Gemini recovery design concepts
During the early ‘60s, project Gemini was tasked to develop and test the technologies that would enable the United States to place humans on the moon … and to get them back safely. Gemini would form the bridge between project Mercury and project Apollo during the space race. Gemini faced engineering challenges that had never been faced before, leading to many design concepts that are considered non-conventional by today’s standards. This was especially true for the entry descent and landing (EDL) system design, for which there was no lack in creativity.
Gliding under the Rogallo wing
Initially, the Gemini capsule was envisioned to land horizontally on solid ground, as the engineers tried to omit the costly logistics of retrieval in the ocean, which was the case for the Mercury capsule. Therefore, a deployable wing was designed, that would allow the capsule to land with the help of an extendable landing gear. The wing was a so-called Rogallo wing, invented by Francis and Gertrude Rogallo in 1948. The Rogallo wing had already been considered for project Mercury, however in the end the design never made the cut.
The Rogallo wing that was considered for Gemini had a frame made out of inflatable tubes, between which fabric sheets were suspended. This allowed the wing to maintain its shape during the flight, while still being able to be stored in a smaller compartment. One of the major advantages of opting for a horizontal landing is that the astronauts would be able to manually control the spacecraft towards a landing strip by manipulating the lines that connect to the Rogallo wing. Such a landing would also require significantly less ground-based support crew and infrastructure compared to a water landing, as was the case for project Mercury.
The development of the Rogallo wing turned out to be more troublesome than anticipated, particularly concerning structural rigidity and the deployment of the wing during flight. Eventually, the concept was fully abandoned in 1964 because major delays in the Gemini program could push back the Apollo launch dates, which was deemed too big of a sacrifice. A combination of several other factors also contributed to the demise of the Rogallo wing on board of Gemini. Instead, Gemini would use a more conventional parachute recovery system with a water landing, similarly to Mercury.[2] This design change made the vehicle 360 kg lighter, allowing more experiments to be carried onboard.[3] Later, during the Apollo project, the Rogallo wing was re-evaluated as a possible landing system, however also abandoned for a ringsail-type parachute.
More gliding parachutes
The Rogallo wing was not the only gliding parachute concept that was investigated for project Gemini. A land landing under other lift-generating parachutes was also envisioned, assisted by a set of rocket thrusters for a soft touch-down. The LeMoigne parasail, Manta-Ray, and sailwing parachutes were all potential candidates that reached the validation phase. Additionally, many tests were performed to investigate the loads on the landing gear at various conditions. Particularly interesting are the tests that aimed to understand the effect of firing landing thrusters either downward or forward to reduce the vertical landing speed. These concepts however never received the same attention as the Rogallo wing for the Gemini capsule, and hence didn’t make it into the final design either. [6,7]
Gemini’s recovery system
Parallel to the development of the Rogallo wing, a more conventional parachute system was being developed for project Gemini, as a backup in case the Rogallo wing development wouldn’t work out. After the cancellation of the latter, this proved to be a good strategy. Eventually all Gemini capsules landed in the ocean, similarly to the previously flown Mercury capsules. Replacing the Rogallo wing with a conventional parachute system resulted in a mass reduction of 360 kg, which was quickly reallocated to more scientific instruments. [3]
After the reentry phase, a 2.53 m wide 20° conical ribbon drogue parachute was deployed in reefed condition to decrease the velocity of the capsule to within the flight envelope of the main parachute. Then, a ringsail-type pilot parachute was used to separate the rendezvous and recovery module from the spacecraft and thereby also deployed the main parachute. A relatively large pilot chute of 5.58 m in diameter was used, as it also served as a main parachute for the aforementioned module after being disreefed. The main capsule itself, together with the astronaut, landed with a single ringsail parachute with a diameter of 25.66 m, which was deployed under reefed conditions. Prior to landing, the capsule transitioned from being suspended by a single attachment point to two attachment points in order to tilt the capsule and thereby decrease the impact loading at splashdown. This was still a new concept that hadn’t been attempted for Project Mercury. [4]
An interesting fact to note is that the parachutes of project Gemini were actuated by the astronauts themselves instead of solely relying on a flight computer. The philosophy of the United States to entrust flight-critical tasks to the astronauts stood in stark contrast with the Soviet Union’s preference to only allow the cosmonauts to carry out as little tasks as possible.
Gemini’s ejection system
The engineers working on Gemini wanted to avoid using a launch escape tower due to its high weight. Therefore Gemini had to be equipped with a personal ejection system for the astronauts, similar to what existed for fighter jets. Gemini’s ejection system was however a very non-conventional one. The ejection system would have to work at high altitudes and at supersonic conditions. At these flight conditions, there was a risk that the astronauts would become unstable upon falling. Therefore each astronaut would be equipped with a ballute that would stabilize him above 2.29 km altitude. When at sufficiently low altitudes, a C-9 flat circular military personnel parachute would be deployed by a rib-less guide surface parachute. [3]
Luckily the emergency ejection system of Gemini never had to be put to use. There were concerns, especially after the Gemini project, that the astronauts would be set ablaze in case of an ejection, as the cockpit was filled with pure oxygen. The ejection system was thoroughly tested under various conditions in wind tunnels, rocket sleds, and drop tests, but never with pure oxygen in the capsule.
Gemini.... With wings
During the early ‘60s, the United Stated Air Force (USAF) was developing a space plane, called Dyna-Soar, that would allow for manned operations in space and controlled reentry. Even though the support towards the Dyna-Soar project was ever decreasing, a small test vehicle was designed to investigate its thermal protection system. These were the ASSET vehicles, which were conically shaped bodies fitted with a delta-wing on their lower side. Eventually the Dyna-Soar project was cancelled because the usefulness of the vehicle was questionable. The results from the ASSET flights would however live on in the form a winged Gemini concept.
In 1966, McDonnel performed a study on the implementation of a delta-wing under the Gemini capsule, based on the results from the ASSET flights. The delta wing and adapter would be separated from the capsule prior to parachute deployment. [5] It is likely that the USAF was involved this study as it could serve as a low-cost alternative to the cancelled Dyna-Soar project.
Eventually the winged Gemini never flew and only remained a concept on paper. Human spaceflight onboard a winged vehicle had to wait until the days of the space shuttle.
Concluding remarks
After project Mercury, there was only very little information on how to bring a man safely back from space, let alone from the Moon. The many engineers that worked on project Gemini had to consider a very wide variety of design options in order to find the most optimal configuration for a reliable and lightweight recovery system. This required much trial and error, leading to numerous design concepts that were abandoned, such as the Rogallo wing or winged Gemini. In the end, project Gemini proved to be crucial to the success of Apollo, by developing the critical technologies that allowed humankind to go to the moon and safely back again.
References
[1] M.J. Mackowski. The Lost Missions of Gemini. 2021. AIAA. Video presentation.
[2] M.J.Ravitzky, S.N.Patel, R.A.Lawrence. To Fall From Space: Parachutes and the Space Program. 1989. AIAA.
[3] H.A.Ray Jr., F.T.Burns. Development and Qualification of the Gemini Escape System. June 1967. NASA. Washington D.C. (USA).
[4] John Vincze. Gemini Spacecraft Parachute Landing System. July 1966. NASA. Washington D.C. (USA).
[5] McDonnell Aircraft Corporation. Winged Gemini. September 1965. Report No. E045.
[6] T.W.Knacke. Steerable Parachutes. September 1969. Symposium for Aerodynamic Deceleration, Braunschweig (Germany).
[7] L.C.Norman, J.E.McCullough, J.C.Cofey. Gemini Land Landing System Development Program. March 1967. Washington D.C.
