Date: 19 Nov 91 03:12:19 GMT From: raveling@Unify.com (Paul Raveling) Subject: X-15 "Pilot Report" OK, since my inquiry last Friday popular demand says it's time to repost my old "pilot report" on the X-15. The original art- icle was inspired by Bill Standerfer's pilot reports on some rare birds, and like Bill's reports, this one is based on the aircraft's flight manual rather than actual flying experience. It discusses a bit about both airframe engineering and piloting techniques, so this edition is cross-posted to both newsgroups (sci.aeronautics and rec.aviation). The flight manual that this information comes from was last revised on December 29, 1961 -- in about 6 weeks it'll be 30 years old. I like to think of the X-15 as the most radical of all self-launching sailplanes, though it DID have a funny sort of aero tow, and its glide performance rivalled that of Mount Saint Helens. (OK, maybe that's a LITTLE severe; it glided a lot like the Space Shuttle.) This PIREP is split into these four parts: 1. X-15 General Description & Walkaround 2. X-15 Cockpit Check 3. X-15 Flight Test: Heading Out 4. X-15 Flight Test: Coming Back X-15 General Description & Walkaround ------------------------------------- The X-15 is mainly a cylindrical fuselage, 49 feet 2 inches long. "Wing root" fairings begin just aft of the cockpit and extend nearly the entire length of the fuselage on each side. From these protrude stubby wings spanning 22 feet 4 inches and horizontal stabilizers. Vertical stabilizers stand both above and below the fuselage, giving the aircraft a total height of 13 feet 1 inch. Let's start a walk-around at the aft end. You're immediately looking into the business end of the XLR-99 rocket engine, which consumes up to 15,000 pounds of anhydrous ammonia and liquid oxygen on a typical flight. The engine's main chamber is made of CAREFULLY shaped and welded tubing that circulates ammonia before it's burned. Besides preheating the fuel, this keeps the engine from melting. Around the rocket nozzle, below it on the ventral fin, and at the aft end of both wing root fairings, are 25 assorted drains, vents, and jettisons. The largest three are the turbopump exhaust and the jettisons for LOX and ammonia. If you use your XRAY eyes to peer past the LOX jettison in the left fairing you'll first see a small spherical tank holding helium at 3600 psi. This particular helium purges explosive gases from the engine compartment, preferably before they ignite. Behind that is the end of a Godzilla-sized piece of plumbing that carries LOX from its tank to the engine compartment. The right fairing looks about the same, but the plumbing there carries ammonia. Lurking behind the XLR-99 is the turbopump and a maze of assorted devices and plumbing. One of the devices is a gas generator, which is the X-15's breed of catalytic converter. When you feed 90% hydrogen peroxide into it, the H2O2 decomposes into superheated steam and oxygen on its catalyst beds. The resulting gases drive the centrifugal turbine in the turbopump, which then drives separate compressors for the ammonia and LOX. Back on the outside, the horizontal stabilizers are the simplest pieces. They have 15 degrees anhedral (or cathedral, if you prefer). They swivel together for pitch control, and move differentially for roll control, since the wings have no ailerons. These, as well as the "rudders", have opposed sets of irreversible hydraulic actuators -- you can make them push, but they won't push back, so you won't feel any air loads on the stick. Cockpit controls make up for this with a system of spring bungees to provide "natural" feel and pitch trim. The vertical stabilizers get more complicated. Their triangular cross section provides adequate room for several hydraulic actuators, and both dorsal and ventral fins are split longitudinally into two parts. Panels on the aft third of the fixed (inboard) sections spread apart to serve as speed brakes. It's best not to use them if you're slow (like subsonic), because the only way to close the speed brakes is to let a fairly heavy air load blow them back. The top of the dorsal fin and the bottom of the ventral fin rotate to act as a rudder. The ventral piece extends below the landing gear, so it has four explosive bolts and an initiator (an explosive-driven piston) to jettison it before landing. Just remember not to jettison the ventral above 300 knots or mach 3.5, whichever is lower. The main landing gear are skids that are locked against the aft fuselage until you're ready to release them. At that time they drop into place and lock, thanks to gravity and air loads. Since they don't have wheels, you don't have brakes. They're so simple and reliable (right?) that they also lack any device to tell you, the pilot, whether they're up or down. The wings have no dihedral, but they do have flaps and rockets. The rockets, two per wing, supply roll control out where there's little if any air. Like the turbopump's gas generator, they're powered by hydrogen peroxide. Almost all of the fuselage's spare volume is split between the liquid oxygen tank in the forward end and the ammonia tank in the aft end. Each is divided into three interconnected chambers and a concentric cylindrical core. The LOX tank's core holds helium for pressurizing the peripheral ends of the two propellant tanks. Propellant feeds out through overgrown sewer pipes at the tank ends nearest the center of gravity. Other tanks for hydrogen peroxide, helium, and liquid nitrogen are crammed into the fuselage at either end of the main propellant tanks and between them. The compartment between the LOX tank and the cockpit also houses two APU's, each geared to an alternator and a hydraulic pump. The APU's are also turbines powered by hydrogen peroxide from their own gas