HAP Sidewinder seeker & warhead mated to a Sparrow motor so they could shoot down some pesky middle east hi-altitude MIGS that could not be downed by Phantoms with standard Sidewinders.
What's really incredible is that Sidewinder was created by a group of guys, but one guy in particular, William McLean, at NWS China Lake. McLean was one of those iconoclasts who didn't just do his government job. He ran what was called at the time "McLean's hobby shop." This was a lab doing all sorts of off-the-books projects that McLean and a few other engineers wanted to do.
One of those was inventing the Sidewinder missile. What's truly amazing is he did it without a formal government contract or funding!
The result was the world's first infra-red homing, guided missile. It was a masterpiece of simplicity. Using just 17 tubes, without complex stability circuits--thanks to the invention of the rolleron, and self-guiding / fire and forget, it was was a world beater of a missile design.
The utter simplicity of the Sidewinder is almost mind boggling.
I can truly understand and appreciate how Sidewinder came about despite official reluctance and even disapproval from my own naval career. It is because of guys like McLean that great things get done.
AIM-9L found its way into another China Lake development effort of the early 1970s known as HAP. The project was undertaken in response to the Soviet Union’s remarkable MiG-25 Foxbat aircraft. The MiG-25 could fly higher—far higher—than any fighter aircraft in the U.S. inventory. In fact, the MiG-25 still holds the Federation Aeronautique Internationale (FAI) absolute world altitude record for a ground-launched manned aircraft. The U.S. and its allies were concerned about MiG-25s that were flying reconnaissance missions over Israel in the run-up to the Arab-Israeli War of1973, and the proposal was made for an air-to-air missile that would be capable of reaching the MiG at altitude. Mike Ripley-Lotee, who worked on HAP, recalled:
"Some guys at China Lake had come up with a concept of taking the brand new Sparrow rocket motor that hadn’t even entered service yet for the AIM-7F and matching it with a Sidewinder front end with the new -9L canards, the big canards, and then some guidance algorithms to allow it to take advantage of the new, improved sensitivity of the seeker for Sidewinder. . . . The whole concept was that an F-4 would have one of these things on a rack and zoom up to about 55,000 feet where it was just flaming out and be in front of the MiG-25, which would be going at Mach 3 or 4, nice and hot, a good signature, get tone from the Sidewinder front end, and fire this thing, and it would have the oomph to engage the MiG-25 up at 70 or 80,000 feet."
The HAP missile also contained components from the ACV, the aerodynamic version of Agile, a program soon to be cancelled. A 1973 technical note on HAP by China Lake engineer Jim Irvine noted that eight missiles were built; six were tested with telemetry warheads, and the remaining two “were built up in a tactical configuration with live warheads.” In an interview decades later, Irvine recalled of the two tactical-configuration missiles that “we put them in the magazines and we left them there,” until, in the late 1970s, “If inally had the missiles torn down.”
The AIM-9L High Altitude Project was a specialized modification of the AIM-9L Sidewinder air-to-air missile, aimed at improving its effectiveness in high-altitude engagements.
The AIM-9L Sidewinder was introduced in the late 1970s as a fourth-generation infrared-guided air-to-air missile. It was a major upgrade over previous variants, featuring: 1-All-aspect infrared seeker (capable of engaging targets from the front rather than just rear-aspect heat signatures). 2-Improved maneuverability for close-range dogfights. 3-Better counter-countermeasures to resist enemy flares and electronic jamming.
However, the missile’s performance at high altitudes (above 50,000 ft / 15,000 m) was not optimal due to several aerodynamic and propulsion challenges. This led to the AIM-9L High Altitude Project, an effort to improve its effectiveness in upper-atmosphere combat.
Why the High-Altitude Upgrade?
By the late 1970s and early 1980s, high-altitude air combat was becoming more critical due to advancements in aircraft performance and missile technology. Several factors contributed to the need for a high-altitude optimized Sidewinder:
Emerging High-Altitude Threats
The Cold War saw Soviet aircraft such as the MiG-25 Foxbat and later MiG-31 Foxhound, which operated at extreme altitudes (60,000+ ft) and speeds over Mach 2.5.
Western air forces needed a high-altitude capable missile to counter these threats effectively.
Limitations of Standard AIM-9L at High Altitude
Aerodynamic Control Issues: At high altitudes, air density is lower, reducing the missile's ability to generate lift and control authority.
Rocket Motor Efficiency: Conventional motors burn less efficiently in thinner air, reducing thrust and overall range.
Seeker Performance: Background infrared interference from the sun and atmospheric effects could degrade target acquisition.
New Fighter Aircraft Capabilities
Aircraft such as the F-15 Eagle and F-14 Tomcat were designed for high-altitude superiority but needed missiles that could match their operational ceilings.
The standard AIM-7 Sparrow (semi-active radar-guided missile) was used for beyond-visual-range (BVR) combat but was less effective at shorter ranges.
