

To be honest i am not that knowledgeable on large UAVs so it could very well be that i am just missing something obvious, but i have asked myself this question for some time: Why are there so few HALE UAVs?

Im sure you must have heard of the P-ISR RQ-180(or whatever its official designation is).

 

[red admiral]

I think that a non complete list of real examples would be:

• U-2
• RB-57F
• Global Hawk
• Grob G-520 Egret (just about as a prop powered example)
• Grob Strato
• Ryan Firefly
• Ryan and Boeing Compass Cope

Most have/had EO imagery and/or ELINT payloads rather than high power radars

Obviously many of these have pilots but as a whole they show the design trade offs needed. They are very specialist designs so you have to decide whether you really need that extra altitude.

 

[flateric]

To be honest i am not that knowledgeable on large UAVs so it could very well be that i am just missing something obvious, but i have asked myself this question for some time: Why are there so few HALE UAVs?

Reading something like Global Hawk Case Study may give anwers to some of your questions. Advanced airframe, use of composites, gust and flutter control for high aspect ratio wings, compact high-altitude powerplants, complicated vehicle thermal control and advanced intelligence suite that able to use all advantages of being HALE are problems to start with.

 

[Jackalope]

I’ve worked on a number of HALE projects using turbocharged piston, turboprop, jet and solar. When considering a jet powered vehicle while using an off the shelf engine you immediately hit a limit on the low speed end. You would love to fly slowly to get endurance, but the minimum Mach for your existing engine might be 0.6. Overcoming this means doing engine development, which is costly and needs to precede the airframe by about five years. Even something as high profile as Global Hawk still does not meet its original target of endurance cruise at 65,000 feet. No customer seems to have the funding or patience for high altitude propulsion development so that’s my response to your question.

 

[Scott Kenny]

To be honest i am not that knowledgeable on large UAVs so it could very well be that i am just missing something obvious, but i have asked myself this question for some time: Why are there so few HALE UAVs? There is ofc the Global Hawk/Triton and two Chinese Projects (Soaring Dragon, Divine Eagle), but if you take the most often quoted criterion of operating altitude >15km, there is nothing else.
There are some MALE drones that are sometimes called HALE (Sokol Altius, Bayraktar Akıncı) and there are some drones that approach the size of a HALE (EUROMALE), but none of them breach the 15km. Pseudo-satellites like the Airbus Zephyr or the BAE PHASA-35 otoh reach even higher altitude and endurance, but have dramatically reduced payload compared to the Global Hawk.
For armed UAVs this situation makes sense to me, higher altitude would be of little advantage while significantly reducing weapons payload. But sensor payload like SAR/MTI radars or SIGINT would clearly profit from the altitude while needing far more payload than pseudo-satellites can offer.
HALE UAVs might not be very survivable against modern air-defenses, but MALEs arent either. HALEs configured as sensor platform could at least have the safety of a large stand-off distance. And considering the current obsession with long-range precision-strike weapons, the sensor output could be just as lethal as an armed UAV.
The point of this post is not to find the one MALE UAV that can fly at 15,001m altitude, im just wondering why there are not more Global Hawk competitors.

Another chunk of fun is that planes like the U-2 and M-55 fly up in what's called "coffin corner" of their safe flight envelope. They have about a 10knot speed bracket between stalling because of going too slow and ripping the wings off as they go trans/super sonic. Even the SR-71 was in the same problem, though was not worried about ripping the wings off. 15knot bracket between stalling and overspeeding for the Blackbird.

You need crazy good engineers to even get there, then you need more crazy good engineers to write the flight control software to keep you there.