Apollo - Saturn IB orbital insertion question

[Danderman]

Since Orion will separate from its launcher a few meters per second prior to orbital velocity, and perform the final injection with its own engine, was there ever a serious consideration during Apollo of doing the same? I believe that the amount of payload provided by Saturn IB would have been increased if Apollo had been loaded with extra propellant and used the SPS for final orbital injection. The downside is that this architecture would have precluded carrying extra payloads such as the ASTP Docking Module, but apart from that, what would have been the show stoppers for Apollo injecting itself into orbit a la Orion?

[edkyle99]

Since Orion will separate from its launcher a few meters per second prior to orbital velocity, and perform the final injection with its own engine, was there ever a serious consideration during Apollo of doing the same? I believe that the amount of payload provided by Saturn IB would have been increased if Apollo had been loaded with extra propellant and used the SPS for final orbital injection. The downside is that this architecture would have precluded carrying extra payloads such as the ASTP Docking Module, but apart from that, what would have been the show stoppers for Apollo injecting itself into orbit a la Orion?

My gut feeling, without running the numbers, is that the payload difference (to the final orbit) would have been marginal.  Manned spaceflight doctrine at the time probably ruled out any complications like these.

 - Ed Kyle

[Jim]

The downside is that this architecture would have precluded carrying extra payloads such as the ASTP Docking Module, but apart from that, what would have been the show stoppers for Apollo injecting itself into orbit a la Orion?

S-IVB disposal. 

[Danderman]

S-IVB disposal. 

Inquiring minds want to know why S-IVB disposal would be any more difficult than the similar procedure for the STS ET. Also, the Apollo baseline was to have the S-IVB simply re-enter at some random point in its orbit, a very messy disposal approach, so why would a suborbital S-IVB be any worse?

[Jim]

Inquiring minds want to know why S-IVB disposal would be any more difficult than the similar procedure for the STS ET. Also, the Apollo baseline was to have the S-IVB simply re-enter at some random point in its orbit, a very messy disposal approach, so why would a suborbital S-IVB be any worse?

The shuttle's trajectory (and payload performance) with OMS burns were designed from the beginning with ET disposal from the get go.   Not the S-IB

[Danderman]

The shuttle's trajectory (and payload performance) with OMS burns were designed from the beginning with ET disposal from the get go.   Not the S-IB

I hope you understand that the answer to the question as to why the S-IVB was not designed for a suborbital "release" of Apollo is not that it wasn't designed for that job.

[Danderman]

The shuttle's trajectory (and payload performance) with OMS burns were designed from the beginning with ET disposal from the get go.   Not the S-IB

Let me try this another way: what modifications, not including software, would have been required for the S-IVB to fly a trajectory where Apollo was left a few dozen meters per second short of orbit?

[Jim]

Probably, none, since it was an abort option.  But using the SM wouldn't increase payload to orbit performance.  It would be better to use the higher ISP of the S-IVB.  Using the SM would only allow for higher orbits but not more mass at lower orbits.

On another note, changes to trajectories are not just simple software changes.  Aeroloads, thermal loads, stability/controllability, RF link margins, range safety, etc  all have to be reanalyzed.  Range safety could be a big one.  There might have been off nominal scenarios where a suborbital S-IVB would have too high of risk for overflight (Europe)

WRT Shuttle trajectories, it is inserted into a 160 nmi orbit, the S-IB is shallower, around 95 to 125 nmi