Jacking and equalizing cylinders on NASA’s crawler transporter hold steady the massive launch vehicle.
Contributed by Ingo Rühlicke, Head of Project and Export Depts., Walter Hunger GmbH and Co Kg
Artemis I is the inaugural moon rocket mission in a program that aims to one day return astronauts to the lunar surface. Perhaps unnoticed, but critical to success, NASA relies on two special crawler transporters to move spacecraft from the vehicle assembly building (VAB) to the launch pads at Kennedy Space Center in Florida. And in the case of Artemis I’s delayed lift-off in September due to Hurricane Ian, back to the VAB for safe shelter.
The massive transporters were originally built in 1965 to carry Saturn V rockets and Apollo spacecraft, along with the launch platform. Later they were used for all Space Shuttle missions. But for NASA’s Artemis deep space exploration mission, using the Space Launch System (SLS) rocket and Orion spacecraft, the load capacity of the crawler transporter had to be increased by more than 50%. To accomplish this task Hunger Hydraulik developed new jacking, equalizing and levelling cylinders with greater load capacity, but also with new features to improve availability, reliability and safety.
The crawler transporters are designed with eight track drives, two in each undercarriage measuring 40 m long and 35 m wide. Original maximum load capacity was 6,000 tons, handled by the hydraulic jacking, equalizing and levelling (JEL) system of 16 double-acting cylinders, four in each undercarriage.
The JEL-system ensured the crawler transporter could lift the fully equipped rocket and launch platform from its parking slot in the vehicle assembly building, keep it level during transport, and lower it onto the launch pad. The 2-m stroke hydraulic cylinders compensated for any deviations, and were especially important when the vehicle moved up the 5° launch pad ramp. Spherical bearings mounted on the rod and cap ends of each cylinder ensured the required freedom of motion during steering and levelling.
Remote-operated valves on each cylinder provided load control, and could also hydraulically shut off a single JEL-cylinder in an emergency and maintain the load with the three remaining JEL-cylinders. A central hydraulic power unit installed in the center of the structure supplied all 16 cylinders.
Now, after 50 years of service, the crawler transporters have been completely overhauled, including new JEL-cylinders. The modifications will support human spaceflight for another 20 years.
The design and specifications for the new JEL-cylinders were developed by NASA in collaboration with Hunger Hydraulik engineers. Besides higher load capacity, critical focuses were on heightened safety and high reliability of the system. Main requirements for the new JEL-cylinder upgrade included:
- Increase load capacity by >50% while maintaining the existing hydraulic power unit. Eight pumps each deliver a maximum 60 gpm flow at 3,000 psi to the JEL-cylinders.
- Use the same installation space and mounting interfaces in the crawler transporter.
- Improve the cylinder-to-structure interface for the spherical bearings.
- Design a multi-level safety system for hydraulic load control.
- Simplify handling and installation of the JEL-cylinders.
- Enhance corrosion protection for the piston rods in the offshore-like environment at Kennedy Space Center.
- Engineer and manufacture a one-to-one scale dynamic test rig.
New cylinder design
Based on the above parameters, the NASA/Hunger engineering team developed a cylinder design that fulfills all these requirements. A risk analysis examined the existing JEL-cylinders as well as the proposed replacement. This considered the effects of potential failures, such as a main seal malfunction, a pipe or hose rupture, or a fracture of a spherical bearing or pin. Results indicated that the new JEL-cylinder design improves system behavior and reduces the consequences of failure, and thus ensures that transport operations are completed without delays or additional risks.
Given preliminary approval, two prototype cylinders were built to prove their manufacturability, performance and quality. The new JEL-cylinders were designed according to ASME standards with a flanged cylinder head and bottom for easy maintenance. To realize a 50% higher load capacity using the existing power unit, designers increased the cylinder diameter accordingly, to a 23-in. bore.
In light of the larger cylinder size, however, space limitations within the existing structure had to be overcome. Solutions included a switch to maintenance-free spherical ball joints instead of the original spherical bearings with pin and clevis; adaptor plates with fast-mounting interlocks were added to the transporter structure; and a hydraulic manifold block mounted on each JEL-cylinder replaced in-pipe mounted safety valves.
Even though the new cylinders lift under power and rely on gravity to lower loads, they are designed as double-acting cylinders. This offers some advantages compared to the old single-acting units. First, the loaded seal is the piston seal, which is surrounded by clean hydraulic fluid and is better protected from environmental influences. Second, the piston rod seal now acts as a secondary emergency seal if the JEL-cylinder must be used in single-operation mode. A special reconfiguration valve in the manifold block activates the emergency mode. To ensure the lowest possible load variation in this emergency mode, the rod diameter was designed as large as possible.
Sealing system upgrade
To hold a load under all circumstances and guarantee stroke movement free of stick-slip and other vibration effects, the seal arrangement in the JEL-cylinders is of a critical importance. The dynamic seals on the piston and in the cylinder head were mainly selected from the Hunger DFE GmbH standard seal catalog and arranged as shown in the nearby graphic.
