Conference 7.286::space

452.0. "ALS engines : back to the future ?" by ITAMKT::MARCOMM () Wed Aug 24 1988 12:42

I type here an article coming fron "Military Space", a newletter, reported
in the August 88 issue of "High Technology Business" magazine.


                 ALS Engines to Be Simple, Cheap

Propulsion for the heavy-lift Advanced Launch System (ALS) may take a
technological step backward to lower U.S. space-launch costs.
In the past, U.S. rocket engines were designed for high performance and 
low weight, with little attention paid to operating costs. But meeting the
ALS program's goal of a ten-fold reduction in launch cost forces engine 
makers to adopt the Shaker philosophy, " 'Tis a gift to be simple".
The shift in design philosophy can be best seen in the liquid oxygen/liquid
hydrogen (LOX/LH2) engines for the ALS core vehicle. The joint NASA/Air
Force program has made a low-cost LOX/LH2 engine its highest technical 
priority. This engine could be used to boost 150,000 pound ALS payloads 
in the 21st century; propulsion technology could also be "spun-off" for 
use on advanced versions of smaller expendable Titan, Delta, and Atlas 
launchers.
In the 1970s, NASA's Marshall Space Flight Center and Rockwell's Rocketdyne
division developed a space-shuttle main engine (SSME). This reusable,
Throttleable engine used a staged combustion cycle and high performance 
pumps to drive fuel into an ultra-high--pressure (3,200 pounds per sq.inch)
combustion chamber. Each SSME can create 470,000 pounds of thrust in a vacuum; 
each costs $30 million.
Fifteen years later, Marshall and ALS engine designers want an expendable, $3
million to $5 million LOX/LH2 engine with 300,000 to 600,000 pounds of thrust.
To achieve this goal, ALS engines will probably use a lower-pressure (2,000 psi)
combustion chamber and a gas generator to drive fuel pumps.
The gas generator cycle was perfected in the 1960s for the J-2 engines on
Saturn V monn rockets. Rockwell's space transportation division is now 
including an improved "J-2S" in its ALS vehicle design. The contrast between 
the SSME's "Indy racer" motor and the ALS's "pickup truck" engine extends
beyond performance, ALS propulsion officials said at a recent Society of
Automotive Engineers aerospace-vehicle conference. Engines for the ALS will
also use simpler machinery and automated "warning light" monitoring systems.
To cut engine costs, Marshall and the Air Force Astronautics Laboratoty (AFAL)
are sponsoring development of lower-cost engine components. AFAL is sponsoring
work on gas generators, combustion chambers, and engine health monitoring 
systems. Results from the engine program will be used for an ALS full scale 
development decision in fiscal year 1992.
Results from Air Force work will be integrated with LOX and LH2 turbopomps,
controls,and valves developed by Marshall, according to a manager of AFAL's
space transportation program (according to ground rules established by the
automotive engineers, all conference presentations were made on a not-for-
attribution basis).
Marshall's components will be based on designs studied in the center' space-
transportation main-engine program. Before the ALS effort focused on expendable 
engines, mai engine studies at Rocketdyne, Aerojet, TechSystems, and Pratt &
Whitney examined a reusable engine that operated at 2,400 to 3,200 psi. "The
chamber pressure will probably be coming down," said a Marshall project 
engineer.
After completing component-level tests at the laboratories, Marshall plans to
test the entire ALS engine at the National Space Technology Laboratories in 
Mississippi. ALS managers are now studying modification requirements for 
NSTL's SSME stand.
Components for ALS engines will be built with new fabrication techniques that
use precision castings to minimize welds. Rocketdyne believes it can cut
50 to 60 percent from the $3 million cost for an SSME thrust chamber. The 
ALS engine chamber will only need eight welds, according to a senior Rocketdyne
ALS official. The four-fold reduction in welds will lower costs and improve 
reliability.
Pumps and ijectors will also use precision castings as well as new materials
that will not need the protective coatings. These improved designs should 
result in a $14-million reduction from the cost of an SSME, the Rocketdyne 
official said.
Another $7.5-million reductionwill come by relaxing engine-design 
specifications. In addition to a gas generator cycle and lower chamber 
pressure, expendable engines will need fewer inspections. Subcontractors
will design components to government specifications rather than the more 
stringent requirements set by Rocketdyne.
Other cost reductions will be achieved if at least 50 engines are built
per year for at least 10 years. With a large production run, laser processing
and nondestructive computer-tomography inspections can cut $5 million from
the cost of each engine.
Marshall and AFAL are also cooperating on designs for ALS booster engines.
This work includes studies of reusable and expendable LOX/Hydrocarbon engines.
Liquid-engines could use methane, RP-1, or propane fuel to create 625,000
to 750,000 pounds of thrust at low altitudes (half the thrust produced by
Saturn F-1s). Marshall's booster program previously examined using LH2 for
a "tripropellant" engine, but the ALS program prefers an engine that only
needs LOX and hydrocarbon fuel.
Engine designers believed they needed LH2 coolant to reduce "coking" - carbon
deposits produced on copper engine parts during combustion. But recent sub-
scale firings at Aerojet and Rocketdyne suggest that coking can be reduced
by using advanced materials that will not react with methane.
AFAL is also examining advanced solid rocket boosters, wich would cost $800.000
each. These boosters would use monolithic fiber casings, eliminating the need
for joints and O-rings. Test firings of boosters up to 36 inches in diameter
suggest that nozzles can be eliminated from small boosters to reduce costs 
even more.



