Re-entry Technology Demonstrators
Several missions have been flown with the goal of validating re-entry simulations and flight testing thermal protection systems. These are listed on this page.
RVX-1
Operator: USAF
Mission: Re-Entry technology demonstrator for ablative materials
Flight: 1959
Status: Completed
Testing for: -
During the ‘50s in the United States, ablative materials were identified to offer great potential for thermal protection systems of reentry vehicles. After the success of the Mark-2 reentry vehicle, which featured a simpler and heavier heat sink thermal protection system, it was time for more ablative materials to be put to the test. The RVX-1 program had the purpose of maturing and validating the ablative protection technologies, as well as the evaluation of a newly developed guidance system to more accurately determine the vehicle’s impact point [181].
General Electric was responsible for the RVX-1 vehicle design, however, two versions existed with each their own thermal protection system: one version from General Electric and the other from Avco. Both vehicles were cylindrical in shape with a blunt spherical nosecone and a flared rear section. The former was 1.70 meters long, 0.51 meters in diameter and weighed 293 kg, while the Avco version was 1.72 meters long and 0.71 meters in diameter. On the General Electric vehicle, three phenolic nylon materials were applied to the cylindrical and flared section. These three ablators were used alternatingly along the perimeter of the vehicle in intervals of 60°, to assess the performance of all three during the same flight. The nosecone itself was covered in a phenolic resin, embedded with small pieces of nylon cloth. The Avco variant of RVX-1 used phenolic refrasil on the cylindrical and flared section, while the nose cone was protected using a quartz-based ablative, called Avcoite [181][182].
There was an onboard recovery system to allow the scientists and engineers to evaluate the materials’ performances after the flight. This included a parachute, of which little information is known, as well as a floatation device, based on that of the Mark-2 reentry vehicle [181].
From January to June 1959, a total of six vehicles were flown, three of both versions, all onboard modified Thor rockets. The first flight occurred on January 23, 1959, although the first successful recovery happened on April 5th, 1959. Although there were only two fully successful and three partially successful flights, both Avco and General Electric proved that their ablative materials could withstand the 6600°C hot environments encountered during reentry. [181]
The RVX-1 vehicles were instrumental in the development of ablative thermal protection systems as these test flights allowed scientists and engineers to analyse the vehicle after the flight. The success of these flights set the stage for a continued research effort towards ablative thermal protection systems through the RVX-2 and the Mark-3 reentry vehicle, and eventually contributed towards the successful use of similar ablative thermal protection materials on manned vehicles such as Apollo and Orion.
RVX-1 before and after launch
RVX-2
Operator: USAF
Mission: Re-Entry technology demonstrator for ablative materials
Flight: March 1959 – October 1960
Status: Completed
Testing for: -
The RVX-1 reentry vehicles matured the technologies for ablative reentry on sub-orbital trajectories, however, their flight conditions were not yet fully representative to that of an operational ballistic reentry vehicle due to the limited capabilities of the Thor-Able rocket. After the development of the more capable Atlas rocket, it was time to put the state-of-the-art thermal protection systems of General Electric to the test in the RVX-2 program. [206]
The RVX-2 vehicle had a conical shape with a spherical nose tip and was at the time the largest reentry vehicle flown. It measured 3.73 meters long, had a diameter of 1.63 meters, and weighed 907 kg. The nose tip was covered with chopped nylon-reinforced phenolic resin, while the vehicle’s frustum was protected by an experimental unreinforced phenolic resin. [1][2]
Three RVX-2 vehicles were launched between March 1959 and July 1959, of which the first two experienced a guidance failure. The third flight was however a great success, where the large reentry vehicle was successfully recovered, as can be seen in the figure below. This successful flight test proved the technology readiness of the ablative materials for high-speed reentry, which was used for the development of the Mark 5 reentry vehicle. [206][207]
Between August 1960 and October 1960, there were another three reentry vehicle flights onboard Atlas rockets, as part of the RVX-2A program. These reentry vehicles were identical in shape to those of RVX-2, however had significantly more experiments onboard to gather scientific data, making the capsule approximately 1225 kg heavy. They featured for example live mice, experimental fuel cells, transpiration cooling, photography equipment, etc … One of these flights tested ablative thermal protection materials of Avco for the Minuteman reentry vehicle. [206]
