Since July 24 is the 48th anniversary of the return from space of Apollo 11, here is a short account the return via parachutes of the Apollo spacecraft.
For all of the earlier work on the Gemini Earth landing system in the mid-1960s—paraglider, parasail, or parachute—virtually nothing about the Apollo program revolved around its Earth landing system. Gone were any extravagant efforts to conclude a land landing; the Apollo astronauts would be rescued at sea after a parachute landing. The Apollo familiarization manual described the system used for this recovery as follows:
The C/M-ELS begins operation upon descending to approximately 24, 000 feet +0.4 second, or in the event of an abort, 0.4 second after launch escape assembly jettison….The apex cover (forward heat shield) is jettisoned by four gas-pressure thrusters. This function is imperative, as the forward heat shield covers and protects the ELS parachutes up to this time. At 1.6 seconds later, the drogue mortar pyrotechnic cartridges are fired to deploy two drogue parachutes in a reefed condition. After 8 seconds, the reefing lines are severed by reefing line cutters and the drogue parachutes are fully opened. These stabilize the C/M in a blunt-end-forward attitude and provide deceleration. At approximately 10, 000 feet, drogue parachutes are released, and the three pilot parachute mortars are fired. This action ejects the pilot parachutes which extract and deploy the three main parachutes….The main parachutes are disconnected following impact. The recovery aids consists of an uprighting system, swimmers umbilical, sea (dye) marker, a flashing beacon light, a VHF recovery beacon transmitter, a VHF transceiver, and an H-F transceiver.
The spacecraft would reach the water landing in the Pacific—for all of the lunar missions—at a velocity of “33 feet per second at 5,000 feet altitude for a normal or abort landing.”
To develop the Apollo landing system NASA contracted with the North American Rockwell Corp. Building on knowledge gained in the Gemini and Mercury parachute landing systems, North American Rockwell undertook a rigorous and extensive design and testing regimen. As Northrop engineer Theodor W. Knacke reported in 1968:
Numerous interesting design details are contained in the Apollo parachute system. The reliability requirement of independent parachute deployment, coupled with large command module oscillations, necessitates divergent drogue parachute and main pilot parachute deployment angles coupled with positive thruster type deployment. The command module oscillations create the possibility of contact between the parachute risers and the hot rear heat shield, and last but not least, the increase in CM weight without an accompanying increase in compartment volume or allowable parachute cluster loads resulted in novel design approaches for parachute packing, storage and shape retention.
Designed for use in both optimum and crisis situations either during launch abort of return from the Moon, this system was fully redundant and handled forces equivalent to 3 g’s without difficulty.
Not all went well with every aspect of the Apollo parachute recovery system. A number of tests failed during the run-up to the missions to the Moon. For example, on September 6, 1963, an Apollo command module boilerplate, No. 3, was destroyed when one pilot parachute was cut by contact with the vehicle and one of its main parachutes did not deploy. Then rigging problems caused the other two parachutes to fail. An investigation led to rigging and design changes on future systems. As in this case, these difficulties were resolved and the program continued.
The most serious operational failure came during the descent of Apollo 15 from the Moon in 1971. During its reentry, all three main parachutes deployed without incident at an altitude of 10,000 feet, but one of the three parachutes deflated while the Apollo 15 capsule was obscured by clouds between 7,000 and 6,000 feet. Regardless, crew and the spacecraft returned safely because redundancy in the system allowed one chute to fail without adverse affect although with slightly higher velocity at impact. Failure analysis found that the parachute lines had been damaged by fuel from the reaction control system (RCS) during return, a normal occurrence but in this instance the parachute assembly was in the way of the RCS ejection ports. The Apollo mission summary reported only: “During the descent, one of the three main parachutes failed, but a safe landing was made.”
As reported at the time: “The most probably cause of the anomaly was the burning of raw fuel (monomethyl hydrazine) being expelled during the latter portion of the depletion firing and this resulted in exceeding the parachute-riser and suspension-line temperature limits.” Based on this anomaly and its occurrence, only once in all of the missions to date, NASA investigators believed that there was only a 1 in 17,000 chance of failure on future missions.
This basic approach worked well throughout Apollo, but NASA engineers still wanted to have a parasail landing system similar to that pursued but not deployed for Gemini. Personnel at the Landing Technology Branch of NASA’s Manned Spacecraft Center tried to adapt the parasail landing system to the Apollo Application Program, the follow-on to the Moon landings. The branch reported that it
expects to have a system that will be adaptable to Apollo. Their present effort is not aimed directly at incorporation of such a system, but rather at developing the technology and hardware necessary for the system itself. They are, however, basing their designs on a spacecraft that is of the CM size and type. The resulting system will most likely consist of a steerable parachute, plus some combination of landing rockets, deployable energy absorbers and stability aids.
Its staff added that it had contracted with four organizations for various aspects of this effort:
- Bendix Products Aerospace Division is developing a computer program to analyze land-landing dynamics.
- North American Aviation is investigating landing gear systems for the Command Module.
- Pioneer Parachute is developing a parasail type of steerable parachute.
- Northrop Ventura is developing a cloverleaf type of steerable parachute.
The primary concern was that this landing system handle a 14,000 pound capsule and be containable within a 1.5 cubic meter space. While this system was considered even less heavy and bulky than a water system, the addition of landing rockets to cushion a landing might push total weight above that already envisioned for the Apollo command module. “Probable weight increase and cost of incorporation must be weighed against the added capability and decrease in cost of recovery operations,” the study concluded.
This statement was the first reference in this recovery literature from the 1960s concerning the very important trade engineers had to make between added weight and reduced recovery operational cost. The Navy was generally quite agreeable during the space race era to deploy their ships for recovery, and NASA was not required to pay for that operation. That made water recovery, at least from NASA’s perspective, not only the most expedient but also the lowest cost method of recovery. Even so, this program concluded without adopting anything more sophisticated than the parachute system used for Mercury, Gemini, and Apollo. It would not be until a return to canopies for some projects in the 1990s that NASA returned to the parasail/paraglider concept for landing.