Why was the skid landing gear located so far aft on the X-15?

Question

The X-15 famously used a pair of steel skids as the main landing gear rather than conventional wheels, which saved weight, allowed the skids to be mounted externally (where wheels and tires would have been destroyed) so they didn't take up internal space, simplified the extension mechanism, etc. However, they were also mounted very far back compared to most conventional tricycle-gear airplanes. Rather than being just a little ways aft of the CG as usual, they were way back under the tail. This caused some unusual behavior on landing - in addition to giving the pilot almost zero pitch control as soon as the skids touched (since they were directly under the tail, the stabilizer had no lever arm to pivot around them), it did weird things to the landing loads. From NASA book SP-60, "X-15 Research Results":

[T]he extreme-aft location of the main landing skids on the X-15 produces dynamic-response characteristics during landing that are as unusual as the gear itself. 

The primary cause of the unconventional response is the craft's downward rotation onto the nose gear immediately following the main gear's touchdown. Significantly, this movement onto the nose gear causes a subsequent rebound onto the main gear, providing a much higher load there than that at initial touchdown. In addition, the nose gear encounters loads that are two to three times greater than at either of the main-gear skids. Another unique feature is that the gear loads achieve about the same maximum level whether the pilot "greases-it-on" or lands with a high rate of descent. These new gear characteristics have not been without problems. Much study and analysis of the dynamic response of the airplane during landing has led to strengthening the gear and back-up structure and modifying the nose gear so as to provide greater energy absorption.

In other words, there was actually a much higher load through the struts after the nose gear touched down when it settled back than the load from initial skid contact. NASA tech memo X-639 (19630002688 in the NTRS) even comments that if the skids were moved forward it would behave more like a regular fighter jet, but doesn't take that thought any further.

So... why did they choose to put the skids way back there rather than close behind the CG? Was it for better directional stability at touchdown with a sucks-at-low-speeds vertical stabilizer and rudder? Or was it just to provide clearance for the ventral fin stub at high landing AoA while keeping the gear short? Or, was it simply that there wasn't room left for landing gear and the required internal structure at a more conventional location? (Or as it often is, a combination of these or other factors?)

Answer 1

Indeed as Peter Kämpf said, the skids were only used for landing; the takeoff on X-15 was done by attaching it to the starboard wing of a modified B-52, known as the NB-52A. This was the only way it could takeoff - it couldn't takeoff on its own.

X-15 being deployed from the NB-52A

With that in mind, it makes sense to place the skids as far aft as possible, since doing so maximizes the clearance for the gigantic ventral fin. However, even that was not enough; a large part of the ventral fin had to be jettisoned (it was detachable) before landing in order to have enough clearance. The fin was deployed with a parachute, and with hope that it would land safely so that it could be reused.

Answer 2

The X-15 landing speed was around 200 mph. The primary reason to have the skids so far back would be directional stability, which would be helped greatly by a trailing parachute.

The placement of the skids did have an effect of increasing the nosewheel load, resulting in a "see-saw" type of loading back onto the rear skids, but the engineers who marveled at this may have underestimated the contributing loss of wing lift (from lowering of AoA) to the "secondary load peak".

Moving the skids closer to the CG would follow conventional wisdom of "taking it on the mains" but would increase the chance of an uncontrollable nose pitch up if the nose wheel hit first, which would have resulted in a heavy tail strike. Touching down on the rear skids first would help lower AoA and reduce airspeed before the plane fully touched down$^1$.

A second set of shock absorbing skids near the CG? Possibly. A landing parachute? Would have helped. But somehow, on the (sometimes) dry lakebed, they managed.

$^1$ a potential design improvement would allow the elevator to "hold" the two point landing position as speed reduced before the nose touchdown. This is how tricycle gear normally works. The Space Shuttle did this slightly better.

