The early progress toward US ballistic nosecone recoveries
During the midst of the Cold War in the early ‘70s, the United States Air Force (USAF) had a strong interest in further developing its ballistic missile capabilities. One of the major challenges that had to be tackled, was to gain a better understanding of the aerothermal heating experienced by the ballistic vehicles during re-entry. The US had already proven that it was capable to return vehicles from space in the previous decade, during the Mercury, Gemini, Apollo and unmanned missions, however these ballistic re-entry vehicles were of a different category. The large ballistic coefficient, high speed and steep descent profile caused extreme heat loads, higher than those experienced by any other types of re-entry vehicles, especially when the range (and thus speed) of intercontinental missiles was ever-increasing.
In 1973, the Ballistic Missile Office (BMO) assigned this task to the Advanced Ballistic reEntry Systems (ABRES) directorate, which initiated the Sandia small re-entry vehicle test programs. The first out of two small re-entry vehicle test programs was the ANT program. The ANT program itself performed ballistic flight tests with two types of vehicles. One type of vehicle was purely ballistic (ANT vehicles), while the other included a recovery system. The latter were called NRVs or Nosetip Recovery Vehicles.
A new high-performance recovery system for ANT
The engineers and scientists in the ANT program were looking for ways to measure the heating effects at different moments of the re-entry phase. They decided that the vehicle had to be decelerated very rapidly at a certain point during re-entry, in order to ‘freeze’ the state of the vehicle such that the heating effects up to that point could be observed after recovery. There was however no recovery system in existence that could operate under those harsh conditions … so they made one.
A heavy ballast was included in the vehicle that would be ejected, providing the necessary deceleration. To prevent the ballast from catching up to the vehicle, it would be explosively fragmented right after ejection. At lower altitudes, a drogue and main parachute were deployed to safely land the vehicles in the shallow waters of the Kwajalein atoll in the Pacific Ocean. The main parachute featured a self-inflating flotation bag that helped the capsule stay afloat after splashdown.
After the design, development and thorough testing of this new recovery system, it was ready to be used for the ballistic test flights on Minuteman boosters in 1977. On each rocket, three to four ballistic cones were mounted, of which some of the NRV type (with recovery system) and others of the ANT type (without recovery system). Using multiple vehicles on each rocket was a cost-effective solution, but also allowed for the direct comparison of the heating effects on the nose tips at the same flight conditions. After four flights of the ANT program (including both ANT and NRV vehicles) between 1976 and 1978, it was time for two new ballistic flight test programs.
The successors to the ANT program
The first vehicles launched for the Interim Recovery System (IRS) program were very similar to those of the ANT program, however improvements were made regarding the parachutes of the recovery vehicle. The first launch of an IRS mission was in 1980 and similarly to previous missions, each rocket would carry three to four vehicles, of which some were to be recovered (IRS vehicles) and some were not to be recovered (ANT vehicles). Later, a new version of the IRS was developed: IRS-II. This vehicle featured a set of deployable drag flaps. This modification of the IRS system proved to be a valuable and cost-effective test bed to characterise the hypersonic flow behaviour of these drag flaps. With the data on the drag flap aerodynamics available, a third and final vehicle within the IRS program was developed: the Advanced Recovery System (ARS) . These vehicles proved that the drag flaps can be used reliably on high-speed ballistic missions.
After that the recovery of small re-entry vehicles was proven to be feasible by the Sandia small reentry vehicle test programs (NRV in particular), the ABRES program was created to prove the recovery technology for full-scale ballistic reentry vehicles. Larger and heavier vehicles would be used. The first vehicle was developed parallel to the IRS program, with which it exchanged multiple recovery concepts, for example, the parachute and mass jettison system. It was called the Large Ballistic Reentry Vehicle 1 (LBRV-1) and flew in 1979 on a Minuteman-I booster. Later in 1983, a second LBRV vehicle flew, called LBRV-2. Its recovery system was modified from that of LBRV-1 and incorporated the main parachute of the High-Altitude Diagnostics (HAD) rocket system. Both missions encountered partial failures during their flights, however both vehicles could be retrieved from the shallow depths of the Kwajamein Atoll, allowing the engineers to gain information regarding the failures.
After the success of the ANT program, the ABRES directorate went onwards with two projects to further increase its capabilities of ballistic vehicle recovery. The first was called the Interim Recovery System (IRS) program and was the second of the Sandia small reentry vehicle test programs, after ANT. The second new project was the Large Ballistic Reentry Vehicle (LBRV) program, which was not part of the Sandia small reentry vehicle test program, in contrast to IRS and ANT.
In retrospect
The ABRES recovery programs like ANT, IRS and LBRV allowed the US to gain experimental data from flight conditions that could not reliably be simulated nor tested on the ground. The operational envelopes of these recovery systems were amongst the most extreme ones of all re-entry vehicles that ever flew. These vehicles would often fly faster than 4000 m/s at altitudes lower than 8 km, resulting in dynamic pressures up to 7700 kPa and g loads well above 100 g.
Although multiple missions encountered failures, they all proved to be extremely valuable for the engineers and scientists that worked on the program. It created a solid foundation for the understanding of aerothermal heating for more recent re-entry and hypersonic vehicles. After the LBRV and IRS missions, ABRES continued to test high-speed test vehicles and technology demonstrators, as it had done before.