Because JEL-cylinders are always loaded in a pushing direction, the main piston seal is a TDA-type oriented toward the piston chamber. A secondary GD1000K-type seal offers additional safety. It seals the piston from the annulus chamber side and permits stroke adjustment during installation and set-up. The seal arrangement in the cylinder head consists of a TDI-type seal and a secondary, externally adjustable seal, called EVD. Together, the sealing system permits the following operating modes:
- Normal operation: The TDA piston seal (with the GD1000K in cascade) seals against load pressure. The annulus chamber only connects to tank pressure.
- Emergency operation 1: The TDI rod seal seals against load pressure in case the piston seal fails. Piston chamber and annulus chamber are hydraulically connected.
- Emergency operation 2: The EVD rod seal seals against the load pressure in case both the piston seal and TDI rod seal fail. Piston chamber and annulus chamber are hydraulically connected.
To guide the piston and rod, as well as to resist any possible side loads, FI/FA-type plastic compound bearing elements are installed. They consist of a POM-PTFE-bronze material and offer minimal friction and stick-slip free motion, especially during low-speed movements. The special shape of the bearing elements grants a 3-mm clearance between the relative moving parts and can, at the same time, directly support the seal elements in the axial direction without any extrusion gap.
To protect against corrosion in the seaside atmosphere at Kennedy Space Center, piston rods are coated with a Ceraplate thermal sprayed metal-oxide coating. It includes a Ni/Cr base layer approximately 150 μm thick and a Cr2O3/TiO2 top layer with thickness about 200 μm. Hardness of the top layer is 950 to 1,050 HV (Vickers hardness), and the surface is superfinished to Ra = 0.15 μm.
This coating provides enhanced corrosion protection during operation, and also when the rods are exposed to the sea atmosphere for extended periods when the crawler transporter sits idle. Ceraplate coating performance has been tested and certified by independent institutes with regards to layer composition, hardness and corrosion resistance according DIN EN ISO 9227.
To improve life and reliability, the JEL-cylinder bearings were upgraded from steel-steel spherical ball bearings with a clevis and pin to spherical ball joints. While the old bearings occasionally cracked, the new design should be more robust and withstand even overload conditions without problems.
The spherical ball joints offer an increased bearing area which reduces contact stresses in the material and the interfaces. This design is also free of bending stresses in all loaded parts. The bearing material is a maintenance-free Hunger H-Glide lining in combination with a hardened steel ball as a counterpart. It allows free tilting of 7° in any direction with a maximum compressive strength in the H-Glide of 160 MPa (23,200 psi). An inner retention pin holds the bearing parts together. To avoid uncontrolled rotation of the JEL-cylinders, bottom-side spherical ball joints are equipped with an anti-rotation device.
Handling and installation
Because of its size, any service and maintenance of the crawler transporter is typically conducted outdoors using mobile cranes for handling heavier parts. In the case of JEL-cylinders, installation requires them to be positioned under the upper load frame structure — which is difficult if the cylinder is hanging on a crane hook. Therefore a lifting fixture was developed to allow easier handling and installation of the JEL-cylinders.
Furthermore, the mounting interface between the crawler transporter and JEL-cylinder was modified with adaptor plates with fast-mounting interlocks. The adaptor plates are only flanged to the transporter structure where good accessibility is a given. This lets technicians lift the JEL-cylinders in position and secure them with fast-locking wedges.
Testing the JEL-cylinders
Proving the performance of the new JEL-cylinders required extensive static and dynamic full-load tests and actual application tests with the crawler transporter. Static testing carried out on a hydraulic test rig at Walter Hunger GmbH & Co. KG allowed full-pressure tests of the JEL-cylinders, not only in the end stroke positions but also mid-stroke. Additionally, all manifold block functions were tested as well as the freedom of motion of the spherical ball bearings.
To measure dynamic performance under full load, a dynamic test rig was built. It included a vertical load frame with moveable mid-support and load cylinders, a hydraulic power unit with two independent controllable hydraulic axes, along with the necessary control and recording hardware and software.
The dynamic test program involved sequences of full-stroke cycles, smaller oscillations and load variations. Also emergency load conditions were tested. During all tests — system parameters, and specific cylinder parameters like pressure, stroke, position, friction, and number of cycles — were recorded for later evaluation. Upon completion of the dynamic test program the JEL-cylinders were dismantled and inspected in detail.
Based on the findings small design adjustments were made to facilitate manufacturing of the production units, along with an evaluation of the operating life, reliability and performance of each JEL-cylinder.
To prove that the new JEL-cylinders would fit within the crawler transporter structure and function as required, the two prototype cylinders were installed in one undercarriage unit — replacing two old cylinders. This let engineers test the new handling systems plus all mounting interfaces.
After initial lifting tests, the crawler transporter was driven the 3.5 miles from the vehicle assembly building to the launch pad and back. This test drive involved cornering, lifting operations and ramp travel. The results were analyzed and necessary changes were subsequently made to the production units.
Next, a complete set of working JEL-cylinders were manufactured. One crawler transporter was completely equipped with the working JEL-cylinders and extensive test rides carried out. With final drive and load tests successfully completed, crawler transporter 2 has now been fitted with the new cylinders and is ready for current and future NASA missions.
Hunger Hydraulics | hunger-hydraulics.com
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