RVX-2 reentry vehicle after successful recovery from launch onboard an Atlas on July 21, 1959
Mark-3
Operator: USAF
Mission: Re-Entry technology demonstrator for ablative materials
Flight: March 1960 - January 1961
Status: Completed
Testing for: -
After the success of the RVX technology demonstrators at the end of the ‘50s, it was time for the USA’s next iteration reentry vehicle: the Mark 3. Its design and geometry were heavily based on that of the RVX-1, featuring a spherical-conical nose cone, cylindrical mid-body, and a large flare at the back to provide aerodynamic stability. The whole vehicle was covered in phenolic nylon to protect itself from the extreme temperatures encountered during reentry. Several versions of the Mark 3 were built, with a slightly different geometry, primarily at the flare. Versions such as the Mark 3 Mod I, Mod IX, and Mod 1A featured a single bi-conic flare, while the Mark 3 Mod IB and Mod IIB featured a uniquely looking double flare. These vehicles were approximately 3 meters long and 0.9 to 1 meter in diameter. Between March 1960 and January 1961, a total of 11 test flights occurred onboard Atlas D, of which 1 was unsuccessful due to a launcher failure before lift-off. The success of the Mark 3 reentry vehicle made it enter service from 1960 to 1965. [208]
Mark-4
Operator: USAF
Mission: Re-Entry technology demonstrator for ablative materials
Flight: March 1960 - January 1961
Status: Completed
Testing for: -
During the late ‘50s, the development of ablative thermal protection materials for reentry vehicles made significant progress in the USA. Companies such as Avco and General Electric tested several materials for their thermal performance during multiple flight test programs such as Polaris and previous RVX flights. After this experimental phase, a new generation of operational reentry vehicles was developed such as the Mark 4 reentry vehicle. [208]
The Mark 4 vehicle design was primarily based on the experience gained from the RVX-1 (also known as Thor-Able-II) program. It was cylindrical in shape with a biconical flared section at the rear and a blunt spherical-conical nose. The cylindrical section of the vehicle measured 0.84 meters in diameter, while at its widest point, the flare was 1.22 meters wide. The earlier-developed Refrasil and Avcoite ablative materials were used on the nose and sides of the vehicle respectively. With a length of 3.22 meters, the Mark-4 was a one of the heaviest and largest reentry vehicles of its time. [208]
Prior to operational deployment, two sub-scale flight test programs were performed, called RVX-3 and RVX-4. The former featured 72% scale versions of the Mark 4 reentry vehicle, while the latter flew reentry vehicles at 94% scale. A total of five RVX-3 reentry vehicles were flown onboard Titan-I rockets between December 1959 and April 1960, while the RVX-4 vehicles were flown once on an Atlas and seven times on a Titan-I rocket. [208]
After the successful demonstration of the sub-scale vehicles, the full-scale Mark 4 reentry vehicle was tested 15 times on Atlas rockets, 28 times on Titan-I, and finally once on a Titan-II between 1960 and 1963. [208]
Full-scal Mark 4 reentry vehicle (right) inside a facility from Avco
Mark-5
Operator: USAF
Mission: Re-Entry technology demonstrator for ablative materials
Flight: August 1960 - Juli 1963
Status: Completed
At the end of the 1950s, the USA was in need of a lightweight, advanced reentry vehicle for their Minuteman and Nike-Zeus rocket launchers. Initially, two different vehicles were to be developed by Avco. However, due to financial constraints and technical difficulties, only one of the two designs made it off the drawing board. The Mark-5 reentry vehicle was significantly smaller than the other reentry vehicles that were being developed at the time, such as the Mark 3 and Mark 4. It featured a similar geometry: a conical nose cone with a spherical tip, a cylindrical midsection, and a flared afterbody to provide aerodynamic stability. The tip was made from an improved Avcoite material, while the rest of the body was covered in phenolic refrain. The back shell however was protected by a layer of Avcoat.
The Mark-5 reentry vehicle did not have any active attitude control mechanism for stabilization. Instead, it tumbled after separation until the point where it hit the atmosphere, where a set of small fins induced a spin to the vehicle. These fins were designed to burn up during reentry and were similar to those used onboard the Mark-4 reentry vehicle.
After ground testing, multiple in-flight test series were conducted in the early ‘60s. Three tests were conducted onboard Minuteman-IA rockets from a Cape Canaveral launch pad between February and June 1961. Simultaneously, 21 silo-launched flight tests were conducted from Cape Canaveral between August 1960 and February 1963, of which 15 were successful. Apart from the flight tests onboard Minuteman rockets, there were also flight tests on the Atlas from May 1961 to July 1963. After the development and testing phase, the Mark 5 entered service on 150 minutemen from 1962 until 1963. [206]
Mark-5 Reentry Vehicles after flight (left) and before flight (right).
