Author Topic: Modification of the SSME for altitude start  (Read 4620 times)

Offline TyMoore

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Modification of the SSME for altitude start
« on: 08/09/2006 07:17 pm »
The selection of the J-2X  engine for the CEV was primarily influenced by the needs of "altitude" starting. The J-2 was originally designed with this need in mind, and each engine has a sphere which  contains a small supply of hypergolic propellant (I'm not sure what they used, but I'll bet someone here knows!)  Some hypergol started motors can even be vacuum restarted several times, as the J-2's used on the S-IV stage of the Apollo missions.

So my question is this, what would it take to make an SSME vacuum startable? Obviously it must contain something like a hypergol ignition system, possibly even externaly mounted pyrotechnics to burnoff the initial hydrogen plume to eliminate the possibility of an explosion. If a hypergol ignition system is used, what hypergolic chemical can be used?

The choice of a vacuum startable/restartable SSME has some advantages because of the engine's high performance and high thrust.

Offline Damon Hill

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Re: Modification of the SSME for altitude start
« Reply #1 on: 08/09/2006 07:36 pm »
The SSME uses spark plugs in the gas generators, just like your car.  I think the J-2 was similar and I know the routinely restartable RL-10 uses redundant spark plugs in its combustion chamber (expander cycle, doesn't have a hot gas generator).  A lot of the SSME's start equipment, probably pumps and pressurization gear was external to save weight--it's a remarkably complex engine with a critical startup sequence; lots of things have to happen with very precise timing, temperatures, pressures, etc.

The J-2X will be substantially improved, with a significantly higher Isp than its predecessor, so I'm not sure this exercise will be necessary.

Also see the discussion, SSME vs J-2 here:

http://www.nasaspaceflight.com/forum/forums/thread-view.asp?tid=1138&start=1

Offline Jim

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Re: Modification of the SSME for altitude start
« Reply #2 on: 08/09/2006 07:49 pm »
J-2 did not use hypergolic start  propellants (F-1 did), it used spark plugs.  The start tanks are fill with LO2 and LH2 and  was more for restarts.

The SSME is a head start engine and requires specific conditions to start.  Altitude starts are difficult because:  1.  Atmospheric pressure is different.  2.  Conditioning the engine would require dumping propellant or carrying purge gas, both which would reduce performance.

Restarting is not currently possible because of water remaining in the engine

Offline TyMoore

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Re: Modification of the SSME for altitude start
« Reply #3 on: 08/09/2006 08:09 pm »
Thanks all for the clarifications. I didn't realize the J-2's were spark started. I knew about the spark starting of the preburners and main combustion chamber for the SSME, but I wasn't to sure about the start conditions. And the requirement for a purge gas (purgent...?) definately makes the engine of limited utility here...

I like the COBRA concept, but I don't think the US is developing it anymore.

I'm not sure what kind of performance the J-2X is expected to have but I understand it is a lot simpler than the SSME.



Offline yinzer

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Re: Modification of the SSME for altitude start
« Reply #4 on: 08/09/2006 10:45 pm »
Ignition and starting are not the same thing - look at your car engine.  Sure, it has a distributor and spark plugs and fuel injectors and all that kind of stuff to "light the fire", but it also has a very large battery and electric motor to spin the engine up to a speed where it can idle.  Rocket engines have similar systems - they need a way to get propellants into the gas generator at a reasonable pressure so it can start.  On large ground-start engines, the propellant tanks are far enough above the engine that the hydraulic pressure will do the trick.  The J-2 used spheres holding compressed gas to spin the turbine until the gas generator lit; I think the SpaceX Merlin uses a solid propellant gas generator to drive the turbine in the same manner.  The compressed gas sphere can be refilled in flight, so the J-2 can theoretically restart as many times as desired; the Merlin can hold 3 start cartridges.  The RL-10 is elegant in that the power to drive the turbine comes from heating liquid hydrogen in the thrust chamber walls (usually with the heat of combustion), but there's enough stored heat in the engine at room temperature to start it.

Now, if an engine is designed to use the pressure from the propellants' weight to start (like the SSME), it can only start while the propellants have weight (on the ground, or under acceleration) - it can't start in free-fall, like it would be after separating from the lower stage.  The SSME is also designed very cleverly to work just so, which makes modification difficult.
California 2008 - taking rights from people and giving rights to chickens.

Offline Damon Hill

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Re: Modification of the SSME for altitude start
« Reply #5 on: 08/09/2006 11:10 pm »
The J-2X lacks the SSME's complex dual preburners/turbopumps, hot gas manifolding, and doesn't have to operate at such high pressures; it's definitely a much simpler engine, and the J-2S with its tapoff cycle didn't even have a separate hot gas generator.

The specifications vary, but I think it's safe to assume that the J-2X will have an Isp of around 450 seconds, very close to the SSME's 455 seconds,
and around 290,000 lbs/thrust, which is almost 50% more than the original J-2.  It's going to be different enough that it hardly seems accurate to so closely associate with the original J-2 to give it the same name.  Well, we'll see how things actually work out; I think only Pratt & Whitney/Rocketdyne really has a clear idea of the details.