A high-altitude optimized AIM-9L could fill the gap between BVR engagements and close-range dogfights
Key Modifications in the AIM-9L High Altitude Project (HAP)
The AIM-9L High Altitude Project (HAP) focused on overcoming the challenges posed by thin air, reduced maneuverability, and seeker interference at high altitudes (above 50,000 feet / 15,000 meters). To achieve this, several modifications were made in three key areas: seeker performance, propulsion, and aerodynamics.
1. Seeker System Enhancements
One of the biggest challenges of high-altitude combat was background infrared (IR) interference from the sun, as well as weaker target signatures due to thinner air and reduced afterburner plumes. The modifications included:
Improved IR Seeker Sensitivity
The standard AIM-9L used an all-aspect infrared seeker, but at high altitudes, targets emit less infrared radiation due to the colder environment.
The HAP variant received advanced signal processing algorithms to enhance its ability to detect low-signature targets.
It was optimized to lock onto targets with weaker heat sources, such as aircraft in supercruise mode (which emit less heat than afterburning aircraft).
Reduced Background Interference from the Sun
At high altitudes, the intensity of IR radiation from the sun is much stronger, which could cause the seeker to struggle with false detections.
The HAP seeker was modified to filter out high-intensity background IR while maintaining sensitivity to aircraft heat signatures.
Better Counter-Countermeasure Capabilities
Standard IR missiles could be distracted by decoy flares at high altitudes due to reduced air turbulence.
The HAP seeker included improved flare rejection algorithms, allowing it to ignore countermeasures more effectively.
2. Propulsion & Thrust Enhancements
At high altitudes, air is significantly thinner, which affects the efficiency of rocket motors. The modifications in propulsion focused on:
Optimized Rocket Motor Burn Profile
The standard AIM-9L’s Hercules MK 36 rocket motor was designed for low and medium-altitude engagements.
At high altitudes, fuel burns less efficiently, and thrust generation can be inconsistent.
The HAP variant’s motor was adjusted to provide better thrust performance in low-density air, potentially through:
Modified fuel composition for improved burn rates.
Longer burn duration to sustain velocity in thinner air.
Sustained Energy for Maneuvering
At high altitudes, a missile loses energy more quickly when maneuvering due to lower air resistance (which normally helps control surfaces work).
The modified propulsion system was designed to retain more kinetic energy, improving sustained turning capability against maneuvering targets.
3. Aerodynamic Adjustments for High-Altitude Control
At high altitudes, the low air density reduces aerodynamic effectiveness, making it harder for control surfaces to generate lift and turn the missile. The modifications included:
Refined Fin & Control Surface Design
The AIM-9L already had double-delta canards for maneuverability, but in thin air, their effectiveness was reduced.
The HAP version have included slightly enlarged or reshaped canards to improve control at high altitudes.
The modifications ensured that the missile could still turn sharply without excessive energy loss.
Thrust Vectoring Considerations
While not confirmed, some experimental AIM-9 versions tested thrust vectoring nozzles for better maneuverability in low-density air.
If applied in the HAP variant, this would have allowed better directional control, even when aerodynamic surfaces were less effective.
Final Conclusion: Why AIM-9L HAP Was Discontinued
The AIM-9L HAP project was ultimately phased out due to: The AIM-9M incorporating most of its improvements. A shift toward BVR combat using radar-guided missiles. Limited test results and lack of operational necessity. The eventual development of AIM-9X, which outperformed it in all aspects.
While never mass-produced, the lessons from HAP contributed to later Sidewinder designs and helped shape the future of high-altitude missile combat.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
![download_(2)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299677-d5b21de1b3f9677336c5b34eb88f681a.jpg "download_(2)[1].jpeg")
![download[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299820-e130d0442d7678b7011dfc57d277f0df.jpg "download[1].jpeg")
![images_(9)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299824-32d853c7ec7ae6cd26e177f2ff23dd95.jpg "images_(9)[1].jpeg")
![images_(7)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299810-0875c18d9d8eef450bc17373f37914c6.jpg "images_(7)[1].jpeg")
![download_(4)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299816-3e0d79cb8e07804e20432d80910e3e1f.jpg "download_(4)[1].jpeg")
![images_(8)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299819-607f17c86c16637d520ed0676005f620.jpg "images_(8)[1].jpeg")
![images_(10)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299829-759092466828a43b71fb1844354d356b.jpg "images_(10)[1].jpeg")
![images_(11)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299833-796629fad4947d93bfafd71632036519.jpg "images_(11)[1].jpeg")
![images_(14)[1].jpeg](https://www.secretprojects.co.uk/data/attachments/299/299835-36f76cdc2821976678f7cb2d39504988.jpg "images_(14)[1].jpeg")
![images[1].png](https://www.secretprojects.co.uk/data/attachments/299/299836-15f121a1f599b1f5e5fed00282905671.jpg "images[1].png")
The AIM-9M incorporating most of its improvements.
A shift toward BVR combat using radar-guided missiles.
Limited test results and lack of operational necessity.
The eventual development of AIM-9X, which outperformed it in all aspects.![download_(1)[1].jpeg](/data/attachments/299/299832-54881312bd9c13535d5c5dc6f870106c.jpg)
