Author Topic: Why the Roll program  (Read 17846 times)

Offline Steve_the_Deev

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Why the Roll program
« on: 12/02/2005 09:00 am »
OK I found out why the fist roll program about T+15 secs?  I contacted Dave "Shuttle Guy"  and here is his explanation:


Hi Steve'
I have registered there as shuttle_guy but have not made the time to post.

My understanding of the Roll is to maintain a zero angle of attack on the Orbiter wings otherwise they would fail. There is not enough dynamic pressure at SRB sep to worry about vehicle attitude. ET sep is the same plus there is no worry about foam loss around the umbilical again due to the low dynamic pressure. After SRB sep foam loss is not a concern due to the low dynamic pressure, no matter how big the foam piece is.

My only problem with Shuttle Guys answer and I admire and respect him but on reentry dont the wings take one heck of a beating too?


What NASA did do which I think is totally cool is after the SRBs were kicked off the Shuttle stack including ET with some cryos still in it.  They rolled the Shuttled so the Shuttle was on top until they dropped of the ET after MECO. Like a Polo player riding his horse!

Offline RedSky

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RE: Why the Roll program
« Reply #1 on: 12/02/2005 12:11 pm »
Quote
Steve_the_Deev - 2/12/2005  4:00 AM
What NASA did do which I think is totally cool is after the SRBs were kicked off the Shuttle stack including ET with some cryos still in it.  They rolled the Shuttled so the Shuttle was on top until they dropped of the ET after MECO. Like a Polo player riding his horse!

Yes, that roll to "heads up" after SRB sep is actually a very recent thing.  For most of the STS program, they stayed heads down through orbital insertion.  I don't remember the reason for the change... but I do recall something about communications... better link with a TDRS or something in a heads up orientation.

Also, while the shuttle "roll program" shortly after launch may have something to do with wings, I think all large guided launchers (i.e., Saturns, Atlas, Deltas, etc) have (or had) roll programs shortly after launch, even though they are wingless and basically symmetrical cylinders to our view.  In reality, I think, the rocket steering software, and perhaps some comm antennas, etc, establish a preferred (required) top side and bottom side to the vehicle for its ride to orbit.  Depending on the orientation of the launch pad, the inclination they are launching to, etc, the rocket will likely have to "roll" shortly after liftoff so that it has its "top side" and bottom side correctly oriented "up" and "down" for the ride.



Offline Rob in KC

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RE: Why the Roll program
« Reply #2 on: 12/02/2005 02:57 pm »
And there was me thinking it was all about the pad being located in the wrong direction.

Offline British NASA

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RE: Why the Roll program
« Reply #3 on: 12/02/2005 05:19 pm »
Quote
Steve_the_Deev - 2/12/2005  4:00 AM

OK I found out why the fist roll program about T+15 secs?  I contacted Dave "Shuttle Guy"  and here is his explanation:


Hi Steve'
I have registered there as shuttle_guy but have not made the time to post.

My understanding of the Roll is to maintain a zero angle of attack on the Orbiter wings otherwise they would fail. There is not enough dynamic pressure at SRB sep to worry about vehicle attitude. ET sep is the same plus there is no worry about foam loss around the umbilical again due to the low dynamic pressure. After SRB sep foam loss is not a concern due to the low dynamic pressure, no matter how big the foam piece is.

My only problem with Shuttle Guys answer and I admire and respect him but on reentry dont the wings take one heck of a beating too?



Well yes, but it seems you asked him about the Roll, and he's answered about the Roll.

Offline Dana

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RE: Why the Roll program
« Reply #4 on: 12/03/2005 10:21 am »
It also probably has to do with weight distribution and also positions the vehicle for the RTLS (Return To Launch Site) abort mode, a truly scary split-S maneuver that thankfully has never been used.
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Offline Rocket Guy

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RE: Why the Roll program
« Reply #5 on: 12/03/2005 03:43 pm »
There are basically four reasons they roll the Shuttle after tower clear, and it's not really about needing to orient the pad south despite what some people say:

a) So the astronauts can see the horizon.

b) to allow better line of sight to the Orbiter S-Band antenna which is located above the crew cabin.

c) To lessen the effects/stresses on the structure.

and

d) To balance off the SSME thrust vector which is into the center of the stack.

Other vehicles roll for other reasons but usually for antenna pointing.