Mark-6
Operator: USAF
Mission: Operational ballistic reentry vehicle
Flight: 1962 - 1987
Status: Completed
With its diameter of 2.3 meters and length of over 3 meters, the Mark 6 Reentry vehicle was one of the largest reentry vehicles developed by the USA. General Electric built upon the knowledge gained during the various reentry vehicle test programs of the ‘50s. Especially the RVX-2 vehicles, which served as a precursor to evaluating the shape and newly developed materials for the Mark 6. In contrast to the more cylindrical body shapes of previous reentry vehicles, RVX-1 and the Mark 6 featured a fully conical body shape with a spherical nose cap. Its tip was made from 60 mm thick phenolic nylon, while the conical body was covered in a layer of a newly developed plastic material. By 1962, the Titan-II rocket entered operational service together with the Mark 6 Reentry vehicle up to 1987. [206][208]
Mark 6 Reentry Vehicle on display at the National Museum of the US Air Force
Mark-11
Operator: USAF
Mission: Operational ballistic reentry vehicle
Flight: 1963 - 1967
Operational: 1963 - 1995
Status: Completed
In 1960, the development of an improved reentry vehicle was initiated by the United States Air Force with a stronger focus on the reduction of the vehicle’s radar crossection when outside the atmosphere. This time, Avco was responsible for the development of the Mark-11.
The shape of the Mark-11 reentry vehicle was similar to that of the Mark-5, featuring a conical nose cone, cylindrical main body, and a flared rear to improve its stability. The vehicle was 2.54 meters long and 0.81 meters wide at its base. Layers of different types of ablative materials were applied to provide thermal protection. The vehicle’s magnesium frame was covered in two types of silica phenolic resin (RaD 58B and RaD 60). Then, a mesh was wrapped around the vehicle to reduce its radar crosssection, after which the final layer of Avoat 2 was applied. Another notable difference with the Mark 5 was its characteristic sharp nose tip, which protected the radar-reducing mesh from heating during ascent. To further reduce the radar crosssection of the vehicle, several thrusters were used to induce a spin and thereby provide stability during reentry and to maintain a consistently small radar signature.
Later, several alternative versions of the Mark-11 were developed, designated by the letters A, B, and C. They featured an aluminium frame instead of magnesium to provide more resilience against ballistic missile countermeasures. The inner thermal protection layer was changed to a tape-wound refrain instead of the silica phenolic resin of the original Mark-11.
The Mark-11 underwent a troublesome test campaign from 28/08/1963 till 12/02/1964, during which five out of six flights onboard the Atlas D experienced a booster malfunction. From the 41 Minuteman IB test flights between 07/12/1962 and 08/12/1967, only six failed. The Mark-11 reentry vehicles entered operational service onboard the Minuteman IB from 1963 until 1973 and onboard the Minuteman II from 1963 until 1995. [206]
Mark 11 re-entry vehicle on display at the Hill Aerospace museum (Credits: kelly Michals)
Mark-12
Operator: USAF
Mission: Operational ballistic reentry vehicle
Flight: 1966
Status: Completed
In the first half of the 1960s, the Mark 5 and Mark 11 reentry vehicles were developed for operational use on the Titan II and Minuteman III rockets. In the second half of the 1960s, multiple new reentry vehicle projects were conceptualized, of which the majority was cancelled during the design phase. The Mark 12 program however continued with the development of a small-scale ballistic reentry vehicle of which multiple could be used on a single rocket, or also called Multiple Independent Reentry Vehicles (MIRV).
The Mark 12 reentry vehicle had a conical shape with a half-cone angle of 9 degrees and featured a relatively sharp nose tip. With a weight of 368 kg, the Mark 12 reentry vehicles had an exceptionally large ballistic coefficient of 10000 kg/m². Although advantageous from a strategic point of view, this high value also causes harsh reentry conditions. A new material was developed for the Mark 12 reentry vehicle’s nose tip, called carbon-carbon composite or reinforced carbon-carbon. Although the development of this new material was troublesome, it proved to be a highly capable material that would later be used on other reentry vehicles such as the Space Shuttle for example. Several test samples of this material would fly onboard the BGRV reentry vehicle as a proof of concept during the Mark 12 development from 1966 onward.
