Figure 10
With the post–air refueling checklist completed, it was time to start the accel to Mach 3+. There were two methods to get the SR-71 through the transonic speed regime and begin the accel. The method used the least often was a level acceleration at twenty-five thousand feet to intercept the supersonic climb schedule.
The favored method was called the “dipsy doodle” and consisted of a climb and descending acceleration to intercept the SR-71’s supersonic climb schedule. It required less fuel to intercept the climb schedule than did the level acceleration, particularly when outside temperatures were warmer than standard.
The following checklist was used for the supersonic accel:
1. Throttles—AB
The pilot advanced the throttles to min-AB and stated, “Throttles min-AB.”
2. Throttles—Max-AB
To start the dipsy doodle, the pilot advanced the throttles to min-AB and began a shallow climb at a constant Mach 0.9. Passing through thirty thousand feet, the pilot firewalled the throttles to max-AB and said, “Max-AB.” At thirty-three thousand feet, he let the speed increase to at least Mach 0.95 and slowly lowered the nose to establish a 2,500- to 3,000-foot-per-minute rate of descent. It was important to exceed Mach 1.05 early in the descent. There’s a transonic region of high drag on the aircraft starting just below Mach 0.95 and up to Mach 1.05. The faster the plane accelerated through this region, the better the fuel consumption.
Going through the sound barrier at Mach 1.0 in the shallow descent was rather benign. The needles of the pitot-static instruments (airspeed, altimeter, and vertical velocity) gave a slight jiggle passing through Mach 1.0, stabilizing quickly. Other than seeing those three needles momentarily jump, there were no other sensations of passing through the sound barrier—no noise, no vibration, nothing!
Continuing to accelerate in the gradual descent, the pilot focused on his rate of descent and how rapidly the KEAS were increasing. He began slowly pulling out of the shallow supersonic descent at around 435 KEAS and smoothly transitioned into a supersonic climb at 450 KEAS. From that point on, all speeds were referenced by either Mach number or KEAS.
Once the airspeed was stabilized at 450 KEAS in the climb, the pilot engaged the KEAS hold function on the autopilot. The autopilot then held the SR-71 at a constant 450 KEAS climb until reaching Mach 2.6. If the crew had not already engaged the automatic ANS feature on the autopilot, this was the time to do so. The pilot was able to devote his attention to more important cockpit indications, switches, and gauges by having the autopilot on.
3. Inlet parameters—Monitor
The forward bypass doors were a rotating band of ports located a short distance aft of the inlet throat. They were located around the circumference of each inlet, at the top and bottom, acting as overboard exhausts for inlet air not required by the engine. Passing through Mach 1.4, the forward bypass doors began to open according to inlet pressures. At Mach 1.6, the spikes unlocked and began slowly moving aft into the inlet. The inlet spike position, forward bypass door opening, and CIP gauges were monitored closely by the pilot throughout the accel. The pilot stated, “Inlet parameters monitoring.”
The checklist contained a chart that cross-referenced several parameters during the climb. For any given Mach number, there was a specific spike position, altitude, and CIP associated with it (see Appendix D, page 201). At Mach 1.7, for example, both spike needles should read two inches aft, the altitude should be around forty-three thousand feet, and the CIP needles should be reading 9.0 psi. By comparing the actual cockpit readings with the checklist’s ideal values, the pilot had a good indication of how well (or poorly) the inlets were performing.
The forward bypass doors modulated from fully closed to one hundred percent open. They were controlled by the air inlet computer (AIC) that measured the duct pressure ratio (DPR) inside each inlet. The DPR was a comparison of air pressure on the outer surface of the engine inlet to air pressure inside the inlet. As the DPR increased, the forward bypass doors modulated open as necessary to relieve excess pressure building up inside the inlet. At Mach 1.4, the forward bypass doors barely opened. However, by the time the aircraft reached Mach 1.7, they were open as much as fifteen to twenty percent. A comparison would be like trying to drink water from a fire hose with the excess water spilling everywhere. That was precisely the purpose of the forward bypass doors on the SR-71, except it was air in excess of the engine’s need going overboard rather than water.