Offline Mark Max Q

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RE: Why the Roll program
« Reply #6 on: 12/03/2005 06:42 pm »
Quote
Dana - 3/12/2005  5:21 AM

It also probably has to do with weight distribution and also positions the vehicle for the RTLS (Return To Launch Site) abort mode, a truly scary split-S maneuver that thankfully has never been used.

What is this maneuver and why is it scary?

Offline David AF

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RE: Why the Roll program
« Reply #7 on: 12/03/2005 07:05 pm »
Quote
Mark Max Q - 3/12/2005  1:42 PM

Quote
Dana - 3/12/2005  5:21 AM

It also probably has to do with weight distribution and also positions the vehicle for the RTLS (Return To Launch Site) abort mode, a truly scary split-S maneuver that thankfully has never been used.

What is this maneuver and why is it scary?

When they make the turn for home there's a risk the wings would fail.
F-22 Raptor instructor

Offline rsp1202

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Offline kneecaps

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Re: Why the Roll program
« Reply #9 on: 03/10/2007 09:40 pm »
Despite being a rather long dead thread, the answer to the question was left unanswered (correctly anyway), the stack rolls to target the correct orbital insertion plane. Since the pads are the same ones used for Apollo they are facing the wrong way to allow the shuttle stack to lauch into the correct plane without the roll.

Secondary reasons are that heads down provides better situational awareness for the crew.

On the roll to heads up towards the end of the ride 'uphill' that was done so they could acquire TDRS comms sooner in the ascent, eliminating the need to keep the Bermuda comm site open. Closing Bermuda saved millions.
Allow subject to scream. In space no one will hear.

Offline Austin

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Re: Why the Roll program
« Reply #10 on: 03/10/2007 10:17 pm »
Also, the SRBs need to be jettisoned over the water...away from populated areas.

Offline Mark Dave

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Re: Why the Roll program
« Reply #11 on: 03/10/2007 11:42 pm »
In documentary footage of the Apollo flights, the big Saturn V also does a roll/pitch program. The thread on the Good video of Apollo 11 you can hear the crew say they had done the maneauver right after clearing the tower. Though with the shuttle it's easy to see the vehicle did such a thing, not with the earlier rockets.

Offline Jim

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Re: Why the Roll program
« Reply #12 on: 03/11/2007 12:33 am »
Quote
MarkD - 10/3/2007  7:42 PM

In documentary footage of the Apollo flights, the big Saturn V also does a roll/pitch program. The thread on the Good video of Apollo 11 you can hear the crew say they had done the maneauver right after clearing the tower. Though with the shuttle it's easy to see the vehicle did such a thing, not with the earlier rockets.

Most launch vehicles have "roll programs" to align them with the flight azimuth.  Just as a crew likes to be heads ups with shoulders perpencucular to the flight path so do most guidance systems.  In fact, many launch vehicle's major axes are aligned like an aircraft with the X-axis running the length of the vehicle.  This orientation may be a little confusion on the launch pad but works perfectly for flight.  Conversely, many spacecraft have the Z-axis vertically.   This is always something to be aware of in launch vehicle/spacecraft integration, especially when it comes point requirements

Offline Dana

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Re: Why the Roll program
« Reply #13 on: 03/11/2007 12:53 am »
Quote
MarkD - 10/3/2007  4:42 PM

In documentary footage of the Apollo flights, the big Saturn V also does a roll/pitch program. The thread on the Good video of Apollo 11 you can hear the crew say they had done the maneauver right after clearing the tower. Though with the shuttle it's easy to see the vehicle did such a thing, not with the earlier rockets.

You have to look at the black-and-white panels painted on the vehicles. Look very closely at some videos and you can see them rotate around.

First, right at liftoff, the Saturn V yawed away from the tower (not usually called out by the CDR the way the other guidance programs were-Pete Conrad on Apollo 12 was the only one that called the yaw maneuver), then it clears the tower and at about 11-12 seconds after liftoff (roughly coinciding with clearing the tower and handing over from LCC to Houston) started the roll program, completed that a few seconds later and then started the pitchover program.

