Conference 7.286::space

652.0. "Advanced Launch System rocket engine defined" by 4347::GRIFFIN (Dave Griffin) Tue Aug 28 1990 20:57

From: [email protected] (Peter Yee)
Organization: NASA Ames Research Center, Moffett Field, CA

Jim Cast
Headquarters, Washington, D.C.                    August 22, 1990
(Phone:  202/453-8536)

Jerry Berg
Marshall Space Flight Center, Huntsville, Ala.
(Phone:  205/544-0034)


RELEASE:  90-113

ADVANCED LAUNCH SYSTEM ROCKET ENGINE DEFINED


     A government-industry board has completed a key milestone 
with its decision defining the type of rocket engine which will 
be designed and built to power the NASA/U.S. Air Force Advanced 
Launch System (ALS).  The two candidates were the gas generator 
power cycle, similar to the J-2 engine employed on the second and 
third stages of the Saturn moon rockets, and the closed expander 
power cycle, similar to the RL-10 engine used in the Saturn I and 
the Centaur upper stage.  The decision went to the gas generator 
cycle.

     The selection by the ALS Space Transportation Main Engine 
cycle selection board was based on work accomplished in two 
phases.  First, three ALS engine contractors performed studies 
defining and evaluating candidate designs.  Then, they reported 
to a joint government-industry engine cycle technical assessment 
team and joint senior review board.  Those panels chose the most 
promising design features for the engine from reports submitted 
by the three contractors, providing a comparable basis on which 
to make the engine cycle decision.

     The Advanced Launch System program will provide, by the year 
2000, a dependable, reliable, high-capacity national launch 
capability.  It is planned as a family of launch vehicles capable 
of delivering a wide range of payloads into low Earth orbit with 
an order-of-magnitude cost reduction from today's launch costs. 

     In support of these ALS objectives, the board's decision was 
based on factors such as production cost, reliability, 
producibility, operations, size, development effort, risk and 
performance.

     The gas-generator type of engine has been widely used since 
early in the history of liquid-fueled rocket development and has 
benefitted from many technology advances over the years.  Its 
basic elements are:

    o  A combustion chamber which generates the engine's thrust
       by exhausting combustion gases at high velocity through 
       the nozzle.

    o  Two turbopumps, which take liquid fuel and liquid oxidizer 
       from tanks at rather low pressure, feed the fuel through
       cooling circuits, and inject both propellants into the 
       combustion chamber at high pressure.

    o  Turbines, powered by hot gas, to spin the turbopumps.

    o  A gas-generator device to provide the hot gas to the 
       turbine(s).  It generates gas by the combustion of a small
       quantity of liquid fuel and oxidizer, diverted from the 
       main flow.

     Definition studies of the Space Transportation Main Engine 
have determined that such an engine can be developed within the 
parameters established for the ALS.  It will use liquid oxygen 
and liquid hydrogen as propellants, be approximately 7 feet in 
diameter and 12.5 feet long, and weigh about 7,000 pounds.  
Supplying approximately 600,000 pounds of thrust, it will power 
the ALS core stage and the launch system's booster stage.

     The Space Transportation Main Engine selection board 
consists of top managers from NASA's Lewis Research Center, 
Cleveland; the Marshall Space Flight Center, Huntsville, Ala.; 
the Johnson Space Center, Houston, Tex.; and the Stennis Space 
Center, Bay St. Louis, Miss.; representatives of the Air 
Force/NASA ALS Joint Program Office; and the presidents and chief 
operating officers of the three contractors currently conducting 
definition studies for the engine.  The 12-member board is 
chaired by Marshall Space Flight Center Director Jack Lee.

     The engine definition contractors are Aerojet, the aerospace 
segment of GenCorp; United Technologies' Pratt & Whitney; and the 
Rocketdyne Division of Rockwell International.  They are working 
under parallel competitive study contracts and have agreed to 
pursue the formation of a teaming arrangement, which will reduce 
the cost and risks of the ALS engine development program.  

     The studies show both engine cycles would provide 
essentially the same projected cost and reliability, while on 
many of the detailed points of comparison, the gas generator was 
favored.  

     The selection of a gas-generator cycle engine, explained 
Board Chairman Lee, will pose a lower development risk than the 
closed expander engine cycle.  "That advantage will give us more 
freedom to concentrate on achieving optimum reliability, 
producibility and production cost, rather than overcoming 
development obstacles," Lee said.

     Future work on the Space Transportation Main Engine will 
include preliminary design beginning this fall, detailed design, 
fabrication, and test of major engine components, and a prototype 
engine program scheduled to begin in 1992. 

    
    Can anyone give a quick description of a "closed expander power cycle"
    rocket engine?
    

    re .1
    
    I wondered about that too. The only reference I have that talks about
    this is British and uses different terminology, but they use the J-2
    and RL-10 to illustrate different turbopump designs.
    
    The 'gas generator cycle' engines burn propellants in a seperate
    combustion chamber to generate enery to drive the turbopumps. Usually
    it is the same propellants as the main engine and they usually burn
    fuel rich to keep temperatures relatively low. The exhaust from the gas
    generator is essentially discarded, generating little useful thrust. In
    some single engine vehicles, e.g. Thor, the turbine exhaust is used for
    roll control of the vehicle. In the J-2 there is what looks like a
    large manifold about 2/3 of the way down the expansion bell so I
    suspect the turbine exhaust is injected into the nozzle to cool the
    lower part (film cooling). The F-1 does this, and you can see the
    turbine exhaust preceed main ignition if you watch the engine start
    sequence in slow motion.
    
    Closed power cycle ('topping cycle' in my book) engines also burn
    propellants in a gas generator, but the fuel rich exhaust is injected
    into the main combustion chamber where it burns along with the rest of
    the propellants. These are sometimes called two stage combustion rocket
    engines, and the SSME is an example of this design. They are more
    efficient in their use of propellants, but more complex.
    
    gary