From: jwatts@stego.rose.hp.com   
      
   In another thread Proponent@gmx.net suggested that the RS-68 might   
   make a better engine for a shuttle derived vehicles this prompted me   
   to go to "Encyclopedia Astronautica" to compare specs.  I was a little   
   confused by what I found.  Here is what I saw   
      
   			SSME		RS-68   
   Thrust(vac):		232,301 kgf. 	337,807 kgf.   
   Thrust(vac):	 	2,278.00 kN.	3,312.00 kN.   
   Isp: 			453 sec. 	420 sec.   
   Isp (sea level): 	363 sec. 	365 sec.   
   Burn time: 		480 sec.   
   Mass Engine: 		3,177 kg. 	6,597 kg.   
   Diameter: 		1.63 m.   
   Length: 		4.24 m.   
   Chambers: 		1. 		1.   
   Chamber Pressure: 	204.08 bar. 	95.92 bar.   
   Area Ratio: 		77.5. 		21.5.   
   Oxidizer to Fuel Ratio: 6.   
   Thrust to Weight Ratio: 73.11978	51.2   
      
   What I find confusing here is part of what I thought I understood   
   about the design trade-offs in the SSME.  I thought the the SSMEs high   
   chamber pressure was chosen to allow a high expansion ratio as sea   
   level and that a high expansion ratio at sea level was needed to   
   improve sea level Isp.  However from these data the RS-68 achieves a   
   (slightly) higher sea level Isp despite having less than half the   
   chamber pressure and less than one third the expansion ratio.  How is   
   this possible?  Is what I thought I knew incorrect?  Or is there   
   something else I am missing?   
      
   I await enlightenment by those far more knowledgeable than myself.   
   --   
   Jon Watts			  916-785-8181   
   Procurve Networking Business	  email:  jwatts@rose.hp.com   
   Roseville, CA			  M/S 5672  Bldg: R3L  Pole:E3   
      
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