Flight testing of the first Mark 12 started in the spring of 1966. The first three flights ended catastrophically with the loss of the reentry vehicle. Ablation of the new heat shield resin caused some of the carbon wrappings to be exposed and thereby inducing an ever-increasing spin until the vehicle disintegrated. After solving the issues associated with the heat shield and several successful flight tests, the Mark 12 vehicle entered service on the Minuteman rocket from 1970 onward [136,214].
Mark-21
Operator: USAF
Mission: Operational ballistic reentry vehicle
Flight: 1986 - Present
Status: Operational
The Mark 21, also known as Advanced Ballistic Reentry Vehicle (ABRV) is the next iteration operational strategic reentry vehicle after the Mark 12A and was developed by Avco for their expertise with previously developed reentry vehicles. It featured a similar conical geometry to that of other tactical reentry vehicles in the late 20th century, with a diameter of 55.4 cm and a shallow half-cone angle to maximize the ballistic coefficient.
In combination with the relatively small nose tip radius, the vehicle’s leading section faces extreme temperatures and heat flux during reentry. Similarly to its Mark 12 predecessor, the Mark 21 reentry vehicle also features a 3D-wover carbon-carbon thermal protection system to deal with the extreme conditions at the vehicle tip. [215]
Up to eleven of these reentry vehicles were deployed on each Peacekeaper rocket until gradual decommissioning, following the START II treaty signed by the USA and Russia in the year 2000. Its design will however live on in the Mark 21A reentry vehicle that is currently being developed.
SHARP
Operator: USAF
Mission: Technology demonstrator for sharp-edged reentry vehicles
Flight: September 1997 - September 2000
Status: Completed
Ever since the dawn of spaceflight, nearly all reentry vehicles have had a blunt body that reduces the heating the capsule experiences during reentry. The large amount of drag that a blunt-shaped vehicle generates is usually not an issue as the goal of a reentry vehicle is to slow down. However, several applications benefit from having a minimal amount of drag, such as hypersonic glide vehicles or hypersonic aircraft. A vehicle with sharp leading edges will however have to deal with significantly higher temperatures, which had not yet been flight-proven by the mid-'90s. Therefore, the Slender Hypervelocity Aerothermodynamic Research Probes (SHARP) were developed by NASA in collaboration with Sandia National Technologies, US Air Force, and US Army. Instead of the typically used ablative materials, three types of ultra-high temperature ceramics were tested in flight. Three missions were proposed, of which only the first two would fly. [231,232]
The first of the two, SHARP-B1, was the ballistic flight of a conical Mark-12A reentry vehicle, modified with a sharp nose tip instead of the conventional rounded nose. It was launched onboard a Minuteman III rocket from Vandenberg on May 21, 1997. The flight was successful and demonstrated the capabilities of the materials to survive the extreme temperature of reentry higher than 2760°C. [232]
On the 28th of September 2000, SHARP-B2 flew a ballistic trajectory similar to that of SHARP-B1. The capsule was also a modified Mark-12A reentry vehicle, however not with a pointy tip. Instead, it featured four sharp-edged strakes made from three different ultra-high temperature ceramics. These strakes were retractable during the flight to allow for post-flight inspection.
A parachute retrieval system was incorporated to retrieve the vehicle safely
. From illustrations, it seems there were three small drogue parachutes, followed by a larger main parachute to allow for a soft splash-down. However, a main parachute malfunction occurred, causing the vehicle to hit the water at three times the foreseen speed. Despite the hard landing, it was successfully retrieved from the shallow depths of the Kwajalein Atoll three days after launch. Although the vehicle suffered little damage from the splashdown, there were numerous cracks in the ceramics due to the severe heat of reentry. [231]
The third phase of the SHARP program that never flew was called SHARP-L1. The vehicle would have been a sharp-edged lifting body that would have served as a more complete technology demonstrator compared to the previous two SHARP flights. Although the SHARP-L1 vehicle never flew, the research and technologies developed for the SHARP project lived on in future hypersonic applications such as the the X-43. [233]
The data gathered from these two flights and particularly the retrieved hardware was exceptionally valuable for the scientists and engineers to validate their computational models and increase their understanding of the behaviour of high-temperature materials on sharp leading edges during reentry, which contributed to the development of hypersonic vehicles such as the X-43 for example.
