Let’s hear it for “Delta Guidance”!

See list attachedOctober 21, 196969-PA-T-130APA/Chief, Apollo Data Priority CoordinationLet’s hear it for “Delta Guidance”!

As part of the Apollo software team's contribution in the search for extra LM hover time and/or payload capability, they are vigorously working on the development of a new descent guidance and throttle control technique. The pay off could be impressive compared to things like trying to decrease LM weight. Specifically, a ΔV improvement on the nominal mission of as much as 100 fps might be realized, which is equivalent to 18 seconds of hover or 300 lbs. increase in descent payload. There are also some other substantial benefits to be gained from this new program formulation. It is the purpose of this memo to make sure you know about this business as well as to give you a report on its status.

Sometime ago a couple of Guidance and Control Division (GCD) people, Tom Moore, Jay Montgomery – and others I am sure – conceived the basic idea of what they called “Delta Guidance.” The unique characteristic of this guidance scheme, as I understand it, is that given a dispersion it attempts to guide the spacecraft back to the nominal trajectory as opposed to looking for a new way of achieving the targeted end conditions like most guidance techniques do. It appears that this can be done with- out significant penalty in terms of payload or undesirable transient tra- jectory characteristics. Their work has been further developed by a group of MIT people, led by Allan Klumpp, which has resulted in a finished set of guidance equations in our hands at this time, which only await the thorough analysis and testing required for final tuning and to develop flight con- fidence. In addition, a complex targeting program has been developed for use in pinning down the various guidance coefficients and targeting parameters.

On October 16 MIT, GCD, and MPAD people got together to discuss and under- stand the program formulation and to layout plans for the analysis work ahead. The specific products we are aiming for are an off-line LUMINARY assembly which can be exercised in the various simulators within a month or so and an agree-to analysis plan which will yield all of the understanding and confidence required to permit addition of this program into the LM space- craft computer for the Apollo l4 flight. Release of that program, I suppose, will not occur until March, which may seem like a long time from now. But it's clear that substantial changes to the descent guidance program – the program controlling the most critical phase of the mission – will certainly not be approved unless we have the absolute confidence of everyone involved that we are doing the right thing. And that is going to take some time.

In addition to the nominal ΔV improvement (that is, increased hover or payload capability) there are some other benefits from Delta Guidance.

1. Although N69 (Δ RLS) corrections during P63 are relatively cheap with the present system, the new guidance technique allows us to perform them with no ΔV cost.

2. Redesignations are improved in two ways. First of all the ΔV required to relocate the landing point a specific distance is markedly reduced. Furthermore, massive redesignations can be performed both long and short without unacceptable loss of landing site visibility.

3. The fact that the guidance is attempting to return the trajectory to nominal means that we are essentially providing a standardized terminal descent for the crew. For example, it eliminates the drooping characteris- tic that sometimes occurs as a result of dispersions or landing radar updates during P64 which in the worst cases could even lead to lunar impact. A standardized terminal trajectory should also have a beneficial effect on crew training in somewhat the same way the standardized rendezvous terminal phase has done.

The second and third benefits just listed will be available if Delta Guidance is implemented, regardless of whether or not we obtain permission from the DPS people to operate their engine in the new way I am going to discuss here. And, they are probably sufficient justification in themselves to implement it, particularly because redesignation apparently will play an important role in providing a point landing capability. However, we can only get the big ΔV saving dangled tantalizingly before you in the first paragraph if we can operate the DPS engine differently than we are currently allowed. Actually we have two choices we can give the DPS people; it doesn't make much difference to us which they choose. The first involves no hardware changes at all, as far as we know, but I am sure the Propulsion people will want to do some qualification testing on the DPS to permit it. The thing we want to do is to throttle the engine from the full thrust position down to 50 or 60 percent thrust (their choice) and back to full thrust periodically during the descent braking phase (P63). With a nominal engine, this throttling would occur about one per minute for a duration of about six seconds each time. Lower thrust engines will do it less often and higher thrust more. The alternate approach involves providing a small throttleable region around FTP large enough to compensate for the engine thrust dispersion. This so- called “shallow throttling” can be used with the same guidance technique and it eliminates the need for throttling through the forbidden zone. Intui- tively, this would seem to be a superior approach since it compensates con- tinuously and directly for the engine characteristic that is giving us all the trouble. However, it only saves ΔV if good engine efficiency is main- tained within the shallow throttling zone. I have heard that in order to do that, some sort of DPS hardware change must be made. (According to Allan Klumpp it involves a precise shaping of some propellent valve pintle, whatever that means.) Engine requalification would no doubt be required for that too. Mr. Apollo Spacecraft Program Manager, if I still have your attention, I would like to urge you to exert whatever influence you can spare toward clearing the DPS for this kind of operation. The benefit to be gained is expected to be worth the cost and effort (converted to lbs./buck) particularly since our informal data sources indicate the DPS can hack it.

One other area requiring immediate attention, which I haven't mentioned so far, involves descent monitoring both onboard and on the ground. The LGC commanded thrust will be entirely different than now which means that some of the MCC displays and Flight Control Mission Rules will become obsolete and will require replacement. It may be desirable to change some of the onboard displays also. Nothing at all has been done so far in this area.

In summary, it appears our guidance people have conceived and are developing a technique for descent guidance which has real advantages over the existing system if it works as advertized. It is possible to get it ready and imple- mented by Apollo 14 provided we place high priority and continuous effort on it. In order to reap one of the greatest benefits, it is necessary that the DPS be qualified to operate in a new way and so that must be vigorously pursued. Why are you still sitting here reading this stupid thing when there is all that important work to be done?