Descent Engine Gimbal Polarity Error

Mr. W. R. Kelly, PP76 April 1967AG 160-67D. G. Hoag Director Apollo Guidance & NavigationDescent Engine Gimbal Polarity Error

References: (a) LTX 500-52, Courseen to Young, 23 March 1967

(b) LM PGNCS Performance and Interface Specification, LSP-370-3, EDP Sheet #12, Paragraph 3.1.4.3(a), 18 March 1966

(c) Functional ICD LIS 540-10001

(d) LGC Electrical ICD LIS 370-10004 (Rev. B signed 25 May 1966), Sheet 39

(e) Memo, Stameris to Hoag, 23 March 1967

(f) Telephone call, Kayton and Hoag, 23 March 1967

(g) Report R-527, Rev. 1, Guidance System Operations Plan AS 206, Vol. 1, January 1967, pages 3-65 and 3-66

(h) Telephone call, Gibson to Miller, 23 March 1967

(i) Letter AG 144-67, Stameris to Smith, 27 March 1967

Gentlemen:

Ref. (a) TWX from GAEC states the AS 206 program, SUNBURST 113, which was delivered from MIT to the GAEC FCI 6 DOF simulation laboratory had a reversal of polarity in the DPS trim gimbal commands. This GAEC TWX further states … “It was determined that this reversal was not cause by a wiring error in the laboratory but was due to a pro- gramming error.”

MIT firmly disagrees with GAEC on this subject and herewith wishes to go on record that the polarity incompatibility was not due to an error in the SUNBURST 113 AS 206 LGC program but was due to the FCI 6 DOF simulation at GAEC being wired backwards from that specified in controlling interface documentation. Apparently LM 1 is also wired incorrectly.

We believe the LM wiring mistake occurred due to confusion between the actual commanded motion of the gimbal from the subject signal and the eventual vehicle attitude response. If the command signal asks for a positive rotation of the descent engine trim gimbal with respect to the vehicle, the response of the vehicle with respect to space is negative angular acceleration – both variables defined in a right handed sense about the appropriate axis. The signal in question is that which commands a rotation rate of the trim gimbal. Its polarity definition should be and has been in terms of this variable and not in terms of the resulting spacecraft motion.

Pertinent controlling interface documentation to define the DPS trim gimbal drive signal polarities are the P&I spec (Ref. b), the Functional ICD (Rec. c), and the Electrical ICD (Ref. d) all of which have both GAEC and MIT signatures. In each of these documents the signals in question are identified as those which drive the descent engine trim gimbal. In each of these documents the signals are labeled in sub- stance as “plus (or minus) pitch (or roll) trim gimbal signals.” The LGC program was consistent with these definitions.

It has been states (Ref. e and Ref. f) that the AS 206 GSOP, (Ref. g) which is written by MIT, supports the polarity convention with which GAEC wired the spacecraft. This is supposedly demonstrated by an equation on page 3-65 which relates positive vehicle angular acceleration to the posi- tive variable “δ”. This variable, however, is not the gimbal trim angle as argued, but instead is the angular difference of the vehicle CG to the thrust vector about the gimbal pivot. The gimbal trim angle command is variable “υ” which is defined on page 3-66, a definition which is consistent with the interface documents. (An equation in the GSOP relating to this command signal is written with an error. Although this error was dis- covered and did not get propagated into the program for AS 206, the GSOP correction will not be made until the next revision of that document which is now in process.)

In order to fix the incompatibility, MIT has been directed by MSC (Ref. h) to make changes in the SUNBURST program so as to send gimbal trim commands according to the polarity with which the spacecraft is wired. This change now exists in SUNBURST, Revision 116. Because this revision is now in violation to the ICD, MIT has initiated an interface change request authorization to GAEC (Ref. i) to issue an IRN making an appropriate ICD change.