NASASpaceFlight.com Forum
SLS / Orion / Beyond-LEO HSF - Constellation => Orion and Exploration Vehicles => Topic started by: CuddlyRocket on 07/31/2006 01:20 pm
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I noticed in the latest iteration on the CEV that the RCS could be used for TEI instead of the OMS.
What else is the OMS used for? Could the RCS substitute for these uses as well? In which case, what is the purpose of having the OMS? Couldn't you just rely on the RCS? Even if you added a couple more RCS clusters for improved reliability and given inferior Isp, the result should be smaller and lighter.
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This was only for a back up to the OMS. It takes too long and requires too many burns . Other vehicles have the same thing. Skylab CSM could deorbit using RCS. So could the shuttle. It is just prudent planning. The CEV OMS is used for orbit insertion, aborts, and earth orbit rendezvous burns as well as the TEI
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The OMS is currently rated as a 10,000 lbf engine capable of producing a delta-V of 5,849 ft/sec. The RCS are 100 lbf engines each (one-hundredth of the OMS) which are capable of producing a delta-V of 237 ft/sec (which is about 25 times less, which fits as the four rear-facing RCS engines would fire simultaneously).
If you had six RCS clusters, they would produce a delta-V of 355 ft/sec, or one-seventeenth of the OMS. So any manouvere that you could do with the OMS could be done with the RCS (subject to reliability), except that it takes longer. (Seventeen times longer in my scenario - or a bit less as the SM would presumably be lighter without the OMS etc.) What manouveres are being performed where you are under such time pressure that you can't take 17 times longer for the burn?
The TEI insertion is the largest delta-V that the OMS is called upon to perform - but who cares if an RCS burn takes 17 times longer? It takes days to get back to Earth anyway. (And notice the g-forces are 17 times less, so should be <1g and therefore no problem for the astronauts enduring them for lengthy periods.)
There's an analogy here with electric-propulsion. It takes longer, but is more efficient, and if you have the time anyway.....
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CuddlyRocket - 31/7/2006 12:59 PM
The TEI insertion is the largest delta-V that the OMS is called upon to perform - but who cares if an RCS burn takes 17 times longer? It takes days to get back to Earth anyway. (And notice the g-forces are 17 times less, so should be <1g and therefore no problem for the astronauts enduring them for lengthy periods.)
There's an analogy here with electric-propulsion. It takes longer, but is more efficient, and if you have the time anyway.....
It is not an analogy with electric-propulsion. Electric-propulsion has ISP's 100's of times more than chemical engine but at less of a thrust. that's why they are more beneficial. The RCS thruster ISP is less than the OMS, therefore requires more propellant for the same delta V.
They do care that it takes longer. The longer RCS burn can't be done all at once because of the low thrust. Because of orbit phasing it has to be done over 3 days. This impacts the consumables that is carried on board.
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Using the RCS to break Lunar Gravity makes you wonder why we need a main engine for the ISS final orbital insertion.
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SMetch - 31/7/2006 3:45 PM
Using the RCS to break Lunar Gravity makes you wonder why we need a main engine for the ISS final orbital insertion.
no, it doesn't. The RCS is a backup.
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Jim - 31/7/2006 6:12 PM
SMetch - 31/7/2006 3:45 PM
Using the RCS to break Lunar Gravity makes you wonder why we need a main engine for the ISS final orbital insertion.
no, it doesn't. The RCS is a backup.
Isn’t the T/W of the CEV’s RCS higher than the Space Shuttle's OMS? The DV for final orbit is very low with Space Shuttle and CEV if it followed a similar trajectory.
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The problem with all this is the extra 1-2 days doing an RCS TEI burn entails - that means 2-3 extra days of supplies that the CSM has to carry for the crew.
Also, if you have no OMS, then what is the back-up when RCS fails? Better to have a good and a bad system, and plan on using the good while keeping the bad in reserve...
Simon ;)
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SMetch - 31/7/2006 9:43 PM
Jim - 31/7/2006 6:12 PM
SMetch - 31/7/2006 3:45 PM
Using the RCS to break Lunar Gravity makes you wonder why we need a main engine for the ISS final orbital insertion.
no, it doesn't. The RCS is a backup.
Isn’t the T/W of the CEV’s RCS higher than the Space Shuttle's OMS? The DV for final orbit is very low with Space Shuttle and CEV if it followed a similar trajectory.
ELV's don't follow the shuttle trajectory. Aborts require higher thrust. Rendezvous and deorbit require high thrust for easier targeting. RCS thrusters have lower ISP.