Here’s a picture of the X-15, showing the rear face of its vertical stabilizer:
If that sentence sounds weird, it’s because you don’t typically hear of a plane’s vertical stabilizer having a rear face: it usually tapers to a narrow edge. The X-15’s stabilizer seems like it would cause a lot of aerodynamic drag, which you’d think should be minimized in a plane that’s trying to go Mach 6+. But then again, aerodynamics get a bit weird when you go supersonic.
So what’s the deal with the vertical stabilizer? Why such an, um, unconventional shape?
Here is what Wikipedia has to say about the wedge tail:
** Wedge tail and hypersonic stability**
The X-15 had a thick wedge tail to enable it to fly in a steady manner at hypersonic speeds.[16] This produced a significant amount of base drag at lower speeds;[16] the blunt end at the rear of the X-15 could produce as much drag as an entire F-104 Starfighter.[16]
A wedge shape was used because it is more effective than the conventional tail as a stabilizing surface at hypersonic speeds. A vertical-tail area equal to 60 percent of the wing area was required to give the X-15 adequate directional stability.
Stability at hypersonic speeds was aided by side panels that could be extended from the tail to increase the overall surface area, and these panels doubled as air brakes.[16]
Below is a YouTube video from a good channel that describes why the tail is the way it is. I have queued it to the correct spot for that bit of info but the whole video is interesting.
I’m assuming once the aircraft (rocket?) gets to the edge of the atmosphere, drag is a minimal issue compared to sea level. I’m surprised though that they didn’t make a minimal concession to streamlining after the wide part, a certain amount of v-shape back. Or was the blunt rear part of the stability advantage?
The video above explains that at hypersonic speeds a tail like that is the only thing that works. A conventional, subsonic tail design, is worthless. Doesn’t work at all.
The X-15 wasn’t capable of a conventional takeoff. It had a conventional nose gear, but the rear landing gear were skids. The skid supports were shorter than the lower tail fin, so when landing, the X-15 had to eject the lower part of the tail fin (the fin ejected with a parachute so that they could recover it afterwards).
If the X-15 had to take off conventionally it would have added a lot of expense to the entire X-15 test program, since the plane would have had to carry enough fuel to get it to altitude, making the plane larger and heavier, not to mention all of the other design changes that would have had to be done. Since the plane was designed to test going high and fast, it made a lot more sense to just carry it up to altitude and drop it.
Yes, drag at top X-15 altitudes (above 250,000’) is dramatically lower than at sea level. But at X-15 speeds, it’s a huge deal at launch altitudes (~40,000’).
Yet the need for controllability trumps all. As Whack-a-Mole’s excellent video explains, this required a wedge-shaped tail with a blunt trailing edge (and a section that protruded inconveniently far below the fuselage).
It is also worth noting that this was a test platform. They had no need to design for efficiency or long range and what not. Some inconvenient drag wasn’t really worth caring about too much (beyond if it caused the plane to lose control).
Well, it tended to be a concern when seeking amazing altitudes and speeds. But they had an impressively powerful engine that didn’t greatly care about some aerodynamic inefficiencies.
Research machines have very different engineering tradeoffs from machines intended to go into series production. Lots of quick-and-dirty “backyard engineering”, albeit from within the most leading-edge sort of “backyard”
Also, turbine powered airplanes have a nice source for bleed air, electricity, and hydraulics. A rocket powered machine has no such source from the main engine(s).
So in effect the whole of ship’s services run off some arrangement of batteries, accumulators, and some sort of APU. E.g. the Space Shuttle’s APUs were a combination of fuel cells for electricity and free turbines spun by catalyzed hydrazine to turn pumps.
For machines like that, you want to avoid large transient loads, like a fin-folding mechanism or ordinary landing gear. I suspect, but don’t know, that the gear lowering machinery on an X-15 was one-time blow-down bottles that were recharged before the next flight.
Yeah, bit I was specifically on about “dropping an actual ICBM out of an airplane, letting it point the right direction on a drogue, abd firing off like it was launching from a silo”.
We did ALBMs too, for a while. And our hypersonic will technically be a ALBM as well.