I'm curious to see how much of the engine will be the traditional welded/brazed tube (I've seen a J-2 at Stennis, beautiful stainless steel welding throughout with interleaved tubing and manifolding), and whether modern milled wall will be used to any degree, and what other improvements to lower manufacturing cost may be used.  One would think 40+ years of experience, new manufacturing techniques, and powerful computer modeling tools would make some diffferences.  Look at how simplified the RS-68 is; that contributed to a faster and much cheaper development cycle.

Offline simonbp

Re: Modification of the SSME for altitude start
« Reply #6 on: 08/10/2006 01:54 am »
Quote
Damon Hill - 9/8/2006  3:57 PM

I'm curious to see how much of the engine will be the traditional welded/brazed tube (I've seen a J-2 at Stennis, beautiful stainless steel welding throughout with interleaved tubing and manifolding), and whether modern milled wall will be used to any degree, and what other improvements to lower manufacturing cost may be used.  One would think 40+ years of experience, new manufacturing techniques, and powerful computer modeling tools would make some diffferences.  Look at how simplified the RS-68 is; that contributed to a faster and much cheaper development cycle.

After watching the minute millings on the walls of a Merlin 1C (the new, regen version) thrust chamber be cut with the greatest of ease on a gigantic CNC machine, and seeing a nozzle for the same where all the tube bends had been done robotically, I certainly have hope that the J-2X can done far less labour-intensively than its predecessor.

The RL-10 is closest current engine to the J-2, and made by the same company. Does anyone know how its manufacture has changed over the past 40 years?

Simon ;)

Offline Jim

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Re: Modification of the SSME for altitude start
« Reply #7 on: 08/10/2006 02:01 am »
Brazed tubes

Offline TyMoore

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Re: Modification of the SSME for altitude start
« Reply #8 on: 08/10/2006 02:33 am »
Thanks for the replies--very interesting stuff.

I should have realized that the term "head start" should mean the natural pressure of the liquid propellant column leading back up to the propellant tanks--and this is the initial propellant feed pressure: the gravity head with the pressure head on top of that.

Yes, I can now see how modification of the SSME would be very difficult, because it would literally require the vehicle to accelerate at one g just to get the engines started, which is very difficult to right after a stage sep. And if your ullage rockets are that powerful, what do you need the SSME for anyway! Doing away with the head start would mean such a drastic systems redesign of the engine that you might just as well "clean sheet" a new engine from scratch.

Since we're discussing modifications of engines, how about regeneratively cooled engines burning hydrocarbons. The tendency of hydrocarbon based fuels to coke in the cooling channels was a problem in some of the early engines, and if I recall was one of the factors limiting the F-1's ability to be reused. However, I was reading an article (I can't find it again, and I don't remember the title...) that seemed to indicate that coking of the fuel was a function of the sulfur content, especially with solid copper or clad cooling channels. The coking products were compounds of copper, sulfur and carbon, which could plug them tight. The conclusion was two fold: reduce the sulfur content of the kerosene based fuel by better refining practices, and gold plating the cooling channels to keep any sulfur away from any copper in the jacket.

Does anyone know if this method is actually used in kerosene engines? And if so, does it seem to work? What does the Russian RD-170/171 use?

Offline Damon Hill

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Re: Modification of the SSME for altitude start
« Reply #9 on: 08/10/2006 03:03 am »
Coatings probably helped; the Russians with all their oxidizer-rich engine designs know a lot about that.  Lowering sulfur content helps, which I think is part of the RP-1 specification.  And keeping the cooling tubes within temperature limits may have the most to do with it; that's a function of design and coolant flow rates.  Pre-chilling the RP-1 and using a synthetic mix of hydrocarbons with zero sulfur and other contaminants would help, but those steps could be a bit impractical and certainly a Sintin approach is rather expensive, which is why the Russians stopped using a synthetic fuel.

Since hydrocarbon rocket engines are virtually never reused, coking apparently has never been that much of an issue in practice.  I think the RD-170 family uses a mix of the above practices, and claim it isn't a problem, to the point that the engines don't require 'de-greasing' after a test firing.

SpaceX is certainly going to find out the realities as they develop their RP-1 engines.  They'll certainly be very aware of the problem.

Offline hop

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Re: Modification of the SSME for altitude start
« Reply #10 on: 08/10/2006 09:42 am »
It's worth noting that quite a number of "non re-usable" Lox/kero engines have demonstrated multiple firings. Areojet claims their Russian surplus NK engines are re-usable, although they were not designed as such.

Offline Jim

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Re: Modification of the SSME for altitude start
« Reply #11 on: 08/10/2006 11:28 am »
Quote
TyMoore - 9/8/2006  10:20 PM
Does anyone know if this method is actually used in kerosene engines? And if so, does it seem to work? What does the Russian RD-170/171 use?

The RD family use oxidizer rich staged combustion to eliminate coking.  The 170 was to be reused for 10 flights.

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