# Why did the X-15 have such a non-streamlined vertical stabilizer? **URL:** \ **Category:** Factual Questions\ **Created:** [November 5, 2022, 2:39am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401 "2022-11-05T02:39:16Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 5, 2022, 2:39am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/1 "2022-11-05T02:39:17Z")
Here’s a picture of the X-15, showing the rear face of its vertical stabilizer: [![](https://media.defense.gov/2015/Oct/02/2001296795/2000/2000/0/151002-F-IO108-012.JPG) ](https://media.defense.gov/2015/Oct/02/2001296795/2000/2000/0/151002-F-IO108-012.JPG) 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? ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [November 5, 2022, 2:49am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/2 "2022-11-05T02:49:21Z")
[Here](https://en.wikipedia.org/wiki/North_American_X-15#Wedge_tail_and_hypersonic_stability) 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]](https://en.wikipedia.org/wiki/North_American_X-15#cite_note-wendell-16) This produced a significant amount of base drag at lower speeds;[[16]](https://en.wikipedia.org/wiki/North_American_X-15#cite_note-wendell-16) the blunt end at the rear of the X-15 could produce as much drag as an entire [F-104 Starfighter](https://en.wikipedia.org/wiki/Lockheed_F-104_Starfighter).[[16]](https://en.wikipedia.org/wiki/North_American_X-15#cite_note-wendell-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. > > — Wendell H. Stillwell, [X-15 Research Results (SP-60)](https://history.nasa.gov/SP-60/ch-3.html) > > 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]](https://en.wikipedia.org/wiki/North_American_X-15#cite_note-wendell-16) ---
**Author:** ![Limmin](https://avatars.discourse-cdn.com/v4/letter/l/f04885/32.png) [@Limmin](https://boards.straightdope.com/u/Limmin)\ **Post date:** [November 5, 2022, 2:59am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/3 "2022-11-05T02:59:34Z")
Here’s another hypersonic vehicle, a nuke warhead. Note the wedge (actually cone) shape: [![](https://upload.wikimedia.org/wikipedia/commons/9/9c/LGM-118A_Peacekeeper_MIRV.jpg) ](https://upload.wikimedia.org/wikipedia/commons/9/9c/LGM-118A_Peacekeeper_MIRV.jpg) This is actually a bunch of warheads on a platform, each individually steerable to different targets. Anyway, the shape is very similar to the X-15’s tail section, and for the same reasons. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 5, 2022, 3:00am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/4 "2022-11-05T03:00:41Z")
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. [![](https://img.youtube.com/vi/7zR26e504uI/maxresdefault.jpg "The Insane Engineering of the X-15") ](https://www.youtube.com/watch?v=7zR26e504uI) ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [November 5, 2022, 4:51am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/5 "2022-11-05T04:51:04Z")
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? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 5, 2022, 4:57am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/6 "2022-11-05T04:57:00Z")
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. ---
**Author:** ![Asuka](https://avatars.discourse-cdn.com/v4/letter/a/dec6dc/32.png) [@Asuka](https://boards.straightdope.com/u/Asuka)\ **Post date:** [November 5, 2022, 8:31am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/7 "2022-11-05T08:31:54Z")
Was that the reason why the X-15 had to be dropped from a B-52, because a conventional take-off would be impossible? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [November 5, 2022, 9:58am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/8 "2022-11-05T09:58:22Z")
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. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [November 5, 2022, 12:14pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/9 "2022-11-05T12:14:02Z")
> [@engineer\_comp\_geek](#): > > 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. Said another way, it was a rocket with two-stage launch system. The B-52 was the jettisonable and fully reusable first stage. Here are some subsequent examples of airplanes as “first stages” for commercial launches: > **[Northrop Grumman Pegasus](https://en.wikipedia.org/wiki/Northrop_Grumman_Pegasus)** > > Pegasus is an air-launched launch vehicle developed by Orbital Sciences Corporation (OSC) and now built and launched by Northrop Grumman. Pegasus is the world's first privately developed orbital launch vehicle. Capable of carrying small payloads of up to 443 kg (977 lb) into low Earth orbit, Pegasus first flew in 1990 and remains active as of 2021\[update\]. The vehicle consists of three solid propellant stages and an optional monopropellant fourth stage. Pegasus is released from its carrier Pe... > **[Scaled Composites Stratolaunch](https://en.wikipedia.org/wiki/Scaled_Composites_Stratolaunch)** > > The Scaled Composites Model 351 Stratolaunch or Roc is an aircraft built by Scaled Composites for Stratolaunch Systems to carry air-launch-to-orbit (ALTO) rockets, and subsequently repurposed to offer air launch hypersonic flight testing after a change of ownership. It was announced in December 2011, rolled out in May 2017, and flew for the first time on April 13, 2019, shortly after the death of founder Paul Allen. The aircraft features a twin-fuselage design and the longest wingspan ever flo... > **[Scaled Composites White Knight](https://en.wikipedia.org/wiki/Scaled_Composites_White_Knight)** > > The Scaled Composites Model 318 White Knight (now also called White Knight One) is a jet-powered carrier aircraft that was used to launch its companion SpaceShipOne, an experimental spaceplane. The White Knight and SpaceShipOne were designed by Burt Rutan and manufactured by Scaled Composites, a private company founded by Rutan in 1982. On three separate flights in 2004, White Knight conducted SpaceShipOne into flight, and SpaceShipOne then performed a sub-orbital spaceflight, becoming the first ... ---
