# Can hypersonic aircraft be agile without the G forces harming the pilot?

I read that high G forces associated with hypersonic flight could kill the pilot.

Is there any way of making this survivable yet keeping agility at hyper sonic speeds (irrespective of the plane's capabilities) and how do they work.

Agility means that you can change the speed vector rapidly. The way this is done in aircraft is to point the wing's lift into the desired direction and then to increase lift as much as possible. The lift force over time results in a kinetic energy change which accelerates the aircraft in the new direction and decelerates the old speed away.

Flying faster means that the kinetic energy goes up with the square of the speed increase. What could turn 90° within 4 seconds at Mach 0.7 will take 200 seconds at Mach 5 when the same acceleration is applied. This makes clear that agility does not survive the speed increase.

What if the force is increased? To regain the same 22.5°/s turn rate would require a lift multiple of 50! This means hundreds of gs for a structure which is weakened by friction heating and a massive increase in wave drag. Since wave drag increases with the square of the speed increase, any hypersonic vehicle attempting to pull many gs will not stay hypersonic beyond the next few seconds! Assuming an L/D of 10, the deceleration would be in the order of 250 g if the wing tolerates 50 g of lift load and things are linearily scaled up. But that would be an L/D of 0.2 - in other words, the wing will be close to 90° angle of attack and the craft will break apart.

Even the more reasonable 6 g of subsonic manoeuvring will mean a 36-fold increase im wave drag at Mach 5, bringing L/D down to less than 2.

Conclusion: There is simply no way for hypersonic vehicles to be agile. The pilot is the least of your problems.

Well, 2 G's is 2 G's, regardless of what speed you're flying. A level (coordinated) turn with 60 degrees of bank is 2 G's, regardless of your airspeed. What changes is your turn rate and turn radius; at 20 knots, a model plane at 60 degrees bank can probably complete a 360 degree turn in a few seconds and a radius of maybe 10 yards. At 300 knots and 60 degrees bank, a conventional jet will need a minute or two and a turn radius of probably a couple of miles. At 2000 knots and 60 degrees bank, an SR-71 might overfly a few states making that same 360! But it's all 2 G's for each of those aircraft.

Now if you want the SR-71 or some similar highly supersonic aircraft to have the turn rate & turn radius of your 300-knot Lear jet, then you'll get a LOT more G's -- which can induce various other issues with your supersonic beast. And if you want either of those to keep their speed while turning in the radius of your 20-knot drone... well, let's just say that you're talking about G forces beyond the realm of all human experience to make that happen!

There are all kinds of discussions that can be had about how hypersonic aircraft can be made more maneuverable without doing things that risk losing control, flaming out the engines, creating unacceptable amounts of drag, and so on, but all of those discussions take place in the context that high speed + high turn rate = more G's, and given any 2 of those factors that you choose (i.e. how fast you want to go & how much G-loading you're willing to sustain), the third is determined.

The max G-loading for a human is pretty well about 9 G's, so pick whatever combination of the other two factors you want. Or go unmanned -- which with today's technology is probably cheaper.

• IIRC SR-71 could, up at the typical cruise flight level, pull only 1.5 G. It simply wasn't intended to do much manoeuvring in cruise. – Jan Hudec Mar 10 '17 at 22:03