Can't remember if it was Jim Lovell or Mike Collins who in a book said the Gemini-Titan II liked to do its navigating with its payload laid over on its side-when you saw the horizon, it was (from the astronauts' perspective) vertical in the windows. They then jokingly went on to say that they could feel the Titan II's nose wander back and forth as it continually reminded itself to go to orbit and not to Moscow! :)
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Offline Thorny

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Re: Why the Roll program
« Reply #14 on: 03/11/2007 01:36 am »
Quote
kneecaps - 10/3/2007  4:40 PM

Despite being a rather long dead thread, the answer to the question was left unanswered (correctly anyway), the stack rolls to target the correct orbital insertion plane. Since the pads are the same ones used for Apollo they are facing the wrong way to allow the shuttle stack to lauch into the correct plane without the roll.

As others pointed out, Saturn V rolled, too. Since the Shuttle (and Saturn/Apollo before it) doesn't launch into the same orbit on every mission, they'd either have to built a rotating launch pad, or rotate the vehicle after launch. And since the vehicle has to maneuver some anyway, it was logical to just roll the vehicle to the desired heading.


Offline dvandorn

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RE: Why the Roll program
« Reply #15 on: 03/11/2007 02:19 am »
The Shuttle stack has to maneuver to specific orientations as it rises through the thickest air to manage stresses on the vehicle.  But, as has been said, most rockets have executed roll programs shortly after lift-off.  And, as has been said, this is due to steering to a specific launch azimuth.

The reasons are slightly more complex, though.  For one thing, the roll program was initiated very early, during vertical flight, often to test engine gimballing.  That's why the roll program would start almost immediately after clearing launch structures.  If something was going to go south with the engine gimbals, they figured a crisp roll of the nearly fully-fueled vehicle would precipitate it, leaving them time to get their abort options in line (and giving them a good idea whether or not the thing was actually going to fly downrange and not fall back down on top of their heads).

For another, while a rocket could conceivably place itself on its azimuth from any roll position, the guidance is designed to align the rocket's body axis co-ordinates to a specified trajectory (specifically, to set angles between the body axis co-ordinates and a stable reference, like local vertical at the launch site).  So, no matter where the rocket starts out in terms of pitch orientation, it's going to place itself based on its assigned body axis definition.

Since your launch azimuth is likely going to vary from flight to flight (or from one launch window to the next), it makes no sense to try and set your pad up so that the desired roll attitude would automatically result from a straight pitchover onto your desired azimuth.  The best solution is to not worry about the pad's roll orientation, just roll the vehicle to the desired position after lift-off.

And, of course, for manned launches, you want to provide specific out-the-window visibility for manual flight maneuvers (like aborts and such).  So, your desired attitude factors in, for example, any need for the pilots to see a horizon during various portions of their flight.

While Apollo crews rode into orbit in a "heads-down" position (feet away from Earth, heads toward Earth), Gemini crews did ride into orbit on their sides.  This supposedly gave the CDR the best vantage for eyeballing Titan II trajectory dispersions.  Mercury crews, I'm not sure -- anyone remember the roll orientation of the Redstone and Atlas vehicles vis-a-vis astronaut orientation?
-Doug

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Offline Stevo

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Re: Why the Roll program
« Reply #16 on: 03/12/2007 05:33 am »
Here's something I've always wondered. For a 28.5 degree inclination orbit, the shuttle rolls 90 degrees after clearing the tower for a due east trajectory. For a 57 degree inclination orbit, the shuttle rolls 145 degrees after clearing the tower for a northeast trajectory (furthest north allowed from KSC). The 1988 NSTS Reference Manual states that the most southerly allowable trajectory from KSC is a 120 degree azimuth (60 degree roll after tower clear) to place the shuttle in a 39 degree inclination orbit.

http://science.ksc.nasa.gov/shuttle/technology/images/launch_sites_8.jpg

My question is this. 39 degree inclinations were flown multiple times in the shuttle program, but they all involved east-northeast trajectories (~120 degree roll after tower clear). Was there a reason why NASA chose to launch on an east-northeast trajectory rather than an east-southeast trajectory with a lesser roll program? My only guess is that this was done because it allowed for utilization of multiple TAL sites rather than just Banjul (if it is even reachable from an east-southeast azimuth).

Steve

Offline Jim

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Re: Why the Roll program
« Reply #17 on: 03/12/2007 09:55 am »
Correct.  The shuttle program did not set up "southern" TAL sites

Offline C5C6

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Re: Why the Roll program
« Reply #18 on: 03/12/2007 10:23 am »
I thought the main reason for roll program was to set the orbit inclination...in page 1 there are other reasons...

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