generators. One token sample of this plumber's nightmare (or is it plumber's heaven?) is what the nitrogen supply does. It cools the No. 2 electronics compartment, the alternators, the APU upper turbine bearings, the stable platform (for inertial guidance), and the ball nose. It purges the hydraulic reservoirs and inflates canopy and electronics compartment pressure seals. And it does some more things mentioned when we inspect the cockpit. And that's simple compared to the helium systems! Walking forward again, we'll pass the cockpit for now. Next comes the nose gear, the proud possessor of two wheels. Of course it isn't steerable -- who needs to steer in that big old dry lake. Like the main gear, it drops by gravity and air loads, but there's a catch: At a positive AOA the nose gear door would have an air load trying to keep it closed, so it gets help. Another initiator gives it a boost, and a small air scoop opens down from it to get the airstream to hold it open. Almost at the nose, eight rocket nozzles supply yaw and pitch control for the ballistic control system. All the way forward is the ball nose, which swivels around two axes to measure angle of attack and sideslip. These angles are worth knowing both at reentry and at low speeds, where the x-15 is least stable. X-15 Cockpit Check ------------------ This is a cozy place. Don't bother with it if you're claustrophobic; admittedly it's a little bigger than a sailplane cockpit, but there's LOTS of stuff in it and you can't see much when the canopy is closed. You get two windows barely larger than slits to peer out of -- the rest is utterly metallic and opaque. One of the nice touches on top is a head brace that folds downward in front of you. That makes deceleration at reentry a fair bit easier to handle. Another necessary nicety is that the windows are double glazed. Heated gaseous nitrogen pumped between the panels keeps them from icing over. The cockpit is unpressurized below 35,000 feet, but it's air conditioned. Above 35K it's pressurized to 3.5 psi by nitrogen. Your pressure suit gets another nitrogen feed to keep it at 3.6 psi. The ejection seat is almost an airplane itself. If you need it, it'll unfold fins, put you in a nice attitude, escort you to low altitude, then blow pieces of itself away and deploy your chute. According to the manual, it will "permit safe pilot ejection up to Mach 4.0, in any attitude, and at any altitude up to 120,000 feet". If you REALLY believe that I'd like to sell you some real estate in Florida. Once settled in the seat, you're looking at about 130 odd gages, switches, lights, and controls of assorted descriptions. First, there's a conventional center stick and rudder pedals. There's also a console stick at your right hand for use when G loads make it difficult to use the center stick. Both are mechanically coupled together and to a system of bell cranks that sum their inputs with those from the Stability Augmentation System (SAS). A horizontal stabilizer position indicator is located on the cockpit wall next to the console stick. This is a must-check item before dropping from the B-52 carrier aircraft and before beginning reentry. A third stick, for the ballistic control system, is at your left hand. When the ballistic control rockets are armed, you can: -- Move it left or right to control yaw -- Rotate it to control roll by firing wing rockets -- Move it up or down to control pitch Other ballistic inputs come from the Reaction Augmentation System (RAS), which is the no-air equivalent of the SAS. The same left-side panel houses the speed brake lever and the throttle. The throttle allows a choice of "off" or any thrust setting between 50% and 100%. The main instrument panel is divided into three sections: Engine instruments on the lower left, APU's on the lower right, and flight instruments at top center. Both engine and APU sections are mainly an assortment of pressure gages, temperature gages, fire warning lights, and sundry switches. The most prominent flight instrument is a big attitude indicator, planted squarely in the middle. It looks fairly ordinary, but it's accurate throughout 360 degrees of rotation around any axis you care to name. Scanning clockwise around the attitude indicator, starting just below it, the flight instruments are... -- Roll rate indicator, calibrated to 200 degrees per second. The X-15 is pretty agile in roll, but you need to limit the rate to no more than 50 degrees per second in some conditions for the sake of stability. -- Altimeter: Looks ordinary, but it has a cutout that unveils warning strips when you're dangerously low -- under 16,000 feet. -- Airspeed: Usable from 100 to 1,000 knots; has a vernier drum to show speed to 1 knot anywhere in this range. -- Angle of Attack: Reads -10 to +40 degrees. Stay below +20 degrees to avoid stability problems, or +17 1/2 after jettisoning the ventral. -- Accelerometer: Reads +12 to -5 G's. Allowed load factor ranges from +3/-2 G with a full load of propellant to +7/-3 G at burnout weight. -- Azimuth indicator: A high class compass. Like the attitude indicator and the next few instruments, its reference is the gyro-stabilized platform in the "Inertial All-Attitude Flight Data System". -- Inertial height (altimeter): Shows altitudes up to 1 million feet. The little hand reads hundreds of thousands, the big hand reads tens of thousands. -- Inertial speed: Calibrated for any speed up to 7,000 feet per second. That translates to 4,150 knots or 4,773 mph. -- Inertial vertical velocity: Reads up to 1,000 feet per second (60,000 fpm) in increments of 100 fps. We'll skip the remaining cockpit clutter, only because it's generally less amusing, though some of it will appear in the test flight. X-15 Test Flight: Heading Out ------------------------------ You'll start with the X-15 already hanging from a pylon on the B-52 carrier aircraft. Don't bother doing a preflight; the taxpayers thoughtfully provided a whole staff to do it for you. Anyway, you're cooped up in a pressure suit, prebreathing 100% oxygen. After entering the cockpit, strap in and and run through the interior check. That's 120 checklist items, ending with "close canopy". By now the B-52 is supplying power, breathing oxygen, nitrogen, and liquid oxygen to top off lossage from the propellant tank. That LOX lossage is due in part to precooling the engine, which you initiate in the captive takeoff checklist. While precooling, LOX flows through almost all of its usual path to the engine, then dumps overboard at the liquid oxygen prime valve. You'll precool for 10 minutes, then start a schedule of 20 minutes off and 7 1/2 minutes on. A few things will need heat during the climb, so be sure to turn on the heaters for the windshield, your face mask, and the nose ballistic rockets (the X-15's, not yours). Also ask the B-52 crew about the hook heater. Once at altitude, go through the 27-item prelaunch checklist. Among other things, you'll switch to the X-15's breathing oxygen, shut off liquid nitrogen but not gaseous nitrogen from the B-52, and start the APU's. Next comes the 29-item precountdown checklist. Among other things, you will: -- Stop LOX transfer from the B-52. -- Cycle the propellants tanks from vent to jettison, then to pressurize. -- Check the flight controls and trim settings. Launch control surface deflections should be 0 degrees for everything except the horizontal stabilizers, which should be 0 to 2 degrees leading edge down. Be careful - if the position's too far off when you drop, the X-15 can pivot on its pylon and slide into the engines just outboard on the B-52. -- Turn on instrumentation. -- Start the final engine precool cycle. -- Set the engine prime switch to PRIME. This opens the liquid oxygen and ammonia main feed valves and the hydrogen peroxide upstream safety valve for the turbopump, and it feeds helium to the engine control and purge systems. It takes 30 seconds to complete priming. -- Depress the turbopump idle button. This opens its H2O2 downstream safety valve, allowing the pump to begin running. When it's brought ammonia pressure up to 210 psi, the turbopump speed control system will take over to keep it at idle speed. -- Set the igniter idle switch to IGNITER. Now you have 30 seconds to finish preparations, drop, and fire the XLR-99's main chamber or to shut down. Don't leave the igniter on longer, or the XLR-99 will be damaged by overheating. What happens when you go to igniter idle is this rapid-fire sequence: 1. The engine is purged with helium for 2 seconds. 2. Three spark plugs are energized in the first stage igniter, which is a small combustion chamber ahead of the 2nd stage igniter and main chamber. 3. Ammonia and gaseous oxygen enter the 1st stage igniter and start burning. The gaseous oxygen is produced by routing liquid oxygen through a heat exchanger that surrounds the turbopump exhaust. 4. When chamber pressure rises sufficiently, gaseous nitrogen from the B-52 flows in to mix with the 1st stage igniter gases. 5. Ammonia and LOX begin flowing to the second stage igniter, which is a larger chamber, where they're ignited by the jet of burning gas from the 1st stage. When the 2nd stage igniter and main chamber build to 150 psi, about 5 seconds later, the jet into the main chamber is sufficient to light it up. You're ready to go, so ask for the countdown -- fast! They're serious about the that 30 second time limit for either lighting up or shutting down. When there's 7 seconds more of idle time allowed, an IDLE END caution light comes on. When time runs out, a NO DROP light tells you to shut it down. So... when you get through the brief countdown and the B-52 drops you, don't waste much time before moving the throttle from OFF to 50%. That'll light the main chamber. Bon voyage! X-15 Test Flight: Coming Back ------------------------------ After you light the torch you're supposed to fly whatever you've been briefed for on this particular flight. Don't be surprised if they tell you to do something that's never been done before, and they can't tell you exactly what to expect. In fact, "Flight Characteristics" is the shortest section of the X-15 Flight Manual. On the way out into space the X-15 is briefly an airplane, but it doesn't take long before it's just a rocket. Gross weight drops from about 33,000 pounds to 15,119 pounds as it becomes an artillery shell on a ballistic trajectory. Then it reenters and comes back as the hottest glider in the Mojave desert. Speaking of hot, every other glider in sight is white to keep it cool. The X-15 is black to keep it cool; it'll radiate lots more heat after reentry than it absorbs from insolation. 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. 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. For final glide, as approaching the 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. 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. Landmarks in the pattern 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: 1. To prevent sand and dust at low altitudes from entering the tanks through their vents. 2. To keep the tanks from collapsing. The vents can't keep up with the rapid change in ambient pressure at low altitude. 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: End of flare; airspeed's dropping past 262 knots. 0 seconds: Set it down when airspeed drops to 200 knots and ride until you stop. With no brakes and no steering, you're out of the loop now. Finally, go through the after-landing checklists to secure everything and convince the folks on the ground that you deserve a beer. -END-