# 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:** 14\ **Page:** 2
**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, 8:58pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/21 "2022-11-05T20:58:38Z")
The ALBM category has gotten real elastic. With “ballistic” being the qualifier stretched to the breaking point. An air-launched nuclear missile is not necessarily ballistic. AGM-28 Hound Dog was an early attempt at a cruise missile that mostly worked. The AGM-86 ALCM / Tomahawk was a follow-on that really worked / works. Neither are ballistic in any sense. AGM-69 SRAM was much shorter in range & not ballistic either. The subsection @gnoitall linked to in the wiki ALBM article was about test launching an actual full-sized Minuteman ICBM from an airplane. This was a lot more about trying to find a survivable basing mode for a land-based intercontinental missile than it was about carrying the missile to high speed, high altitude, or close to its target. IOW, using the airplane as a first stage booster loosely speaking. Which is where this digression from X-15 design got started. As @AK84’s cite said, the Russian Kh-47M _Kinzhal_ gets to the speed and range of a decent long range missile, but even it isn’t really ballistic as in exo-atmospheric sub-orbital trajectory. Even the first generation sub-launched Polaris and land-based Atlas had comparable or greater ranges. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [November 6, 2022, 6:23am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/22 "2022-11-06T06:23:56Z")
True, but then again since the 1980’s, how many purely Ballistic missiles\* have been developed, except by Iran? The American ATACMS, the Russian Iskander and Yars, the Indian Agni and the Pakistani Shaheen series all have some significant maneoveiring capability for the RV. \*I am ignoring North Korea since there isnn’t as much information available on what they exactly have as far as RV are concerned. ---
**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 6, 2022, 12:14pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/23 "2022-11-06T12:14:08Z")
Good point. Although even going back to the US Atlas & its Soviet contemporaries, the missile + warhead was never strictly ballistic like an artillery shell is. They always had guidance enroute and it was only fairly far down-track (80-90% of the total trajectory?) that the warhead became simply an unguided rock falling with great speed from a high place. As you say, it’s different now. Nearly everything of the last 20-30 years has some degree of guidance and maneuvering during the final approach to the target. To me the key parameter of “ballistic” is that the trajectory is highly lofted. It doesn’t power through the atmosphere to get where it’s going. Instead it gets well above the atmosphere and coasts for a sizeable portion of the total range. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [November 6, 2022, 5:26pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/24 "2022-11-06T17:26:44Z")
> [@LSLGuy](#): > > To me the key parameter of “ballistic” is that the trajectory is highly lofted. It doesn’t power through the atmosphere to get where it’s going. Which is like the Minuteman I in that airdrop test, and unlike most of other long-range air-to-surface missiles listed. I guess “ballistic” is used in those cases to mean “not staying airborne by aerodynamic lifting surfaces”. Which, bringing it back to the X-15, I have to wonder how lift-effective its wing structure was during the period it was in atmosphere, and how much of its staying airborne was “given enough thrust, a rock will fly.” ---
**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 6, 2022, 5:48pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/25 "2022-11-06T17:48:31Z")
> [@gnoitall](#): > > I have to wonder how lift-effective its wing structure was during the period it was in atmosphere, and how much of its staying airborne was “given enough thrust, a rock will fly.” The X-15’s wing loading at max gross weight is about 50% higher than the roughly comparably-sized F-16C. At light weights the X-15’s loading is about 25% greater than the F-16’s. As well the F-16 has much higher lift aerodynamics, so one square foot of F-16 wing creates a lot more lift than one square foot of X-15 wing. Both as a result of 30 years of aero-engineering progress and as a result of not needing to compromise the design for very high speeds. Bottom line: IMO the wings were certainly helping during ascent, but it was pretty much going to go wherever it was pointed, driven by pure thrust. During the glide home, obviously the wings were big enough to give the pilot a reasonable expectation of arriving at ground level at a survivable descent rate. But still, like the later Space Shuttle, I’m sure it was a thrilling ride going down as well as it was going up. There were two landing accidents in the program. In each case they never got the rocket engine really running after the drop from the B-52 for one reason or another and so very, very quickly they were running out of altitude. The pilots in each case sorta-successfully landed on the emergency return lakebed, but with enough vertical speed it broke the aircraft in two. They each survived & the aircraft was rebuilt & reused. See [![](https://img.youtube.com/vi/rSxDPhF-HSk/hqdefault.jpg "The X-15 Makes an Emergency Landing") ](https://www.youtube.com/watch?v=rSxDPhF-HSk) and [![](https://img.youtube.com/vi/STBE7JpQMeA/maxresdefault.jpg "X-15 Crash Landing (1962)") ](https://www.youtube.com/watch?v=STBE7JpQMeA) ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 6, 2022, 7:43pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/26 "2022-11-06T19:43:08Z")
some observations of the 1st video. It was still in trial mode. It didn’t look like it impacted hard enough to bend up the sub-frame. The ventral fin couldn’t have folded as I suggested earlier because it would hit the skids. ---
**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 6, 2022, 7:53pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/27 "2022-11-06T19:53:51Z")
> [@LSLGuy](#): > > But still, like the later Space Shuttle, I’m sure it was a thrilling ride going down as well as it was going up. There was a modified Gulfstream that they used to train Shuttle pilots. In order to simulate how the Shuttle handled in flight, they would put the landing gear down and deploy the thrust reversers. There is of course the common “like a brick” phrase used to describe how the Shuttle flew. Another phrase I heard was that at altitude, it basically glided “like a Steinway”. The X-15 was slightly better than that, but not by much. ---
