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It’s so fascinating, seeing the testing that is involved in the development of a weapon system or even a single component to bolster a strategic goal. I was not disappointed when I discovered a report covering the Hellbender and Mk11 series. This was a series of six tests that had the purpose of evaluating the ability of a Mk61 Mod 7 (B61-7) to serve as an interim solution to an Earth-Penetrating Weapon (EPW).

Although Department of Energy Laboratories were already involved in the development of a high-performance Strategic Earth Penetrator Weapon (SEPW), the defense community expressed an interest, in November 1986, in modifying an existing stockpile weapon for less coast and more immediate use. Buried installations had been identified in the Soviet Union that were not easily challenged by weapons in the current stockpile.

The series was six tests, four of these tests were conducted as simple drop tests and were given the “Hellbender” designator from Los Alamos National Laboratory. The other two were configured around a potential delivery vehicle and were designated as Mk-11 tests.

Starting on January 27th, 1987, the first test was to demonstrate that, with minimal structural modifications, the Mk61-7 gravity bomb could survive and function as a penetrator weapon. The specific test was judged to be the minimum levels that would be useful for an interim EPW, consisting of impacting the Mk61-7 into a moderately hard soil at a suitable velocity for penetration and energy coupling. Although the Mk61-7 was built for low-altitude, high-speed drops on hard irregular targets, it cannot survive earth penetration environment without modification, to realize this test, it was modified to accept a steel nosecone in place of the standard energy-absorber radar nose. The energy-absorber nose would create an unacceptable blunt shape for earth-penetration, the penetrator nose was a conical and machined from a heat-treated billet of HP-9-4-20 steel, with a special cavity machined also for a special data recorder to record the impact environment. This steel was selected over other available materials because of its high strength and fracture toughness. The nose was designed to adapt to the aluminum center case with two tape joints. Two parallel joints were required for optimum strength because radial clearance was limited. The parachute system was also removed with the tail section modified to accommodate vertical suspension from the underside of a helicopter. The test unit was then dropped, free-fall onto a dry lack bed known as Antelope Lake at the Tonopah Test Range (TTR).

Although many of the perceived underground targets could be held at risk with a soil penetrating weapon, a second test was created to demonstrate the utility of the Mk61 against the same targets during the winter season (potential buried Soviet installation). The parameters for the other tests were therefore chosen to simulate the environment that a weapon would encounter when impacting a deeply frozen-soil target. A very hard location at TTR known as Brown’s Lake was selected as the simulated frozen target. Although the dry lake bed was not actually frozen during the test, terradynamic calculations showed that it was considerably harder to penetrate than a typical soil target frozen to a depth of 6 feet.

This test would increase the velocity which required it to be dropped from a high-altitude at 17,000 feet, the requirement eliminated the use of a helicopter. As a result, the Mk61 was modified to be carried from a DOE Twin Otter aircraft, this style of delivery also decreased the penetration depth due to its angle of impact.

The last test of the Hellbender series was the only test that actually incorporated the design that was preferred by LANL, an all-steel case. Up until this point, only the nose had been the steel penetrating component with the center case remaining aluminum. Obtained from Y-12 and Oak Ridge, it was machined from a solid billet of 4340 steel because the preferred HP-9-4-20 steel could not be procured to support any of the Hellbender tests.

Significantly different from the Hellbender tests, the two Mk-11 tests consisted of an air-drop and a rocket-boosted re-entry test. To fit inside the Mk-11C re-entry aeroshell, the conical steel-nose design had to be shortened about 6 inches. The new steel nose was filled with a 315lb depleted-uranium ballast for mass property consideration. The result was a slight ogive profile compared to the conical hellbender nose.

The standard Mk-11C aeroshell was modified slightly for this test, externally, the composite skin was machined down to the bare heat-shield for proper reentry simulation. Internally, the substructure was cleaned out to accept penetrator mounting features including a nose cup, a center case ring, and a rigid aluminum support cone. To provide area support for the Mk-11C shell, the void between the penetrator and the shell was filled with a dense polyurethane foam. The normal Mk-11C spin rockets were used for spin stabilization of the vehicle. The mating spacer section was attached to the test unit to accommodate parachute extraction from the cargo bay of the C-130 aircraft. Standard Mk-11C gas-actuated ball locks were used to attach the spacer to the reentry body.

The second Mk-11C aeroshell was modified for a proper reentry simulation and penetrator mounting features. All new hardware was fabricated for mounting the penetrator inside the shell. A new in-flight telemetry was designed to accommodate unique rocket system functions. In addition to monitoring the same set of dynamics instrumentation, the circuitry was used to separate rocket motors, fire the second stage rocket, release the test vehicle, turn on the Mk61 electronics, initiate the impact recorder, and start a special flight stabilization device. The telemetry package also transmitted general diagnostic information and provided rocket-safing features, including an environmental-sensing switch. Due to instability seen in the air-drop test when the standard un-modified spin-stabilization system was used, a new stabilization system was developed. This stabilization device was called an Active Rate Reducer (ARR). It used high-pressure nitrogen to create counter thrust that would damp out instabilities during flight. It was delivered as a complete assembly from the missile contracting agency.

The Mk-11C rocket test was launched into a ballistic trajectory using a Talos rocket motor. The second stage, improved Honest John rocket motor was fired several seconds after apogee to accelerate the payload to a realistic reentry velocity. The aft end of the Mk-11C aeroshell came to rest about 6 inches below the soil. This suggests that the aero structure did not remain attached to the payload very long and probably had a negligible effect on penetration performance.

This was definitely an interesting find and makes me want to know more about the possibilities of a penetrator-type weapon being utilized with reentry vehicles. The overall conclusion of the tests was that the Mod 7 could, with slight modification, be used as an interim solution to their EPW problem, I’m very confident though that this would eventually lead to the Mod 11.