A Common Aero Vehicle (CAV)
Model, Description, and Employment Guide
27 January 2003
Terry H Phillips
AFRL and AFSPC
The Name: In the 1994-1995 timeframe, briefings about a new reusable launch vehicle (RLV) architecture called the Military Spaceplane (MSP) system were made to numerous flag officers. Consistently capturing the flags’ imagination was the force application mission using what was called a “hypersonic weapon” released by the MSP system. General Joseph Ashy, Commander of AF and US Space Commands, in particular, stated this was the mission to transform AF Space Command into “Space Combat Command”. As further research into ground attack weapons for the MSP system was made, it became obvious the term “hypersonic weapon” was not a good description. The “hypersonic weapon” was conceived to be a carrier vehicle for penetrator warheads and any suitable weapons or submunitions already being developed for the aircraft community. In early 1996, a meeting was held at TRW’s Colorado Springs, Colorado facility to name this new weapon and lay out a plan for its eventual design, test, acquisition, and employment. The basic concept was for a maneuvering reentry vehicle using common guidance, navigation, and control (GN&C) as well as a common aerothermodynamic shell, to deliver a wide variety of submunitions, unitary penetrators, or intelligence, surveillance, and reconnaissance (ISR) platforms or sensors. Since the concept used a common aero shell, the decision was made to call the new weapon the Common Aero Vehicle or CAV. The same CAV would also be common to a large number of launch systems, including RLVs, expendable launch vehicles (ELVs), retired Inter-Continental Ballistic Missiles (ICBMs), and air launch from a variety of platforms. The “Aero” term was short for “aerothermodynamic shell” and not for “aerospace”, as mistakenly used in some documents. Early briefings by prime contractors even used the title “Common Aeroshell Vehicle” before they began using the shortened Common Aero Vehicle
Earlier Programs: Early work with Sandia National Laboratories had resulted in Phillips Lab’s MSP Technology Office showing graphics of a very simple, flap controlled, biconic hypersonic weapon. Meetings with TRW, Boeing, Lockheed-Martin, Wright Lab’s Munitions Directorate and Phillips Lab’s Ballistic Missile Technology Office showed a large body of research existed on much more sophisticated maneuvering reentry vehicles which could be adapted to the CAV concept. Boeing had the most actual flight test experience with programs such as Boost Glide Reentry Vehicle, Maneuvering Control and Ablation Studies (MARCAS), Advanced Control Experiment (ACE), Advanced Maneuvering Reentry Vehicle (AMaRV), and Technology Demonstration Maneuvering Reentry Vehicle (TDMaRV). All of these programs had direct applicability to CAV, especially AMaRV. AMaRV flew several times in the late 1970s and early 1980s and demonstrated profiles similar to those a CAV would fly. Lockheed-Martin had two programs, MSTART and High Performance Maneuvering Reentry Vehicle (HPMARV) which were directly related to CAV. HPMARV, in particular, had
detailed computational fluid dynamics (CFD) and wind tunnel analyses, even though the vehicle never flew. Boeing and Lockheed-Martin were both provided small amounts of funding over the next few years to mature their CAV designs and recommend employment, test and acquisition options.
MSP Line Item Veto: In 1998, AFSPC had put together an MSP acquisition
initiative for the Fiscal Year (FY) 2000 Program Objective Memorandum (POM).
This initiative contained initial funding for the entire MSP system, including CAV. MSP had congressional interest, and steady congressional adds had been provided to the MSP Technology Office over the years. In 1998, the President exercised his new line item veto for the first time. One of the programs he line item vetoed was MSP. Weapons in space were a contentious issue with that administration and MSP (and by extension CAV) received a black eye. The actual MSP RLV was renamed Space Operations Vehicle (SOV) at the direction of the AFSPC Commander, and the MSP POM initiative died a quiet death. When the Supreme Court overturned the line item veto on constitutional grounds, Congress dictated the returned money could be used for either Space Maneuver Vehicle (SMV) or CAV. When the money arrived at the MSP Technology Office, it came with instructions from the Office of the Secretary of Defense (OSD) that the money was to be spent only on SMV, not CAV. For the next 2-3 years, any public mention of CAV or other space weapons was not allowed, and work performed on CAV was done quietly and out of the limelight.
