Ballistic missiles

Within the ANT program there was a distinction between non-recovered and recovered vehicles, of which the latter was referred to as the Nose Recovery Vehicles (NRV). These were equipped with a two-stage parachute system that allowed for physical retrieval of the flight data on board of the vehicle after a soft splashdown in the ocean. [81]

In order to decelerate the vehicles as quick as possible, a heavy ballast was jettisoned at a predetermined time, such that the nose tip of the vehicle would not be further affected by aerothermal heating.  This ballast was detonated milliseconds after jettison to prevent a collision with the vehicle. After this initial deceleration, a 0.29 m diameter nylon ribbon drogue chute was deployed, followed by a 0.66 m diameter rib-less guide main parachute. At the crown of the main parachute, a self-inflating flotation bag was attached to keep the vehicle afloat after splashdown. [81]

The NRV recovery system was tested and qualified using numerous testing methods. Subsonic wind tunnel testing of both drogue, main and extraction lid were performed to characterise their drag performance. The main parachute was tow-tested behind a truck to quantify and validate the filling rates of the flotation bag. Subsequently, the vehicle was repeatedly drop tested in water to validate the functionality of this flotation system. Twelve trans- and supersonic rocket sled tests were used to test the supersonic deployment sequence of the entire system at high dynamic pressures. Finally, the recovery system was flight-qualified after being launched to the relevant entry conditions using a Sandia STRYPI VII rocket in 1975. [82]

The Interim Recovery System (IRS) was developed to recover ballistic missile nosecones. For this They created a system consisting of three stages of which two are parachutes[83,84]. The recovery system was designed to start at 6700 m after the nosecone has gone through the re-entry phase[83].

The system starts by first separating about 63% of the vehicle mass thereby increasing its ballistic coefficient [83]. After this, a two-stage parachute system is deployed. Starting with a 0.48 m ribbon parachute made out of Kevlar[83,84]. The parachute is made out of Kevlar to handle the deployment conditions. The target conditions for the drogue parachute are going supersonic with a maximum dynamic pressure of 431kPa [83]. This drogue parachute had a packed weight of 1.0 kg. Following the drogue parachute, a 0.91 m guided surface parachute is deployed with a flotation device at the top. Landing with a target velocity of 37 m/s. The main parachute had a packed weight of 0.91 kg[83].

The IRS system was tested in multiple ways. First in a supersonic wind tunnel, these test went to Mach 2.5 with a maximum dynamic pressure of 72kPa. After this five captive-sled test were performed. This meant that the nosecone was attached to a sled which reached a maximum velocity of 213 m/s. Finally ten free-flight tests were performed where the nosecone were ejected upward resulting in a parachute deployment at Mach 2.5 with a dynamic pressure of 339 kPa.

The LBRV-2 vehicle was a 59 kg conical vehicle with a base diameter of only 54.6 cm. It flew in 1983 onboard on a minuteman rocket and was recovered 8000 km downrange at the Kwajalein atoll. The LBRV-2 recovery system was different from that of LBRV-1. It featured a Kevlar 1.96 m wide 20° conical ribbon parachute, of which the design was based on that of the High Altitude Diagnostics (HAD) rocket. This main parachute was extracted by a Kevlar 0.41 m diameter flat ribbon parachute. Prior to parachute deployment, a ballast mass would be jettisoned from the vehicle to provide an initial deceleration at high velocities where the parachute can’t operate [87].

The flight of LBRV-2 went largely successful, although some issues were encountered after the deployment of the parachute. Because the parachute attachment point was located upstream of the vehicle’s centre of gravity, the vehicle was experiencing a rolling and coning motion. This caused one of the bridle lines to get stuck behind a guide rail and made the vehicle even more unstable. The vehicle splashed down in a lagoon under a very undesirable angle, causing it to break into two parts. Due to the rough landing, the parachute canister also sheared off. Eventually, both the parachute and the test vehicle were successfully retrieved from the shallow depths of the lagoon at Kwajalein Atoll. [87]

During the late ‘70s and 80’s, there was an interest among the US Air Force to understand the aerothermal flow phenomena that occur during re-entry and at hypersonic speeds. In order to validate the thermal design of the existing ballistic re-entry vehicles, a series of flight test programs were initiated: the Sandia small re-entry vehicle flight test programs. A novel recovery system had to be designed to retrieve the test vehicles such that they could be examined. The Nose Recovery Vehicle (NRV) and Interim Recovery System (IRS) programs concluded in the successful development of a recovery system capable of recovering re-entry vehicles with extremely large ballistic coefficients, by using a mass-jettison system and two parachutes, including a flotation device. [88]

The IRS-II missions were the successors of the IRS flights. The vehicle and operations were identical to those of the IRS missions, with the exception of a set of drag flaps that were added. These drag flaps are initially opened to 23° to double the vehicle’s drag. Thereafter, 63% of the vehicle’s mass was jettisoned at high velocity from the back of the vehicle to decrease its ballistic coefficient. 0.08 seconds later, the ejected mass is explosively fragmented to prevent a collision with the vehicle. The drag flaps were extended further to 45°, after which a Kevlar, 0.48 m wide, 20° conical ribbon drogue parachute was deployed. A guide-surface parachute will eventually land the capsule safely at 30 m/s in the ocean. Flotation of the capsule is guaranteed by a ram-air inflated bag attached to the top of top of the parachute.  The recovery sequence and timing can be seen in figure on the right. [88]

The Advanced Recovery System (ARS) concept, which uses a deployable drag flap to decelerate ballistic nose cones, was proven during the IRS-II program. The flights of ARS used a similar vehicle and recovery architecture compared to IRS-II, however at different flight conditions. The vehicle architecture can be seen in the figure on the right. [88]