The purpose of each inlet spike was to control and position the supersonic airflow inside the throat of the inlet for optimum performance and to prevent supersonic air from entering the engine. One axiom true for all aircraft flying supersonic with turbojet engines is that supersonic airflow and its corresponding shock wave must not reach the engine’s compressor. If supersonic airflow did reach an engine’s compressor, it would flame out the engine. Supersonic aircraft utilize various means of stopping supersonic air from approaching the compressor. In the F-4 fighter, a moveable ramp at the front of the inlet prevented the supersonic airflow from entering the inlet.
In the case of the SR-71 cruising, the entering Mach 3.0 air had to slow down to Mach 0.6 before reaching the compressor. That would roughly be the equivalent of slowing air down from 2,100 miles per hour to 600 miles per hour in a distance of around twenty feet. This creates a tremendous amount of pressure and heat inside the inlet—pressure plus heat equals energy.
Starting at Mach 1.6, each spike moved aft one and five-eighths inch per 0.1 Mach the plane accelerated, up to a maximum travel of twenty-six inches aft. The spikes were hydraulically actuated and had to be able to withstand air pressure exceeding fifteen tons under certain airflow conditions. The easiest way to understand how the inlet spikes and forward bypass doors operated is to consider the spikes as dumb (they moved fore and aft strictly on a Mach schedule) and the doors as smart (they reacted to the existing DPR and were computer controlled).
4. Aft bypass—Set (A)
There was an inherent problem in having the forward bypass doors open more than necessary. Excess air exhausted overboard created a tremendous amount of drag on the aircraft as the slower, exiting air hit the supersonic airstream. In some cases, the induced drag was significant enough that the mission had to be aborted because of the excessive fuel consumption required to overcome the increased drag. To optimize performance, the AIC controlling the forward bypass doors was programmed to run on a relatively tight schedule, keeping the door closed down as much as possible without creating inlet problems. Crews referred to this as the inlet “schedule.” On some SR-71s, the forward bypass doors were scheduled tighter than on others.
At this point in the accel, it was necessary for the forward bypass doors to bleed overboard a large amount of air, which tended to keep the doors fifteen to twenty percent open—way too far for efficient operation. Each inlet system included a second set of doors, called the aft bypass doors, to help close down the forward bypass doors. Positioning the aft bypass doors was a manual operation by the pilot; there were no automatic controls. Located just outboard of the throttles were individual left and right aft bypass control switches, with fixed positions of either CLOSE (shut), A (fifteen percent open), B (fifty percent open), or OPEN (one hundred percent open). In future discussions, pay close attention to whether the reference is to the forward or aft bypass doors.
This may sound confusing at first. The aft and forward bypass door results worked in opposite directions to each other. If you manually opened the aft bypass doors, the forward bypass doors would tighten down; if you manually closed down the aft bypass doors, the forward bypass doors would open up. If you think in terms of maintaining a constant air pressure inside the inlet, opening up one door (to let air escape) would allow the other door to close down. The advantage was that opening the aft bypass doors to exhaust excess air did not create any drag. The aft bypass doors kept the excess air inside the inlet, surrounding the exterior of the engine, cooling it down, and exiting the air in front of the afterburners for additional thrust.
As the aircraft reached Mach 1.7, the pilot manually opened the aft bypass doors from CLOSE to A position, allowing the forward bypass doors to close down more, and stated, “Aft bypass set A.”
5. CG & trim—Monitor
This was a reminder for the pilot to continue to monitor the CG as well as the pitch trim of the aircraft. During the accel, the automatic fuel tank sequencing slowly moved the CG aft. As another means of cross-checking that the CG gauge was reading correctly, the pilot monitored the pitch trim indicator. Since the pitch trim worked automatically to keep the aircraft in trim with the autopilot engaged, any unusual pitch trim readings were a clear indication of an erroneous CG indicator. The RSO could always use his circular slide rule to determine the actual CG location, assuming each of the individual tank readings was accurate. The pilot confirmed, “CG and trim monitoring.”
6. IGV switches—LOCKOUT
The inlet guide vanes shifted automatically from the axial position to the cambered position between a CIT of 85 to 115 degrees Celsius (approximately Mach 1.7–2.3) and had to be shifted by 150 degrees Celsius. As each engine’s IGVs shifted independently, a noticeable yawing motion of the aircraft took place until both engines shifted to the cambered position. It was at that point that hot temp devs really slowed the accel down, requiring more fuel. After both IGVs shifted, the pilot placed the switches to LOCKOUT, preventing the IGVs from shifting back to axial until required later on in the descent. The pilot stated, “IGVs locked out.”
