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Author
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Topic: Apollo service module over-sized engine bell
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Captain Apollo
Member Posts: 260
From: UK
Registered: Jun 2004
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posted 06-08-2015 06:30 AM
   
Was the SPS engine bell on the Apollo service module over-sized? I have read it was due to early design decisions related to a direct ascent method — in essence the design was fixed. But with weight such an issue why not change it once lunar orbit rendezvous was chosen? |
Robert Pearlman
Editor Posts: 43576
From: Houston, TX
Registered: Nov 1999
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posted 06-08-2015 07:27 AM
Aerojet was awarded the contract and authority to begin work on the SPS prior to the decision to switch to lunar orbit rendezvous. A redesign would, of course, have required the time for the technical details to be worked out, but it could have also meant that the contract would have to be recompeted, delaying the service module even further. |
MadSci
Member Posts: 230
From: Maryland, USA
Registered: Oct 2008
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posted 06-13-2015 12:00 PM
Interesting question. I admit, I had often looked a that big bell and wondered the same thing. Thanks for the information! |
GACspaceguy
Member Posts: 2516
From: Guyton, GA
Registered: Jan 2006
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posted 06-13-2015 12:44 PM
Engine bell design is an interesting tradeoff for an engine that operates in the atmosphere. If you look at the lift off footage of the Saturn V you see that the exhaust gases are in a parallel stream. However, when the vehicle is high in altitude the back end of the vehicle is engulfed in the ejected gases. This is because the size of the bell final diameter is based on the local pressure, the “back pressure” which at sea level is 14.7psi. The bell is optimized for when the vehicle needs its maximum thrust, at lift off and there is a backpressure of 14.7psi. When the vehicle gains altitude the pressure decreases, thus the back pressure reduces and the exhaust gases are “under expanded” such that when they exit the confines of engine bell they expand rapidly and roll around the back end of the vehicle. So how does this play into the SPS engine? That engine is designed for a back pressure in space, which is zero. Therefore the engine bell is designed for that condition and the bell becomes very large for the maximum thrust of the expanding gases with a steady state back pressure of zero. This can also be seen the the RCS thusters, the bell is very large for the actual size of the engine itself. I trust this helps.
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Jurg Bolli
Member Posts: 994
From: Albuquerque, NM
Registered: Nov 2000
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posted 06-13-2015 04:16 PM
Excellent explanation, thanks. |
Spoon
Member Posts: 148
From: Cumbria, UK
Registered: May 2006
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posted 04-26-2016 11:41 AM
quote:
Originally posted by Robert Pearlman:
Aerojet was awarded the contract and authority to begin work on the SPS prior to the decision to switch to lunar orbit rendezvous.
Apologies for bumping an old thread Robert, but would you be able to point me in the right direction regarding reading up on this further? |
Headshot
Member Posts: 891
From: Vancouver, WA, USA
Registered: Feb 2012
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posted 04-26-2016 06:18 PM
Were there some emergency launch scenarios that might have required the SPS to fire while in the Earth's atmosphere, albeit at a high altitude? |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 04-26-2016 10:13 PM
Yes. |
Kizzi
Member Posts: 32
From: Manchester, England
Registered: Apr 2012
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posted 05-30-2016 03:03 PM
I've found this source. North American Aviation won the contract to build the CSM, and also the second stage of the Saturn V launch vehicle for NASA. Because the CSM design was started early before the selection of lunar orbit rendezvous, the service propulsion engine was sized to lift the CSM off of the Moon, and thus was oversized to about twice the thrust required for translunar flight.  |
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