Srinath V. Iyengar, P.E., United Launch Alliance, email@example.com
United Launch Alliance (ULA) launches Atlas V from Cape Canaveral and Vandenberg Air Force launch facilities. Liquid hydrogen (LH2) and liquid oxygen (LO2) are the primary cryogenic propellants for these launch vehicles. Large quantities of these propellants are stored on the launch site. Cryogenic propellants present leak, fire and explosion hazards throughout their handling, starting from the time the commercial transport trailers enter the launch pad and continuing through storage tank filling, propellant transfer to launch vehicle and prelaunch operations until liftoff. Range Safety regulations provide safety requirements for hardware, software and operations to ensure safety throughout the ground processing.
Atlas VThe Atlas V 400 and 500 series space launch vehicles are the latest evolutionary versions of the Atlas launch system and were placed into service in 2002. Atlas V uses a standard Atlas Booster, zero to five strap-on solid rocket boosters, a Centaur and one of several Payload Fairings. The Atlas Booster and the Common Centaur shown in Figure 1 contain the cryogenic propellants. The Atlas booster contains RP-1 (non-cryogenic) and LO2. The Common Centaur contains LH2 and LO2. The Atlas Booster contains approximately 280,000 kg of LO2 and RP-1. The Common Centaur contains approximately 20,000 kg of LH2 and LO2.
Ground facilities are required to store and transfer cryogenic propellants to the Space launch vehicle. Figure 2 shows the Atlas V launch facility at Cape Canaveral. Both LH2 and LO2 are stored on the launch pad. The propellants are stored in both cylindrical and spherical tanks as shown in Figure 2. The storage capacities of these tanks are significantly larger than the launch vehicle since there is a loss through boiloff and propellants are needed for multiple launch attempts.
Cryogenic propellants arrive at the launch pad in road trailers. Due to the large quantities of propellants required, a number of trailers deliver the propellants to the launch pad. The propellants are then transferred to the large storage tanks on the launch pad. Prior to each launch, the propellants are transferred to the launch vehicle. Cryogenic Operations for Space launch Vehicles consist of four major phases.
- Transportation of cryogenic propellants to launch facilities
- Filling of storage tanks at the launch facility
- Storage in the launch facility tanks
- Transfer between storage tank and launch vehicle
Safety requirements are applicable for each of these phases. The safety requirements are imposed by various government regulatory agencies such as the Department of Transportation, Federal Occupational Safety, US Air Force Range Safety and Federal Aviation Administration.
HazardsCryogenic propellants present many hazards during the prelaunch operations. LH2 has a boiling point of 20.3K and LO2 has a boiling point of 90.2K. The low temperatures present a significant hazard to personnel during handling operations. The consequences could be damage to skin and or respiratory ailments if exposed to large amounts of the vapor. A second hazard is the low latent heat of cryogenic fluids. The rapid vaporization leads to increased pressure in the tanks for a potential tank rupture. Loss of vacuum in a tank or vacuum jacketed (VJ) line can cause rupture if relief devices do not function or not there.
A third hazard is leaks. Leaks can be in the liquid state or gas and can occur through the tank connection joints or the transfer line joints. Hydrogen in gaseous condition even at a low 4 percent concentration is explosive. The fourth hazard is spills. For cryogenic transfer, hoses are connected between trailers and tank connections. Spills can also occur during transfer between storage tanks and launch vehicle if there are failures. Table 1 summarizes the hazards during the four phases, consequences and controls.
Safety requirements are implemented in all operation phases. ULA contracts the supply to major distributers in the US. LH2 and LO2 are transported in road trailers to the launch facilities. The trailer capacities range from 10,000 gallons to 15,000 gallons. LH2 and LO2 have unique requirements due to their physical properties such as density and flammability. Since the trailers are on public roads, a major requirement is that the transport trailers have to be designed per DOT regulations to ensure public safety. The launch pad is managed by the US Air Force and the Air Force Range Safety regulations AFSPCMAN 91-710 also require compliance to DOT regulations to ensure safety on the launch pad. DOT requirements are outlined in the Code of Federal Regulations Title 49 and specific sections in it such as Section 178.338.
Filling of storage tanks is a dynamic hazardous procedure. There is a significant hazard of leaks and spills. The transfer process involves a number of steps:
- Connecting trailer hoses to the storage tank fill connects
- Preparing the trailer for transfer
- Preparing the storage tank for transfer
- Disconnecting the hoses
The transfer process is jointly performed by the trailer personnel and ULA personnel. Range Safety document AFSPCMAN 91-710 provides the requirements to ensure safety during operations. Since personnel are present in this phase, the procedures and training play a vital role to ensure safety. The procedures include cautions and warnings that draw the attention of personnel and elevate their concentration on the task ahead.
United Launch Alliance launches Atlas V from both Cape Canaveral and Vandenberg Air Force Base. At any launch pad, the launches could occur a month apart or there could be several months before the next launch. The cryogenic propellants may remain in storage tanks during this period. It is important to keep some propellants in the tanks to ensure the tanks remain cold because repetitive warming and cooling of the entire tank could subject the tanks to high stress levels. The tanks are inspected regularly in accordance with an In Service Inspection Plan per the Range Safety requirements. The tanks are also required to be certified over periods ranging from 10 to 20 years to assure continued safe operation. The LH2 and LO2 tanks are located far apart to keep the fuel and oxidizers at safe distances.
Transfer from storage tanks to the launch vehicle occurs through VJ lines. The storage tanks are pressurized to transfer the propellants. The VJ lines are first purged with an inert gas and then cooled down with low flow prior to the high flow transfer. During the test phase it may be required to fill the launch vehicle and then drain the propellant back to the storage tank. Sometimes the transfer may be needed during a launch attempt if the launch is scrubbed for several days. These transfers are performed remotely through software controlled functions. Transfer is performed by tank pressurization and operating the shut-off valves. Software is considered critical and treated as a software safety critical function per the Range Safety regulations. This classification ensures robust development process and rigorous software testing prior to implementation. VJ lines are equipped with relief devices to relieve pressure in case of propellant vaporizes rapidly. VJ lines are designed per ASME B31.3 and industry practices.
Cryogenic safety is a major concern throughout launch vehicle launch processing. The safety concern exists due to the large cryogen quantities stored on the launch pad and used during the transfer operation. The potential hazards are unintended releases (leaks and spills) resulting in personnel injury, fire or explosion. Safety is achieved by imposing design requirements like ASME pressure vessels and piping codes, relief devices, locating the oxidizers and fuels apart with significant separation distances, performing hazardous operations remotely and following written procedures. This paper is an abbreviated version of a paper of same title presented at the International System Safety Conference in 2013.
- United Launch Alliance, Atlas V Launch Services User’s Guide March 2010. This publication is in public domain and available at www.ulalaunch.com.
- US Department of Labor, 29 CFR part 1910 Occupational Safety and health Standards
- US Department of Transportation, 49 CFR part 1910 Transportation
- US Air Force, Air Force Space Command Manual 91-710 Range Safety User requirements Manual, Launch Vehicles, Payloads and ground Support Systems requirements. 1 July 2004. This publication is in public domain and available at www.e-publishing.af.mil.
- US Department of Transportation, 14 CFR part 1910 Part 400 Commercial Space Transportation
- S.V. Iyengar. Cryogenic Safety for Space Launch Vehicles during Ground Operations. International System Safety Conference 2013