Long burns - calibrated in real time?

[Not Myself]

I am guessing there are probably several here who could answer this one.

My question concerns the longer burns that would have taken place on a lunar mission, but would apply to the initial burn (the one starting with the rocket on the ground) for other flights as well.

So to go to the moon and get back, you need a big honkin' rocket to get off the ground and into earth orbit, then to go on its way to the moon (with these two initial phases possibly combined into one).  Upon arrival, you need to apply the brakes, and then add some power to head back to earth a while later.  If re-entry takes place straight away, then no need to go into earth orbit, in which case, I count three or four major burns, depending on whether you have an initial earth-orbit phase, or whether you just go straight from the ground into a trajectory towards the moon.

As these burns were no doubt subject to variability of the engine output, slight errors in the spacecraft orientation, atmospheric conditions (on the initial burn), and things like this, was the timing and power of the burn adjusted in real-time to ensure the spacecraft was where it needed to be and headed in the right direction by its end?  Or were these burns conducted by the initial plan, with any sources of error compensated for with smaller course corrections later?

[Chew]

was the timing and power of the burn adjusted in real-time to ensure the spacecraft was where it needed to be and headed in the right direction by its end?  Or were these burns conducted by the initial plan, with any sources of error compensated for with smaller course corrections later?

They were adjusted in real-time, but since few burns are perfect, course corrections were scheduled into the missions.

[ka9q]

There were several distinct sources of error that had to be accounted for.

One was the error in total burn delta-V due to small variations in propellant flow rate, specific impulse (engine efficiency), thrust ramp-up and tail-off times, inaccurate vehicle mass estimates, etc. Errors in major burns by the big Apollo engines (the big SPS on the back of the CSM and the LM descent and ascent engines) were measured in real time by the inertial guidance systems on each spacecraft. The "residuals" were trimmed, often by hand, after shutdown using the reaction control system (RCS) thrusters.

Another source of error was in the inertial guidance systems themselves, i.e., if they did not measure acceleration accurately and in the correct directions they would indicate residuals different from the actual values. To minimize these, the performance of the guidance systems were closely monitored throughout flight and their gyroscopic platforms frequently realigned to the fixed stars.

A third were caused by small, unmodeled forces such as urine dumps, dumps of excess CSM fuel cell water, and even steam emissions from the cooling sublimators on the LM. These and other small errors were usually detected by earth radio tracking, and mission control would work up periodic "mid course corrections" to remove them. These corrections were scheduled in advance for specific times, but were often skipped as unnecessary when the errors were small enough. Most of the mid course corrections that were done were also small enough to be performed with the reaction control thrusters.

[ka9q]

Since you asked about all of the big engine burns, including those starting on the ground, this would cover the Saturn V launch vehicle as well as the Apollo spacecraft.

The Saturn Instrument Unit (IU) housed its own guidance system completely separate from that on Apollo, though the Apollo guidance system could take over if the Saturn's failed. (Only one guidance failure occurred in the launch phase of an Apollo/Saturn flight, and it occurred "the other way"; Apollo 12's spacecraft guidance system momentarily lost power and "tumbled" due to two lightning strikes right after liftoff, but the guidance system in the Saturn V was unaffected. Otherwise the mission would certainly have been aborted.)

The IU follows several guidance algorithms during the launch phase. Because much of first stage flight takes place through the atmosphere, special rules apply that are quite different from those in use during powered flight in the vacuum of space. For one thing, the "angle of attack" -- the relative direction of the wind past the launch vehicle -- must be kept very small or the aerodynamic forces will rip the entire stack apart. Outside the atmosphere, angle of attack is irrelevant.

Also, the vehicle accelerates so quickly, reaching Mach 1 only about 60 seconds after launch, that drag minimization is a major factor in optimizing the launch trajectory. This requires a "lofted" trajectory that would be unnecessarily inefficient if an atmosphere weren't present (e.g., when launching from the moon). There may be other considerations such as avoiding hazards to coastal cities and offshore oil platforms, and making it possible to recover from an engine failure (as the recent Falcon 9 flight just did.) For these reasons, the Saturn first stage (and even most first stages today) fly a pre-programmed trajectory carefully designed and optimized ahead of time on the ground.

Special care has to be taken during staging so that no sudden steering commands will cause the stages to hit each other after separation.

Only after the rocket has left most of the atmosphere and staged is "closed loop" guidance usually initiated. Then the onboard system continuously computes, in real time, the most efficient trajectory from where it actually is to where it wants to be.

[Not Myself]

Wow, what a wealth of information!  Many thanks!

I will ask a follow-up question when I have a chance, but don't have the time right now.

[Not Myself]

So to summarise what I think I have learned here,

a) At no point in the flight is it is as simple as, "Point your nose in this direction and burn the engines at X% strength for Y seconds", there is always some real-time monitoring.

b) In the "get off the ground" phase, the spacecraft will typically follow a pre-defined trajectory, and real-time monitoring ensures that it moves back when it deviates from this path.  (How vigorously to do so must be part of the plan.)

c) One out of the atmosphere, the spacecraft normally switches over to a mode whereby it doesn't worry about the trajectory, but about reaching the end goal.

Does that sound right?

Follow-up question - the US space shuttle, and also Buran, clearly had active aerodynamic control surfaces, although I think these only get used on the way down.  Most rockets would appear to have passive design features geared to providing desirable aerodynamic performance - presumably lets why they're long, thin, and pointy at one end, and I don't think those fins are there to make them look cool.  But do these rockets have any active aerodynamic control surfaces, or is 100% of the active control from the engines and thrusters?

[Donnie B.]

As far as I'm aware, the only active aerodynamic control surfaces were on the Launch Escape System, and those would only deploy in the event of an abort (hence were never used during any actual flights).  Others may have better information, though.

Of course, the entire CM was a control surface of a sort during reentry.  It was operated by the mass distribution and by reaction jets that changed the angle of attack on the fly.

[Echnaton]

As far as I'm aware, the only active aerodynamic control surfaces were on the Launch Escape System

The Saturn V had fins with flight control surfaces.

[Al Johnston]

That shows the fin had two pieces, but there doesn't appear to be any actuator to move the aft section...

[Echnaton]

A little more reading indicates that the fins were for stabilization only.  Not flight control, which could be accomplished better by engine gimbaling, which may have also eliminated the need for fins altogether. 

[Al Johnston]

Indeed: proposed later versions of the Saturn V did without the fins

[raven]

I think they were added because Von Braun loved winged rockets.

[Count Zero]

My favorite clip of a rocket guidance system in action is this newsreel from 9 years before the first manned moon landing.  At the 0:58 second mark, you see the Polaris SLBM thundering out of the water angled ~40 degrees to the right, then correct and start climbing up and to the left.

[Not Myself]

Thanks to all for the answers.

My favorite clip of a rocket guidance system in action is this newsreel from 9 years before the first manned moon landing.  At the 0:58 second mark, you see the Polaris SLBM thundering out of the water angled ~40 degrees to the right, then correct and start climbing up and to the left.

Unfortunately, I find myself at the moment in a jurisdiction which does not consider free and unfettered access to all the material on the internet to be a good thing.  I will have to watch the video some other time.

[Echnaton]

Thanks to all for the answers.

My favorite clip of a rocket guidance system in action is this newsreel from 9 years before the first manned moon landing.  At the 0:58 second mark, you see the Polaris SLBM thundering out of the water angled ~40 degrees to the right, then correct and start climbing up and to the left.

Unfortunately, I find myself at the moment in a jurisdiction which does not consider free and unfettered access to all the material on the internet to be a good thing.  I will have to watch the video some other time.

You need a good VPN.