7. Aft bypass—Set (B)
When each IGV shifted, the respective forward bypass door opened to twenty percent or more. The resulting drag required the pilot to place the aft bypass door in the B position (fifty percent open). That normally closed the forward bypass door to around ten percent open. For the remainder of the acceleration, the forward doors would slowly but surely close down. The pilot kept a close eye on the spike positions, door positions, CIPs, and Mach number and compared them to known values to determine how the plane was performing and if inlet problems were developing.
8. Exterior lights—Off
The SR-71 was now high enough that there was no need for exterior lights. The pilot turned them off and said, “Exterior lights off.”
9. Pitot heat—OFF
The pitot tube was hot enough. The pilot turned it off and stated, “Pitot heat off.”
10. Above FL 500, DEF systems—Set
The RSO operated the DEF systems as briefed and said, “DEF systems set.”
11. Radar—As briefed
The ASARS radar was set as briefed. The RSO stated, “Radar ON/OFF.”
12. IFF—Mode C OUT
The RSO placed the Mode C switch in the OUT position, preventing automatic altitude reporting from the transponder, and said, “IFF Mode C out.” As far as air-traffic control was concerned, the plane simply evaporated at FL 600.
13. Aft bypass—Set (A)
The SR-71 was now around sixty-one thousand feet. The pilot did not automatically move the aft bypass switch into the A position (fifteen percent open) but monitored the forward bypass doors. When the forward bypass doors approached closed, he moved the respective aft bypass switch to the A position and said, “Aft bypass set A.”
14. KEAS bleed—Monitor
In a few minutes, the aircraft would level off at Mach 3.0. At Mach 3.0, the maximum airspeed limit was 450 KEAS, and the aircraft was already climbing at 450 KEAS. The pilot needed to gradually reduce the KEAS to be well under the limit. Starting at Mach 2.6, the constant 450 KEAS climb speed the autopilot had been holding began to bleed down automatically by the autopilot. The bleed schedule called for a ten KEAS decrease for each 0.1 Mach number increase above Mach 2.6. The pilot confirmed, “KEAS bleed monitoring.”
The RSO updated the INS altitude to the mid-leg altitude, between initial level-off and the start of the descent, and stated, “INS altitude updated.” Anticipating the level-off, the pilot consulted his checklist cruise charts (Appendix D, Page 201) for the Mach 3.0 maximum range profile with fifty thousand pounds of fuel remaining and found the following parameters: altitude seventy-one thousand feet, 410 KEAS, fuel flow for each engine around twenty-one thousand pounds per hour. Approaching Mach 2.97, the pilot disengaged the KEAS hold function of the autopilot and slowly lowered the nose of the plane by rotating the pitch wheel forward with his index finger, leveling off close to seventy-one thousand feet. Simultaneously, he retarded the throttles until each fuel-flow gauge read around twenty-one thousand pounds per hour to hold Mach 3.0; this was a good starting point.
The pilot made very small corrections with the pitch wheel and throttles to hold the altitude and Mach. The CIT gauge was the pilot’s only indication of whether the outside temperatures were hotter or colder than standard. If he saw 328 degrees Celsius on the gauge at Mach 3.0, he knew it was a standard day temperature of -56.5 degrees Celsius outside; if it was hotter than standard, he knew it would cost him fuel. The ANS continued to navigate the SR-71 around the route.
16. Aft bypass—Set
The pilot now had a choice to make. He had to decide whether to leave the aft bypass doors in the A position or move them to the CLOSE position. He evaluated how the forward bypass doors were scheduling at Mach 3.0 cruise. If they were too tight for his comfort level, he placed the aft bypass in the CLOSE position. If the forward bypass doors were too far open for good fuel economy, he placed the aft bypass doors in the A position. Talking to other crews who recently flew the same plane helped the pilot get an idea if the doors were scheduled to run loose or tight. Unfortunately, the SR-71 often had a mind of its own!
17. Aft bypass—CLOSE
Any time the SR-71 was at or above Mach 3.05, the aft bypass was always placed in the CLOSE position, and the pilot confirmed, “Aft bypass close.”