**Author:** ![3AxisCtrl](https://avatars.discourse-cdn.com/v4/letter/3/ecd19e/32.png) [@3AxisCtrl](https://boards.straightdope.com/u/3AxisCtrl)\ **Post date:** [November 5, 2022, 1:26pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/10 "2022-11-05T13:26:18Z")
> [@md-2000](#): > > I’m assuming once the aircraft (rocket?) gets to the edge of the atmosphere, drag is a minimal issue compared to sea level. 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). ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [November 5, 2022, 2:08pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/11 "2022-11-05T14:08:42Z")
And then there was this crazy thing (which never went into operations): > **[Air-launched ballistic missile](https://en.wikipedia.org/wiki/Air-launched_ballistic_missile#Air_Mobile_Feasibility_test:_C-5_w/_LGM-30)** > > An air-launched ballistic missile or ALBM is a ballistic missile launched from an aircraft. An ALBM allows the launch aircraft to stand off at long distances from its target, keeping it well outside the range of defensive weapons like anti-aircraft missiles and interceptor aircraft. Historically, once launched the missile was essentially immune to interception due to a lack of capable anti-ballistic missiles, with those few that did exist being limited to known static positions. This combination ... ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 5, 2022, 5:20pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/12 "2022-11-05T17:20:29Z")
I’m surprised they jettisoned the dorsal stabilizer instead of folding it like they did with the YF-12A. Or deploy the skids from the stabilizer. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 5, 2022, 5:39pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/13 "2022-11-05T17:39:55Z")
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). ---
**Author:** ![3AxisCtrl](https://avatars.discourse-cdn.com/v4/letter/3/ecd19e/32.png) [@3AxisCtrl](https://boards.straightdope.com/u/3AxisCtrl)\ **Post date:** [November 5, 2022, 6:14pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/14 "2022-11-05T18:14:45Z")
> [@Whack-a-Mole](#): > > Some inconvenient drag wasn’t really worth caring about too much … 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. ---
**Author:** ![3AxisCtrl](https://avatars.discourse-cdn.com/v4/letter/3/ecd19e/32.png) [@3AxisCtrl](https://boards.straightdope.com/u/3AxisCtrl)\ **Post date:** [November 5, 2022, 6:15pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/15 "2022-11-05T18:15:31Z")
> [@Magiver](#): > > I’m surprised they jettisoned the dorsal stabilizer … I think “dorsal” should here be “ventral”. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [November 5, 2022, 6:46pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/16 "2022-11-05T18:46:10Z")
> [@Magiver](#): > > I’m surprised they jettisoned the dorsal stabilizer instead of folding it like they did with the YF-12A. Or deploy the skids from the stabilizer. 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. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [November 5, 2022, 7:42pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/17 "2022-11-05T19:42:17Z")
ALBM are in service in Russia and China, The Russians have [used them in Ukraine.](https://en.wikipedia.org/wiki/Kh-47M2_Kinzhal). ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [November 5, 2022, 7:42pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/18 "2022-11-05T19:42:26Z")
Very cool find Whack-a-Mole. I watched the entire thing. Fascinating. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 5, 2022, 7:49pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/19 "2022-11-05T19:49:12Z")
> [@3AxisCtrl](#): > > I think “dorsal” should here be “ventral”. yes, you are correct. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [November 5, 2022, 8:34pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/20 "2022-11-05T20:34:28Z")
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. > **[AGM-69 SRAM](https://en.wikipedia.org/wiki/AGM-69_SRAM)** > > The Boeing AGM-69 SRAM (Short-Range Attack Missile) was a nuclear air-to-surface missile. It had a range of up to 110 nautical miles (200 km; 130 mi), and was intended to allow US Air Force strategic bombers to penetrate Soviet airspace by neutralizing surface-to-air missile defenses. > The SRAM was designed to replace the older AGM-28 Hound Dog standoff missile which was tasked with the same basic role. The Hound Dog was a very large missile that could only be carried in pairs by the B-52, so s... [Next page](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401.md?page=2)