Ballistic Missile Technology (BMT) Office: Technology for follow-on ICBMs
was, to a large extent, directly relevant to CAV. Inertial Navigation System/Global Positioning System (INS/GPS) components and antennas were developed which were ideal for CAV. Two (of three attempts) successful Missile Technology Demonstration (MTD) tests were made using a modified Pershing reentry vehicle (RV) to deliver Eglin AFB-designed unitary penetrators in White Sands Missile Range. MTD-1 penetrated 31 feet into 2500 pounds per square inch (psi) weathered granite after impacting at over 3000 feet per second (fps). For reference, hardened concrete measures 5000 psi. MTD-1’s INS/GPS
navigation system performed flawlessly. MTD-2 had a launch vehicle malfunction resulting in launch vehicle destruction. The larger MTD-2 unitary penetrator was so tough, however, it was recovered and used successfully on MTD-3. The BMT Office also invested in advanced vehicle technologies including structural, aerodynamic, thermal, trajectory, and booster/interface analyses and systems engineering on flight controls, GN&C, range safety, telemetry, antennas, and power. The BMT Office remained the pilot light for CAV, and kept the technical effort alive through judicious investments. At the direction of the AF Research Lab (AFRL) Commander, a CAV integrated product team (IPT) was formed by the BMT Office from industry and government experts. The CAV IPT remains active today integrating CAV activities and providing technical expertise and advice to senior leaders.
AMaRV as CAV-L and HPMARV as CAV-H: Two basic concepts for CAV have
emerged. AMaRV was a modified biconic design using split flap control. CAV-L
means low performance CAV. AMaRV-like CAVs have a hypersonic lift to drag ratio (L/D) in the 2.0-2.5 range. HPMARV was a lifting body design applicable to high performance CAVs or CAV-H with hypersonic L/Ds in the 3.5-5.0 range. Hypersonic L/D almost directly correlates to down-range and cross-range gliding ability. The higher L/D lifting body designs such as HPMARV, Boeing wave rider designs, and NASA Ames’ and Sandia’s Slender Hypersonic
Aerothermodynamic Research Probe (SHARP) L1 all provide much more cross-range and thus offer superior footprints for employment. Unfortunately, they also put much higher demands on thermal protection systems because of the sharp leading edges required to get the higher hypersonic L/Ds. Ultra high temperature ceramics (UHTCs) such as the halfnium-diboride used on SHARP B1 and B2 and planned for L1, may be a possible solution to sharp leading edges. Use of AMaRV and HPMARV names is just a convenience for users and in no way denotes only Boeing is capable of producing an AMaRV-like CAV and only Lockheed-Martin is capable of producing an HPMARV-like CAV. Both, and other contractors such as Orbital, have their own designs for both CAV-L and CAV-H.
Boeing AMaRV or CAV-L
Lockheed-Martin HPMARV or CAV-H
Conventional Air Launched Cruise Missile (CALCM) have been priced in various sources at $0.8-2.5M each. CAV’s original goal was cost competitiveness with the AGM-158A Joint Air to Surface Standoff Missile (JASSM). JASSM, however, came in at less than $400K, a price CAV cannot match. Best estimate of CAV costs from the prime contractors is ~$1.5M per CAV.
