History of the HMT / LTT Trailer Program
The High Mobility Trailer (HMT), later called the Light Tactical Trailer (LTT), has had a complex and troubled history.
The original HMT that began production in January 1997 failed to meet objectives and the entire fleet was deadlined. However, after modifications that solved safety and stability problems, the trailer survived, with a new name, as the Light Tactical Trailer (LTT). The LTT came to be considered an outstanding off-road capable trailer, well matched to the HMMWV prime mover.
M1101 Light Tactical Trailer, towed by a HMMWV.
History of the High Mobility Trailer (HMT) or Light Tactical Trailer (LTT) Program
In 1984, the US Army began producing the 1 1/4-ton HMMWV to replace the M151 series 1/4-ton truck (jeep) and companion M416 1/4-ton trailer, the Commercial Utility Cargo Vehicle (CUCV), and other light trucks as the Army’s primary light tactical vehicle. Each HMMWV was expected to replace a set of two M151s and trailers. As the HMMWV proliferated in the Army and Marine Corps, it was used to tow the M101 3/4-ton Cargo Trailer.
With the sun behind it, an M1038 High-Mobility Multipurpose Wheeled Vehicle (HMMWV) tows an M105 Cargo trailer, Osan AB, Korea, 23 Jan 2001.
The M101 trailer dates back to 1952, originally towed by the M37 3/4-ton truck and later the M880 / M1008 / M1009 CUCV trucks. The HMMWV could reliably tow the M101 on paved roads, but cross-country travel behind the highly mobile HMMWV was found to be problematic. The M101 had a narrower track width (tire spacing) than the HMMWV, causing stability problems, and its suspension did not provide adequate wheel travel and ride dynamics, causing a loss of mobility in the HMMWV/M101 system. To avoid rollover accidents as well as excessive component wear, HMMWVs were ordered to slow down in off-road travel, reducing its effectiveness and mobility.
While the Heavy chassis HMMWVs could also tow the M105 1 1/2 Ton Cargo Trailer, that trailer was not viewed as a solution to the "HMMWV trailer problem."
TACOM was directed to develop the High Mobility Trailer, with the same track as its HMMWV prime mover. The HMMWV and trailer mobility had to be at least 90% of the mobility of the HMMWV alone. For the most demanding portion of the mission
profile, this translated to a requirement for a 15-mile/hour average and 20-mile/hour maximum cross-country speed while fully loaded. To reduce time and costs of development and acquisition, Project Manager for Light Tactical Vehicles (PM-LTV), procured the HMT as a commercial-off-the-shelf, nondevelopmental item (COTS).
After a competition, in October 1993 a five year contract was let to Electrospace Systems (Richardson, TX) for development of three HMT versions that track the same as the HMMWV, use HMMWV wheels and runflat tires, and have independent suspension arms with sufficient travel to match HMMWV movements:
- Light cargo trailer with a 1,500-lb. payload (3/4 ton)
- Heavy cargo trailer with a 2,500-lb. payload (1 1/4 ton)
- Trailer chassis with a 2,700-lb. payload
Production was subcontracted to Silver Eagle Mfg. (Portland, OR). In January 1997 they began to build trailers with delivery of 7,563 HMTs scheduled for the Active, Guard and Reserve Army. Small problems were fixed, all requirements were met, and conditional materiel release was obtained in July 1997. The contract was restructured in 1996 for a total of 6,700 units. Electrospace was acquired by Raytheon and became E-Systems Inc. All 6,700 trailers were delivered by the end of July 1999.
Problems Deadline the High Mobility Trailer
Unfortunately, all was not well with the HMT. By November 1997, performance testing revealed that some HMMWV rear bumpers and crossmembers were cracked by the forces exerted while towing the HMT over rough terrain with full loads. During follow-on HMMWV bumper testing, an HMT aluminum drawbar completely failed, separating the HMT from the HMMWV. In March 1998, after determining that the aluminum drawbar design did not have an adequate safety margin, TACOM issued a Safety of Use Message, deadlining the 1,700 unit fielded HMT fleet until a fix could be developed and tested. A steel drawbar kit was developed as a replacement and was subjected to extensive laboratory and field testing. Additional problems surfaced in cross-country terrain testing, as cracks and deformities in the HMT's surge brake’s inner slide, outer housing, and lunette assemblies appeared. These tests showed that the HMT could be safely towed at up to 12 miles per hour, with a full payload, without evidence of brake actuator wear or fatigue, but that was still well below the HMT's required speed.
