US crewed capsules

Mercury was the first US crewed capsule. The capsule was designed for a single person and flew both sub-orbital and orbital flights. The capsule flew on many different rockets such as Little Joe, Big Joe, Redstone and Atlas. The latter two carrying a crew.  As the first two crewed flights were suborbital the re-entry velocity was much lower than in orbital flights. This reduced the requirements for the heat shield. The first two Mercury flights were thus flown with a passive heat shield. For a safe landing, Mercury had a single drogue and a single main parachute. To ensure crew safety in case of a parachute failure, a backup parachute was included. 

The capsule would land in the water where it would wait for the US Navy to retrieve the crew. To dampen the impact of the landing, a skirt was included which was deployed just before landing.  

Some more daring concepts were proposed, that aimed to launch Gemini further than low-Earth orbit. One proposal would use a Gemini spacecraft to explore the potential landing sites for the Apollo missions by performing a lunar fly-by. Another more challenging concept proposed to expand on the existing Gemini designs to use them as the return vehicle for the Apollo program. At some point, even the feasibility of a Gemini mission to Mars was investigated by General Electric, featuring a nuclear-powered space plane, similar in appearance to the Dyna-soar [110]. There have been plenty more Gemini mission proposals that never saw the day of light, however, for the scope of this blog post, only a selection was covered. Because most of these concepts were merely proposals, there is not much detailed information available on the recovery systems that these would have used. It is however very likely that much of the already existing Gemini recovery system would have been used, as most concepts only deviated mildly from the flown capsules.

The entry velocities of Apollo were in the ballpark of 10 km/s, whereas the Gemini re-entry was in the order of 7 km/s. The record for the fastest manned entry would be on Apollo 10 with 11.09km/s. This increase in velocity led to two significant issues. First, there is more velocity to bleed off before the capsule reaches the ground, and second, the thermal loads on the ablative heat shield are much higher and, therefore, would become too heavy [3]. To mitigate this problem, Apollo flew a controlled trajectory, pitching up after atmospheric entry. This led to a horizontal atmospheric leg during the entry flight. Apollo did not pitch up high enough to leave the atmosphere and thus did not fly a skipping entry.

After the entry into the atmosphere was completed, at 7.3 km the forward heatshield would be jettisoned, and at the same time, a 2.2 m pilot chute is deployed from the heatshield to extract it from the wake of the CM. Shortly after, two 5 m diameter conical-ribbon parachutes are deployed by a mortar. These drogue parachutes remained attached until 3.3 km until they were disconnected for the CM, and three 2.2 m pilot chutes were deployed. These pilot chutes extract the three 25.5 m main parachutes. As proven on Apollo 15, even if one of these main parachutes fails, the spacecraft can still land safely [2]. Just before splashdown into the ocean, floatation devices would be deployed to allow the astronauts to wait for the recovery ships.

New methods had to be developed to get all these parachutes stored in the command module in the available volume. This involved packing the parachute in a steel-reinforced container and applying a ram force to get the parachute packed [2].

Parachute system 

The Dragon 2 parachute system is quite similar to the other crewed capsules as it has a drogue parachute and main parachute system. The drogue parachute is a cluster of two parachutes for redundancy reasons. The main parachute is a somewhat unconventional cluster of four main parachutes. Even though it is not the largest parachute cluster, that honour goes to the Kistler K-1 parachute cluster, it is still quite unique. It is unknown why SpaceX went for a four parachute cluster. One of the reasons to increase the number of parachutes of the clusters is if a parachute is already designed and build. However, SpaceX states that the parachute system for the Dragon2 has been completely redesigned from the Dragon capsule as the requirements are different. The only reason to increase the number of parachutes in the cluster then is to decrease the size of a single parachute. This would be mainly a production-related reason.

Testing

The Starliner parachute system has gone through extensive testing, which included system tests, drop tests, and a pad abort test. 

Boeing has performed several drop tests from both helicopters and balloons. These various tests proved the sequence of events required to deploy the parachutes. Helicopter tests primarily showed the operations of the main parachutes as the drop was performed from 3.4 km. The balloon, which could fly up to about 11.5 km, allowed for testing the full parachute sequence including the drogue parachute. 

During the pad abort test, Boeing demonstrated the full sequence of the parachute system. The objective of the test was to show that the launch abort system can safely extract astronauts in case the rocket fails. During the test, it could be seen that one of the main parachutes did not deploy. It was later determined that the parachute was incorrectly rigged and that the pilot chute was not attached to the main parachute due to a missing pin. This meant that, even though the pilot chute was ejected from the mortar tube, the main parachute was not pulled out. One of the philosophies of using a parachute cluster is to add redundancy, meaning that the capsule is still able to land safely under its two parachutes. Therefore, Boeing deemed the test a success.

Landing

The capsule will land in the water and will be retrieved there by the US Navy. This makes the system very comparable to the Apollo landing system. At some point, however, Orion was to touchdown on land. This was to be done using an inflatable landing system.

As the capsule lands in the ocean, no landing system is required. The only additional hardware includes floatation devices to force the capsule to float in the correct orientation. At some point during the development, Orion (then part of the Constellation program) was equipped with an airbag landing system to allow for land landings. The idea was that a land landing would allow for easier reusability for the Orion capsule.

This is very comparable to the Boeing Starliner capsule. It was considered not to have an airbag system on the outside of the capsule but on the inside. These airbags were placed into the astronaut seats. This resulted in an airbag system per person, leading to a lower overall landing system mass.

New Shepard is a reusable US commercial crew vehicle designed by Blue Origin. It is designed to perform suborbital flights carrying research payloads and private astronauts. The booster separates from the capsule 2mins 45sec into the flight after which the capsule passes through the Karman line spending a short period in space before beginning its re-entry. Due to the vehicle’s suborbital trajectory and low apogee, minimal heat shielding is required. At an altitude of roughly 2000m [22], the capsules three drogue parachutes deploy followed shortly by a cluster of 3 main parachutes. Finally, the vehicle fires a retro rocket shortly before touchdown to ensure a soft landing in the West Texas desert. Blue Origin has prioritised safety with this vehicle, ensuring that every system is redundant this includes the parachute system which it claims is capable of safely landing the capsule even if just one of three main parachutes deploys. In this case the vehicles retro rocket that fires just before touch down in order to ensure a safe landing is able to be adjusted to give a higher maximum thrust to compensate for the higher decent rate [23].