Wew, pant, finished !

Any comment ?
 

    >  a gas generator to drive fuel pumps.
    
    What's a gas generator, how does it work, and why is it better than
    turbopumps?
    
    Willie

    Big Dumb Booster lives again. The idea of making the engines so
    cheap that recovery costs more than a new engine surfaced in the
    late 60s and early 70s. A major proponent was TRW. They designed
    and built the engines for the Apollo Lunar Module. They had to be
    reliable above all else and that meant simple (at least to TRW).
    The resulting engines were extremely reliable and cheap. The LM
    descent engine was used for a while in the second stage of Delta
    ELVs.
    
    As proof of concept, TRW designed what was essentially a scaled
    up LM engine that required no expensive machining or assembly. They
    had parts made up and contracted to a shipyard to assemble the engines
    which were then fired up in a test stand. Apparently they worked,
    but this was the beginning of 'shuttle-or-bust' so NASA wasn't
    interested. I guess they weren't sufficiently gold plated to interest
    the Pentagon.
    
    The BDB is a good approach and the SSME development has stretched
    LH2 technology far enough that it should be possible to build a
    big, dumb, cheap LH2 engine.
    
    One other comment...
    
>for joints and O-rings. Test firings of boosters up to 36 inches in diameter
>suggest that nozzles can be eliminated from small boosters to reduce costs 

    I suspect this is some sort of misprint or comment taken out of
    context by the original author. If not, I'd like to know how they
    plan to build SRMs without nozzles (and have them operate with any
    efficiency).
    
    gary

    Might it be that the "no nozzles" comment means "no steerable nozzles"?
    
    Willie

    Thanks for entering that info.
    
    According to AW&ST, the Air Force has selected three contractors
    to go ahead with ALS development pending a full-scale development
    decision and selection of a prime contractor.  The three winners
    are Boeing Aerospace, Martin Marietta/McDonnell Douglas, and General
    Dynamics.  Target payloads are 110,000 to 140,000 lbs. to LEO
    equatorial and 150,000 lbs. to polar orbit.  Initial operational
    capability is projected for FY 1998, which is why Shuttle-C may
    be produced, to provide an interim heavy-lift vehicle for the mid-1990s.

    re .1
    
    A gas generator is usually a simple pyrotechnic device that generates
    lots of hot gas that is then used to drive turbopumps or pressurize
    tanks (in a pressure fed design). 
    
    The alternative is usually to bleed some exhaust gasses from the
    main combustion chamber and use it to drive the turbopumps. This
    complicates the design of the combustion chamber and the turbos
    have to be able to withstand the combustion temperatures (gas
    generators are usually designed to burn at a much lower temperature).
    Other schemes have small combustion chambers to drive the turbopumps.
    The exhaust from the turbos is clearly visible on some of the older
    missiles, e.g. Thor, Atlas, Jupiter where the turbo exhaust is used
    for roll control
    
    The SSMEs are even more complex. If I remember correctly, some of
    the propellant is burnt in a precombustion chamber and the turbos
    are driven by some of the exhaust from this. The gas is then fed
    back into the combustion chamber.  [I may have details of this wrong,
    its been a long time since I read up on this]
    
    The gas generator approach is much simpler although you can't do
    some types of operation easily with gas generator designs (e.g.
    restarting engines, throttling engines). They are well  suited to
    first stages if you assume that any anomolies in performance can
    be compensated for by the upper stages. I don't know what the Ariane
    engines do, but they do rely upon the 3rd stage to compensate for
    any preformance variation in the first two stages. The Soviet Proton
    is similar in this respect. Incidently, one report in AW&ST claims
    that the Proton first stage engines are gravity fed!! That certainly
    qualifies it as BDB.
    
    Since the SSMEs operate for the entire ascent and must be able to vary
    thrust to meet the shuttle's ascent profile, gas generators aren't
    really practical for them.
    
    gary
                                

    Yeah, I have a comment:
    
    HHHHHHHHOOOOOOOOORRRRRRRRRRAAAAAAAAAAAYYYYYYYYY!!!!!!!!!!!!!
    
    FINALLY, maybe, people are seeing the light. Like I said in previous
    notes, we need airline type maintainability, and turnaround times
    - not gold plating. We need launching punch at low cost - forget
    the frills. 
    
     I was a little confused by .0 - are people planning to make the
    engine reusable? I hope so. Imagine what would happen to airline
    travel if each engine had to be thrown away after each use.
    
     It's time for (gawk! I cannot believe I'm typing this) a little
    business common sense in launch vehicle design.
    
    			Gregg