In August 1999, PM-LTV asked the Army user community to waive the cross-country speed requirement and accept a 10-mile-per-hour cross-country speed restriction, noting that 10-mph was significantly better than the 6-mph restriction applied to the M101 trailer. The user community refused and TACOM looked for a design change that would allow 15-mph travel with low risk. Extensive software simulation of the surge brake assembly performed by TACOM, AM General, and SAIC (contractor) and backed up by data gathered in test trials at Aberdeen Proving Ground, determined that there were loads on the surge
brake that it was not designed to withstand.
The pressure was on TACOM to fix the HMT problems. An October 1999 GAO report was critical of the HMT program. The report was used by US Senator Harkin to demand action. An ABC News broadcast used the HMT problems as an example of Federal government waste and mismanagement. TACOM took up the challenge and used a December 1999 workshop at APG to identify all innovative alternatives for resolving the HMT issues. After developing many alternate scenarios and considering all factors, the PM-LTV decided to modify the existing surge brake to increase its strength rather than adopt a new brake design. In early 2000, Army Materiel Command ordered 5,696 HMTs and 854 chassis trailers into storage until the revised design could be finalized and the modification applied to the existing HMT fleet.
Light Tactical Trailer (LTT) Emerges from the HMT Program
Light Tactical Trailer (M1101 or M1102) with Soft Top Kit, towed by a HMMWV.
The foundry at Rock Island Arsenal (RIA) produced prototypes of revised surge brake inner and outer housings, cast from a harder alloy steel. Six HMT units, light and heavy, were fitted with the new brakes and were run at APG test courses to replicate the HMT's crosscountry mission profile – over 4,000 miles for each trailer – followed by a system test to verify that all safety concerns were resolved. PM-LTV completed testing on 31 Oct 2000, without any safety incidents, and
identified corrective actions to address the minor test incidents that did occur. Both the light and heavy HMTs achieved
average cross-country speeds of over 18 miles per hour, exceeding the requirement. Both also exceeded the 1,500- and 2,500-lb. payload requirements. The tests showed that the revised surge brake assembly was a safe and durable product and did not cause damage to the HMMWV/HMT interface. RIA produced surge brakes to replace all original HMT surge brakes. TACOM Commanding General approved Full Materiel Release on 16 March 2001. All HMTs were modified and returned to the field by September 2002.
Additional contracts for more High Mobility Trailers were awarded following the solution to its problems. To distinguish the revised design, fully fielded trailer from its troubled original version, HMT was renamed the Light Tactical Trailer (LTT). Silver Eagle Manufacturing Co. was awarded, in April 2004, a new five year contract for up to 12,800 LTT units. In 2008, the quantity was increased to 64,677 over another five year period. Also in 2008, Schutt Industries, Inc. got a smaller LTT production contract for the Marine Corps version.
The new LTT version had field problems and MWOs within the normal range of equipment problems. For example, Silver Eagle recalled 700 M1101/M1102 trailers due to wheel bearing spindle nuts that may be over-torqued. In addition to MWOs applied to retrofit the trailers and cure defects, some towing HMMWVs were required to have MWO 9-2320-280-7, Crossmember Kit for light HMMWVs and MWO 9-2320-280-6, Bumper Reinforcement Plate for heavy HMMWVs (except M1123 and HMMWVs with Tow Pintle Extension Kit).
After all, the Light Tactical Trailer finally achieved recognition as an outstanding off-road capable trailer, well matched to the ubiquitous HMMWV.
The history section was largely adapted from High Mobility Trailer: Diverse Team Surmounts Design Problems to Produce a Trailer Capable of Living Up to Its Name, PM: Nov-Dec 2001 and United States General Accounting Office, Report to the Hon. Tom Harkin: Army Purchased Truck Trailers That Cannot Be Used as Planned, October 1999.