**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 6, 2022, 10:32pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/28 "2022-11-06T22:32:48Z")
The wiki on that vehicle is interesting. > **[Shuttle Training Aircraft](https://en.wikipedia.org/wiki/Shuttle_Training_Aircraft)** > > The Shuttle Training Aircraft (STA) was a NASA training vehicle that duplicated the Space Shuttle's approach profile and handling qualities, allowing Space Shuttle pilots to simulate Shuttle landings under controlled conditions before attempting the task on board the orbiter. The STA was also flown to assess weather conditions just prior to Space Shuttle launches and landings. > NASA developed the STA using the Grumman Gulfstream II as the underlying aircraft platform. During the early phases of t... One of the details it mentions is the so-called outer glide slope is flown at 300 kts in a 20 degree dive. For comparison, 20 degrees is a pretty common dive angle for dropping dumb or smart bombs from a jet fighter. Then at 2000 feet above the runway elevation they nose up to a more traditional descent angle and the speed rapidly decays as the itty-bitty wings try to hold the weight off the ground. The idea, as with every landing in an airplane, is to run out of speed and altitude just _after_ you run out of unprepared ground short of the runway, but not too long after. Sounds sporty. There was a fascinating post-mortem written by NASA investigators about a mechanical failure in the shuttle _simulator_ cockpit instruments during a practice sim session where had the same thing happened in the real vehicle they’d almost certainly have landed (crashed really) well short of the runway. A close call of a different nature. I stumbled on that report probably 10 years ago. A few minutes just now Googling for the same report came up empty-handed, but golly there are a lot of shuttle gurus who have written intelligently on the vehicle, the missions, and everything else. A space geek could spend a lot of time reading that stuff. ---
**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 7, 2022, 11:18am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/29 "2022-11-07T11:18:31Z")
> [@LSLGuy](#): > > One of the details it mentions is the so-called outer glide slope is flown at 300 kts in a 20 degree dive. For comparison, 20 degrees is a pretty common dive angle for dropping dumb or smart bombs from a jet fighter. Then at 2000 feet above the runway elevation they nose up to a more traditional descent angle and the speed rapidly decays as the itty-bitty wings try to hold the weight off the ground. The idea, as with every landing in an airplane, is to run out of speed and altitude just _after_ you run out of unprepared ground short of the runway, but not too long after. Sounds sporty. 20 degrees down angle is indeed pretty harsh. AIUI, a typical descent on a commercial aircraft is more like 3 degrees. I used to have a space shuttle landing sim on my phone. If you turned on all the HUD aids, it wasn’t hard to follow the prescribed path from 50K feet down to the runway - but setting it down in the right spot, and gently enough, to score a “safe” or “perfect” landing was devilishly hard. Here’s a replay (not mine) of someone executing a “perfect” approach and landing (fast-forward to about 2:40 to catch just the final approach and touchdown): [![](https://img.youtube.com/vi/3D3c_ZRfKyk/hqdefault.jpg "F-Sim Space Shuttle - Perfect Landing - 900K Score [HD+]") ](https://www.youtube.com/watch?v=3D3c_ZRfKyk) ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [November 7, 2022, 7:07pm UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/30 "2022-11-07T19:07:22Z")
> [@Whack-a-Mole](#): > > Below is a YouTube video Great video from the top, thanks! ---
**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\ **Post date:** [November 8, 2022, 12:50am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/31 "2022-11-08T00:50:38Z")
I take it that, decelerating from Mach 20+ to landing, the Space Shuttle apparently didn’t need a wedgetail fin? ---
**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 8, 2022, 1:29am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/32 "2022-11-08T01:29:49Z")
I think the difference is the X-15 was meant to test flying a plane in the atmosphere (even if really high up, still in the atmosphere) at hypersonic speeds. The Space Shuttle just needed to drop like a rock(ish) through the atmosphere. It mainly needed to shed speed the whole time so came in at a high angle-of-attack and stayed that way for most of the re-entry. It’s tail probably did little to nothing like that (because it probably was not in the airstream). When the Space Shuttle finally wanted to “fly” and get to a runway it’d lower its nose and it was now at a comparatively slow speed and its tail fin would work like a tail on most planes. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 8, 2022, 2:58am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/33 "2022-11-08T02:58:13Z")
> [@EnolaStraight](#): > > I take it that, decelerating from Mach 20+ to landing, the Space Shuttle apparently didn’t need a wedgetail fin? It had a heat shield. it could bulldoze through the atmosphere like a meteor… ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 8, 2022, 5:34am UTC](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401/34 "2022-11-08T05:34:39Z")
The shuttle did manoeuvre at hypersonic speeds during descent. Its cross range capability also occurred during hypersonic flight. From what I can gather, some of the post entry interface control was still done with thrusters, but transitioned to control surfaces as it dropped into the thicker atmosphere, but still at hypersonic speeds. Interestingly, the speed brake opens to 81% at Mach 10. So here the tail fin is opening as seen on the X-15 pretty much as it starts to need control. The X-15 is credited with contributing a significant amount of knowledge used in the space shuttle. and the tail configuration is explicitly noted in [this nice paper](https://www.researchgate.net/publication/296652080_Space_Shuttle_Design_and_Lessons_Learned). [Previous page](https://boards.straightdope.com/t/why-did-the-x-15-have-such-a-non-streamlined-vertical-stabilizer/974401.md?page=1)