The recent loss of the Space Shuttle Columbia is forcing NASA to rethink its manned space program. Future government payloads and NASA missions could rely more on unmanned commercial launch vehicles.
Building up the unmanned rocket fleet
|A technician checks the cryogenic tanks that will store propellant in the second stage of an Ariane 5 rocket.|
According to experts, there should be 30 to 35 launches of telecommunication satellites into geostationary orbits per year, but the trend will be toward heavier satellites in the 4.5 to 5.5-ton range. These are the missions the Delta IV and Ariane 5 were built for: getting heavier, sometimes multiple payloads into orbit. And those added capabilities should make them suitable for many of the NASA missions previously scheduled for Space Shuttles.
The Ariane 5 family
The A5E/CA Ariane 5 is the latest in the Ariane family of expendable launch vehicles. It can place a single 5.9-ton payload or two payloads weighing 6.9 tons into geosynchronous transfer orbits (GTO). Its central component is the cryogenic or core stage. It consists mainly of an aluminum alloy tank containing two compartments for extremely cold liquid hydrogen (LH) and liquid oxygen (LOX), which serve as fuel. The two compartments hold 157 tons: 132 tons of LOX and 25 of LH. The core incorporates a thrust frame that handles forces created by the Vulcain 2 engine, and twin solid-propellant boosters.
The Vulcain 2, built in France by Snecma and new on the Ariane CA, develops 303,500 lb of thrust, a 20% improvement over the Vulcain. The improvement is mainly due to a higher oxygen-to-hydrogen ratio and it's enough to let the new Ariane 5 CA increase the payload it can put into GTO by 1,760 lb. The increase in thrust also led to a redesign of the LOX turbopump, combustion chamber, and nozzle extension.
Oxygen and hydrogen turbopumps are powered by hot gas produced in a small gas generator burning hydrogen and oxygen at a low mixture ratio. This ensures the gas isn't too hot. To pressurize the LOX tank, a subsystem carries liquid helium through heat exchangers near the engine where it's converted to a gas and sent to the LOX tank.
Each Vulcain 2 can last through 20 starts and fire for 6,000 sec, which seems like overkill since the Ariane 5 will only need the engine for 575 sec and a single start. The extra capacity, according to Snecma, lets engineers test fire each motor for a few hundred seconds, ensuring each is similarly calibrated. The extra power and reliability could also come in handy for upgrades and to make sure missions have a safety margin.
Each booster holds 240 tons of propellant (ammonium, polybutadiene, and aluminum), and can be steered up to 6* by a nozzle actuation unit. Compared to the previous Ariane 5, the CA 5 uses boosters with slightly larger fuel capacities and lighter engine nozzles. Each booster generates 540 tons of thrust at liftoff, which quickly increases to 600 tons of thrust, as much as an Ariane 4 at lift-off. Boosters remain with the core until it has reached 30 to 45 miles, at which point they detach and follow their own trajectories, burning out after a total runtime of 129 sec and parachuting back to Earth for recovery.
|Medium+ launchers are further delineated by a pair of numbers in parentheses. The first is the diameter of the fairing in meters. The second is the number of GEM-60 strap-on boosters it carries|
The Delta IV Family
To match various payloads and mission needs, Boeing builds a variety of Delta IVs: the Medium, three variations of the Medium+, and the Heavy. All are built around a common booster core (CBC), the first stage that carries the new Boeing Rocketdyne RS-68 engine. The engine burns LH and LOX, producing 650,000 lb of thrust. It is 30% more efficient than liquid oxygen/kerosene engines and is also environmentally friendly, generating only steam as a combustion by-product.
The second stage is similar to that used in the Delta III, but with larger fuel tanks for the Pratt & Whitney second-stage RL10B-2 engine. Versions of the P&W RL10 have been used on Delta launchers for more than four decades.
The Delta Medium consists of one CBC and a second stage topped with a faring over the payload. It can take a 9,285-lb payload to GTO. (The Delta III can lift 8,400 lb into GTO, which is twice the payload of the Delta II.) The Medium uses a 4-m (13-ft-diameter) fairing to protect payloads during assembly, launch, and ascent. Once in space, the composite fairing splits in half and falls away. Satellites and other payloads are put on a payload fitting (PAF) and inside the fairing away from the launch pad, which cuts on-pad time to about six to eight days. Like the entire second stage, the Delta IV Medium fairings are follow-ons to Delta III fairings.
Medium+ launchers also have the CBC and second stage, but to accommodate heavier payloads, Boeing straps two or four booster motors to the CBC. And to house larger payloads, the Medium+ uses a 5-m (16.6-ft-diameter) fairing as well as the 4-m one. A Medium+ launcher can place between 10,230 and 14,475 lb in GTO.
|Technicians check filament-wound containers that will either carry fuel or be used to pressurize the cryogenic fuel for Ariane launch vehicles.|
The strap-on booster engines, built by Alliant Techsystems, Magna, Utah (www.atk.com), consist of a machine-wound graphite fiber and epoxy casing, rocket engine, and solid propellant. Unlike the main and second-stage engines, solid-propellant motors cannot be stopped and restarted, nor can the thrust be controlled. It's either firing and generating thrust or it's off. Alliant builds several strap-on GEMs (graphite epoxy motors) for Delta launchers, including the GEM 40 and GEM 46 (the number is the diameter of the motor in inches). The Medium+ uses two or four of the new 42.5-ft-tall GEM 60s, depending on payload and mission requirements. A GEM 60 weighs a little over 73,000 lb, carries almost 66,000 lb of fuel, and generates 191,000 lb of thrust for about 90 sec. The engine, little more than a carefully shaped nozzle and combustion chamber, can be fixed, or adjusted in flight to control the direction of thrust.
The Delta IV Heavy, the big kid on the block, combines three CBCs and a second stage, along with three 5-m fairing options to lift up to 28,950-lb payloads into GTO. The composite fairings vary in length and can handle single or multiple payload missions.