In order to meet this price, CAV must be kept as simple as possible. RLVs have an attitude control system (ACS) to maintain attitude control while exoatmospheric or flying at high angle of attack (AOA) in the atmosphere. The Space Transportation System’s (STS’s) Shuttle orbiter, for example, uses its ACS all the way down to mach 2 before transitioning to all aerodynamic flight controls. This is required on RLVs because their thermal protection systems (TPS) are reusable. The vehicles must be flown at AOAs up to 45 degrees (higher for some designs) to spread the heating out over the windward side of the RLV. CAVs use a completely different strategy for thermal control in the atmosphere. Rather than reusable TPS, CAVs use ablative TPS that absorbs a considerable amount of heat just in the phase changes from a solid to a liquid and eventually to a gas. Since the TPS is ablating and eroding, the aerodynamic shape of the CAV is changing slightly, but not enough to affect flight controls. Unlike RLVs that reenter at high AOAs, CAVs reenter at low AOAs and fly at close to maximum L/D to give the highest possible down-range and cross-range distances. An RLV flown like this with current technology TPS would burn up due to the high heat loads developed. Since the CAV is flying in a region where aerodynamic forces are optimized, no ACS is required after atmospheric interface at ~50 nautical miles (nm) or 300,000 feet. Aerodynamic control alone is sufficient to maintain control up to ~20 degrees AOA. Since maximum L/D is normally 10-15 degrees for CAVs, aerodynamic control is easily maintained. Deleting the ACS saves weight, volume, and cost. If the CAV is being deorbitted, an upper stage or deorbit module provides the change in velocity (delta V) to deorbit and provides attitude control until atmospheric interface. At atmospheric interface at ~50nm, the upper stage or deorbit module is jettisoned. We will discuss deorbit modules and upper stages in more detail later.
To properly balance the aerodynamic forces, most CAV-L designs use fixed yaw plates to impart stability in the atmosphere. CAV-H designs usually have moveable yaw flaps for more control of their aerodynamically complex shapes. Center of gravity control is very critical to maintaining aerodynamic stability. The other control surfaces for CAVs usually consist of two movable flaps on the rear of the CAV on the windward side. On CAV-L, these flaps are normally located together because of the conical shape. On CAV-H, the flaps can be separated to provide more control authority because of the larger acreage available on lifting body shapes. These flaps can operate together to provide pitch control and trim and operate independently to provide roll control. These flaps could be powered either by hydraulics or electro-mechanical actuators (EMAs), with EMAs seemingly the preferred solution. Thermal batteries or the latest available lithium-based batteries would power the EMAs and the GN&C system. During
atmospheric flight, the CAV sees large variances in flight control movement effectiveness due to the increasing density of the atmosphere as the CAV descends, and flight control gains must be carefully controlled. These relatively simple flight control systems are still capable of generating very high g forces and CAVs can exceed 100 gs lateral acceleration.
As mentioned earlier, the GN&C system is based around an INS/GPS system. Because of the plasma being generated by reentry, blackout of the GPS system is an issue. Antenna designs to overcome this problem as well as tolerate high heating are being investigated. TPS additives that cut down on plasma formation are also under investigation. Since the system is INS/GPS, the Kalman filtered INS smoothes out any loss of signal from the GPS and updates itself whenever GPS is available. Circular errors probable (CEPs) of less than 10 feet are possible using GPS and the CAV’s GN&C system.
Thermal Protection System: TPS technology is currently the most severe
limitation on CAV performance. CAV provides optimum performance when able to operate in a heavy heatload environment for approximately 3000 seconds. With 3000 second TPS, CAV is virtually unconstrained on what profiles can be flown. Downrange and crossrange distances can be optimized and the generic CAV-L described elsewhere in this document (hypersonic L/D 2.4, 1800 pounds, 500 square inches aero reference area) has approximately 2300 nm crossrange and up to 15,000 nm downrange capability when reentering from orbit. Current technology TPS, however, can only survive approximately 800 seconds in the high heatload environment and this requires the CAV to be flown much more conservatively to survive. This could include flying at a higher AOA of 15-20 degrees from mach 25 down to mach 5 rather than flying at maximum L/D of 10 degrees. It also requires flying at higher bank angles to increase sink rates and decrease time of flight. Depending on profile, this may entail S-turning across the ground track to keep on course while in a steep bank.
The integrated heat load over time and the peak stagnation heat rate drive TPS system design. If a CAV were built today, TPS would probably be carbon-carbon (CC) nose and movable control surfaces to minimize erosion, and carbon-phenolic (CP) ablatives on the rest of the vehicle. The thickness of the material would be adjusted depending on the environment around the CAV. Windward TPS would be slightly thicker, but nothing like the difference between RLV windward and leeward TPS because of the RLV’s high AOA reentry. TPS weight is a concern, and estimates of 40-60% of the aeroshell’s total weight being TPS
have been published. Technology investments in TPS to both increase the length of time it can survive the CAV environment and to lighten its weight should have very high priority.
