X-15 pilot report, part 4:
X-15 Flight:  Flying the Mission and Returning
X-15 just after engine start

After you light the torch you need to fly precisely in the face of some unusual challenges. Let's say the mission at hand is a more or less full bore altitude flight.

Part 1 is to go to full throttle and pull up to just the right climb attitude.X-15 pulling up into climb With 57,000 pounds of thrust from the rocket, maybe even 60,000 on a good day, you start at 2 G's acceleration while your tanks are full and your gross weight is about 33,000 pounds. As you burn fuel and oxidizer, with weight dropping to barely over 15,000 pounds, acceleration doubles to 4 G's. Rocket motor performance can vary depending on lots of factors that sound small, but sometimes they add up instead of cancelling each. If you compound that deviation by missing your planned climb attitude by as little as 1 degree, the results begin to look astronomical. Flying with precision that would give most pilots enormous pride, your imprecision can easily produce a 30,000 foot overshoot or undershoot in maximum altitude.

All that precise flying happens on the gages. Let's say today's mission profile puts you in a climb attitude of 42 degrees. If you glance out the windows in this attitude you see nothing but dark blue sky that's quickly getting blacker as you climb. With no visual cues and thrust overpowering gravity your sense of balance lies -- it feels as if when you pulled up the rotation never stopped and you've gone past vertical, to an inverted attitude! Those instruments connected to the gyro-stabilized inertial platform are what you need to believe to stay in touch with reality.

Illustration of X-15 flight path As you climb out of the atmosphere the aerodynamic flight controls lose their effectiveness. Here you have to transition from flying with the right hand side stick (and rudder pedals) to flying with the left-hand side stick, for the hydrogen peroxide thrusters. That's unless you have the luxury of flying the #3 X-15 -- its adaptive controller, the MH-96, lets you use only a single side stick and automatically blends aerodynamic controls and reaction controls.

On today's altitude flight either you cut the engine after an 82 second burn or the X-15 burns exhaust enough ammonia and LOX to burn out by then without your help. That's another variable that makes a big difference in peak altitude, if you have power on for an extra second you can expect a sizeable overshoot. Now you're weightless, still climbing in a ballistic arc. In a couple minutes today's flight tops out at about 300,000 feet over the high deserts of Nevada and southern Califoria.

Colorado River, X-15 photo from 210,000 feetAs you pitch over you can now catch a spectacular view of the southwest U.S. The view shown here is the Colorado River Valley from 210,000 feet.. In the words of Robert White, "My flights to 217,000 feet and 314,750 feet were very dramatic in revealing the earth's curvature ... at my highest altitude I could turn my head through a 180º arc and wow! - the earth is really round. At my peak altitude I was roughly over the Arizona/California border in the area of Las Vegas, and this was how I described it:  looking to my left I felt I could spit into the Gulf of California.  Looking to my right I felt I could toss a dime into San Francisco Bay." If the coast is clear that far north, you can just about make out Puget Sound, nearly a thousand miles away..

Coming back down you carefully establish the correct attitude for reentry into the atmosphere as a very fast glider with a truly crummy glide performance. REAL gliders are painted white to help them keep cool, by reflecting solar energy. The X-15 is painted black to help it keep cool, by radiating the heat that builds up rapidly from air friction. A few parts of the nose, wings, and stabilizers briefly hit temperatures up to 1,200 degrees and glow red hot during reentry.

Pulling out of the reentry is one of the maneuvers that lets the pilot know this is no ordinary realm of flight.  If this altitude flight topped out at 350,000 feet you can expect to be coming out of reentry at Mach 5.4 in a 40-degree nose-down attitude.  Pulling out of this dive requires pulling an average of 5 g's for about 20 seconds.  If you only need to turn through a mere 10-degree heading change at Mach 5.3, expect to pull 3 g's for 20 seconds.

There are a few unusual but modest control couplings. At low angles of attack, roll inputs couple to a favorable yaw.. Above mach 2.6, roll response gets quicker as angle of attack increases.  At least that's what the manual says -- Scott Crossfield reports that there's virtually no roll/yaw coupling, she rolls nicely.

As the X-15 slows down and drops, stability degrades and allowed yaw angles decrease. Below about 40,000 feet and mach 0.5, minimum control speed is determined by stability; above that point it's governed by buffeting at the tail. Stability margins allow you to fly AOA's up to 20 degrees, but the pre-stall buffet starts at 13 degrees.

Stability problems can be evil. Conventional wisdom says that you're NEED the electronic stability augmentation systems (or the SAS functions built into the MH-96) or your chances for a survivable reentry are puny. Hypersonic stability trouble can bite in big ways, including the sort of inertia coupling that earlier killed Mel Apt in the X-2.

Going back to being a glider approaching the landing pattern, expect a sink rate of about 150 feet per second (9,000 fpm) at mach 0.75. That gives a max L/D of almost 5:1 at 40,000 feet, but realistically you can expect closer to 4:1.

Finally, you have to land at Edwards Air Force Base. Field elevation is 2,200 feet for a somewhat groomed runway on the dry lake bed. Approach will be sort of a 360 overhead pattern; it'll actually be a tightening spiral because true airspeed drops while you descend at a constant indicated airspeed.

Illustration of X-15 approach path

Landmarks in the pattern on this flight are:

145 seconds to touchdown, 28,900 feet: High key point

This is 2 miles short of the approach end of the runway and 1.5 miles to its right. You should hit it at 300 knots and roll into a 45-degree banked turn to the left. You'll maintain 300 knots IAS and the 45-degree bank until just before you flare.
108 seconds to touchdown, 20,900 feet:
270-degree key point: "Crosswind leg",
90-100 seconds to touchdown, below 17,000 feet:
Pressurize the propellant tanks. They were switched to Vent at burnout, but they need pressure now for two reasons:
75 seconds to touchdown, 14,100 feet:
Low key point, "downwind abeam" (opposite approach end of runway, about 3 3/4 miles from it).
46 seconds to touchdown, 8,500 feet:
90-degree key point, "base leg".
30 seconds to touchdown, 5,500 feet:
Jettison the ventral.
19 seconds to touchdown, 3,500 feet:
Roll out onto the runway heading and drop the flaps. You're still at 300 knots.
15 seconds to touchdown, 3,000 feet:
Drop the gear and begin to flare. The flare is a 1.5 G pullout.
8 seconds to touchdown, on the deck, just above 2,200 ft field elevation:
End of flare; airspeed's dropping through 262 knots.X-15 landing, paced by F-104
0 seconds:
Set it down when airspeed drops to 200 knots and ride until you stop. With no brakes and no steering, you're almost a passenger now.

Well, not entirely. You can actually steer a little bit by using the stick as if you're banking: The stabilators will shift weight from one landing skid to the other and will steer you in the direction you've moved the stick while you're fast enough.  When the speed drops off, force generated by the stabilators drop too, and you do finish the rollout as a non-steering passenger.

Finally, go through the after-landing checklists to secure everything.  As the B-52 offers its salute with a flyby, convince the folks on the ground and the ones who get out of the chase planes that a modest celebration is in order.  Or maybe a big celebration, this flight may well have been another milestone in aviation history.
X-15 on lakebed after landing, B-52 flyover

This pilot report is split into these four parts:

    1.  X-15 General Description & Walkaround
    2.  X-15 Cockpit Check
    3.  X-15 Flight:  Heading Out to Launch
    4.  X-15 Flight:  Flying the Mission and Returning

Related to all four sections:

    Photo credits and pointers to related resources

Send questions and comments on the SierraFoot X-15 pages to Paul Raveling.
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