Answer 3

As already explained in another answer, the main constraint driving the placement of the main landing gear on the X-15 was to avoid tailstrike at landing. But there was also another important effect appearing at landing that precluded locating the main landing gear just behind the CG like in a conventional airplane. A simple overview of the aerodynamic behaviour of the X-15 helps understanding it.

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The X-15, like other aircraft of comparable role and era, was designed to collect data at hypersonic speeds, data which was later used in the space shuttle development. And the X-15 was an engineering masterpiece. It was powered by one rocket engine producing 70'000lbf/300 kN of thrust (comparable to an A320), reaching speeds of 4500mph/7000kmh and an altitude of 60mi/100km. It was piloted, among others, by Neil Armstrong. Its nose wasn't pointy like in other supersonic airplanes, but blunt so that the shock wave remained detached from the airplane, lowering its heating. Its skin was made of a nickel alloy which resists extreme pressure and heat but is difficult to machine. It was launched attached under the right wing of a B-52, and when the 45'000ft altitude was reached the X-15 detached from the mothership and ignited its rocket engine. After less than one and a half minutes (!) it was already flying at 160'000ft and Mach 6. It was a marvel designed to smoothly fly at hypersonic speeds.

Anyway, at the low subsonic speeds typical of landing, the X-15 was nothing more that a draggy pipe with some stocky protuberances. After having reached its target speed and height, its rocket engine was exhausted and the X-15 could only glide back toward Earth. The small wing produced lift only in limited quantity, and as a consequence its lift to drag ratio was quite low. It had, therefore, to glide at a quite high angle of attack. So at low subsonic speeds the X-15 was more like a flying squirrel than a hypersonic marvel.

Due to complexity, weight, and precious space occupied inside the fuselage, wheeled landing gears were not an option on such an experimental airplane. Tires' heating would have been an insurmountable problem, too: on the SR-71 for example, the tires of the main landing gear "contained aluminum and were filled with nitrogen. They cost $2,300 and would generally require replacing within 20 missions"; plus the "air-conditioning system was also used to keep the front (nose) landing gear bay cool" (source: Wikipedia). The skin-friction heating increases more or less with the square of the Mach number and since the X-15 was flying at twice the Mach number of the SR-71 it meant a far higher airframe temperature. Main landing gear in the form of simple skids were therefore the only viable solution.

Putting the landing gear just behind the CG like in most conventional design was also not doable: to avoid tailstrike at the high angle of attack of the approaching phase, their struts should have been very long, heavy and bulky (think about the Concorde). So the skids had to be located far back on the fuselage.

But there was also another important aspect to drive the placement of the skids. During approach, in order to compensate for the high AoA and high sink rate, the horizontal stabiliser had to be set at a quite negative deflection to trim the aircraft. Upon touchdown, both the AoA and the sink rate went immediately to zero and the horizontal stabiliser suddenly found itself generating a huge download. Now, if the main landing gear were located just behind the CG like in a conventional design, this huge download on the horizontal tail would have made the aircraft pivot around the main landing gear and sink the tail into the ground. The skids had therefore to be placed well behind the CG to avoid this effect. Theoretically there was an optimal distance behind the CG, somewhere halfway between tail and CG, but it would have been not enough to avoid tailstrike and the skid were finally placed at the far end of the fuselage. This gave the X-15 a quite peculiar landing: upon touchdown with the skids the nose was forced down quite abruptly; then nose suspension got compressed and then extended back pushing the nose up; this up-nose movement downloaded on the skids suspension which then got compressed and extended back as well. After a couple of back and forth jumpings, the X-15 came to slide on the lakebed. This video from Wikipedia shows this behaviour quite well.

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Other interesting information about the longitudinal placement of the skids can be found in this NASA report.

Answer 4

It is worth pointing out that they didn't get a fleet of X-15's, so there were likely extensive constraints on technology, analysis, and and modifications. If the company only makes 3 as opposed to 300 the economy of scale doesn't support the more in-depth stuff.

When I first saw the question, it looked like the skids were set to protect the tail, and for directional stability.