Note that a CAV with 800 second TPS is still an operationally useful weapon system. The CAV would have a cross-range of approximately 900 nm, and its footprint would thus be 1800 nm wide. Operational flexibility would be reduced
since the CAV would have to be launched into an inclination which placed CAV very close to the ground track passing through the target. Launch inclination limitations could mean some targets would be difficult to reach.
CAV Size: CAV is a generic concept and could be sized to accept virtually any munitions or ISR payload. A rough analogy would be the MK 81-84 series of general-purpose bombs designed by the Navy and adapted by the AF and most foreign air forces. The MK 81 weighs 250 pounds (lbs), the MK 82 500 lbs, the MK 83 1000 lbs, and the MK 84 2000 lbs. It would be possible to build CAVs in several weight classes, and indeed CAV designs from 1000 to 4000 lbs have been proposed by various contractors. A more apt comparison, however, might be the AF’s current Tactical Munitions Dispenser or TMD in SUU-64/B,
SUU-65/B, and SUU-66/B configurations. The TMD is a clamshell dispenser using aluminum linear shaped charges to split its body section and separate the submunitions it carries. The aeroshell/body sections are identical and only tail fins, fuzes and payloads are different. The weight of the TMDs varies from 710-960 lbs depending on payload and all the payloads occupy the same volume. The size chosen was a compromise to maximize commonality and minimize cost. The success of the TMD is borne out by the recent decision to upgrade all TMDs with an INS guided Wind Corrected Munitions Dispenser kit for improved medium altitude release accuracy.
Initially, CAV size will be a compromise between weight and weapons effectiveness. Weight determines how many CAVs can be lifted and is also related to weapons effectiveness. CAV with a penetrator payload has a big advantage over current aircraft released penetrators because it can impact at virtually any velocity desired. Most conventional penetrators impact at between 800 and 1500 feet per second (fps). The current physics limit on conventional penetrators is approximately 4500 fps. Above that velocity, the penetrator goes plastic on impact and spreads itself molecules thick over the impact surface. CAVs with penetrator warheads, then, must slow down to 4500 fps or less before impact. Young’s equation shows penetration into hardened, 5000 psi concrete of
40-60 feet for an 800 lb, MK 83 class penetrator. This is several times deeper penetration than possible even with the BLU-113/B penetrator used by the 4700 lb GBU-28 dropped on the last day of Desert Storm. Because of the added velocity, a CAV with a relatively small penetrator can be as effective as much larger, slower penetrators delivered from aircraft.
Because CAV can be effective as a penetrator with an 800 lb, Mk 83 class, BLU-110/B size penetrator, CAV initial sizing will most likely be around this size payload. For this guide we can postulate the weights used in the CAV-L and CAV-H models that follow. Thus CAV-L would weigh approximately 1800 lbs and the more complicated CAV-H approximately 2000 lbs. Actual weights of CAVs would vary depending on payload, just as they did for the Tactical Munitions Dispenser mentioned earlier, but 1800 lbs and 2000lbs are good conservative estimates for planning use. CAVs are long and pointy to maximize hypersonic lift to drag ratio and designs vary in length from 107 to 144 inches (in). Width or diameter on CAV-L is approximately 36 inches while the lifting body of CAV-H is typically around 48 inches. For planning use and wargames, then the following dimensions can be used:
Length 107-144” 107-144”
Diameter/Width 36” 48”
Weight 1800 lbs 2000 lbs
Payload 800-1000 lbs 800-1000lbs
On the following pages are three degree of freedom (3DOF) models of both a CAV-L and CAV-H developed for the government. They are non-proprietary and non-ITAR and can be used without permission or restriction. They give very similar results to the proprietary and ITAR models used by the prime contractors based on their actual designs. Since they do not represent an actual vehicle design, their use and distribution are not controlled.