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Forty-four years ago this month, the USAF/NASA X-24A lifting body was flown to a speed of 1,036 mph (Mach 1.6) by NASA Research Pilot John Manke. It was the highest speed ever attained by the rocket-powered lifting body.

A lifting body is an unconventional aircraft in that the vehicle generates lift without the benefit of a wing. Rather, the aircraft produces lift by the manner in which its fuselage is shaped.

In the early days of manned spaceflight, there were two schools of thought regarding the preferred mode of entry from orbital flight. One camp favored ballistic entry where the predominant flight force was aerodynamic drag. This was in contradistinction to lifting entry where both aerodynamic lift and drag forces were generated.

Ballistic entry is the more simple approach, but affords little control of the endoatmospheric flight path. This stems from the fact that the landing point for a ballistic entry is largely dictated by the entry vehicle’s velocity and flight path angle at entry interface.

While operationally more complicated, lifting entry provides a positive means for controlling the entry vehicle’s flight path and thus its landing point. At hypersonic speeds, even a small amount of lift markedly enhances entry vehicle downrange and crossrange capability.

Part of the complication of designing a lifting entry vehicle stems from the need to deal with high levels of heating during entry flight. The vehicle’s shape not only dictates its aerodynamic capabilities, but its aerodynamic heating characteristics as well. Thus, issues of flight path control and airframe survivability are interrelated.

The heyday of lifting body flight research spans the period from 1963 to 1975. For the record, the lifting bodies flown in that era include the following vehicles: M2-F1, M2-F2, M2-F3, HL-10, X-24A and X-24B. Each of these aircraft were piloted. All lifting body flight research was conducted at Edwards Air Force Base, California.

The X-24A was developed by the Martin Company under contract to the United States Air Force. A single X-24A was produced. It measured 24.5 feet in length and had a gross weight of 11,450 pounds. Airframe empty weight was 6,300 pounds.

Though unconventional in shape, the X-24A incorporated full 3-axis flight controls. The aircraft was powered by the venerable XLR-11 rocket motor. Consisting of four rocket chambers, the XLR-11 was rated at 8,500 pounds of sea level thrust. Maximum burn time was on the order of 140 seconds. All landings of the squat little ship were conducted deadstick.

The X-24A displayed generally good handling characteristics, but had to be flown precisely. Angle-of-attack had to be maintained between about 4 and 12 degrees. Flight at lower and higher angles-of-attack encountered undesirable aerodynamic control and cross-coupling characteristics.

On Monday, 29 March 1971, X-24A (S/N 66-13551) fell away from the B-52B mothership in an effort to fly a maximum speed mission. NASA research pilot John Manke was at the controls. Manke accelerated the aircraft in a climb and reached a record speed of 1,036 mph (Mach 1.6). Interestingly, it was also John Manke who had previously flown the X-24A lifting body to its highest altitude of 71,407 feet on Tuesday, 27 October 1970.

The X-24A, like all of the lifting body aircraft, contributed significantly to the decision to land the Space Shuttle Orbiter deadstick. The lifting bodies, as well as X-aircraft such as the X-1, X-2, and X-15, proved conclusively that an unpowered aircraft could reliably (1) manage its energy state all the way to landing and (2) precisely control its touchdown point.

The X-24A flew a total of 28 flight research missions. Following its final flight, the X-24A was then converted to the radically different-appearing X-24B configuration which flew 36 times. Today, the X-24B is displayed in a place of honor in the United States Air Force Museum at Wright-Patterson Air Force Base in Dayton, Ohio.

Posted in Aerospace, History

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