Fig. 3. Program 437AP (left) compared with standard Thor IRBM.

Figure 4 (below) superimposes the outline of the payload section of Corona-M over the visible portion of the Program 437AP payload vehicle. In the left hand image, their re-entry vehicles are aligned, at the top of the stack. Clearly, the RVs match, as expected. In the right hand image, they are aligned at the elevation of the interface between the missile forebody and the 437AP payload. Their cylindrical camera sections are about the same height. The diameter of the 437AP was greater, 60 inches (1.524 m) vs. 50 inches (1.27 m). It was about 13 inches (0.33 m) shorter than Corona-M.

Fig. 4. Outline of Corona-M superimposed over Program 437AP vehicle.

Table 1 compares the volume of the payload sections of Corona-M and 437AP, below the base of the H-30A RV's thrust cone. Corona-M was about 67.2 ft3. The 437AP was about 78.6 ft3, assuming there was usable volume beneath the half-foot tall black band at the top of the cylindrical section; if not, then its volume would have been about 68.8 ft3. With or without the black-banded section, the 437AP had roughly the same payload volume as Corona-M; therefore, it seems likely that it could have accommodated the major components of its imaging system.

Table 1: Comparison of payload volume

Payload Module Section Corona-M Pgm 437AP Cylinder diameter, ft 4.17 5.00 height, ft 3.50 3.80 volume, ft3 47.72 74.61 Conical fairing below RV thrust cone bottom dia., ft 4.17 5.00 top dia., ft 3.34 3.00 height, ft 1.75 0.31 volume, ft3 19.44 3.98 Vehicle total volume, ft3 67.16 78.59

The payload section of Corona-M was integrated with the Agena stage of its launch vehicle, which provided it with electrical power, attitude control, and communications (telemetry and command and control). The 437AP had no Agena, because it was launched on a sub-orbital Thor IRBM. It seems unlikely that those systems would have fit within its payload section. If not, then a clue to their likely location may be found in Austerman's report that the 437AP included an "aft bulkhead" and a "modified General Electric Mark II nonrecoverable re-entry vehicle." Figure 5 (below) compares the 437AP with a drawing of a standard Thor IRBM. The G.E. Mark II re-entry vehicle consisted of a blunt nose cone, below which a conical payload compartment extended into the missile forebody, occupying a portion of the space above the airborne guidance system.

Fig. 5. Program 437AP (left) compared with standard Thor IRBM.

The payload compartment of the G.E. Mark II RV appears to have been the only space other than the 437AP's payload section, that could have housed the electrical power, attitude control, and communications systems. Figure 6 (below) superimposes its outline onto the 437AP launch photo. Could this have been what Austerman meant by the 437AP's "aft bulkhead?"

Fig. 6. Program 437AP with outline of payload compartment of G.E. Mk II RV.

The empty volume of the payload compartment of the G.E. Mark II RV was about 18 ft3. It's dimensions appear to have been limited by a conical bulkhead of similar size, at the top of a Program 437 Thor IRBM forebody, shown in Figure 7 (below). Employing a modified G.E. Mark II RV payload compartment on the 437AP vehicle, seems consistent with the "complete interchangeability" of payloads reported by Austerman. Whether it was actually present on the 437AP, and its purpose, remains to be determined.

Fig. 7. Program 437 Thor IRBM forebody without payload, on display at Vandenberg AFB. Source J. Page. Originals here and here.

Another part of the G.E. Mark II RV that may have been been used on the 437AP, was its nosecone. In Figure 3, the conical fairing between the 437AP's cylindrical section and its H-30A RV, bears a strong superficial resemblance to this nosecone. The rounded tip of the G.E. Mark II RV nosecone subtended an included angle of 105 deg, resulting in a slope of 37.5 deg relative the top of the missile forebody. Measurements of the 437AP launch photo yield about the same angle. If this was part of a G.E. Mark II RV nosecone, then about 40 percent of the surface area of its heat shield must have been removed to provide an opening for the H-30A RV. It makes more intuitive sense for a new fairing to have been designed and built from scratch, which may well have been done. There is no way to be certain.

The H-30A RV probably required minimal modification for Program 437AP. Since 437AP launches were sub-orbital, its retro-rocket would have served no useful purpose; therefore, it almost certainly was removed, perhaps replaced by an inert replica.

The H-30A would experience its maximum rate of deceleration at a much lower altitude than its orbital counterpart, resulting in a far greater maximum rate. Flight 1 is estimated to have peaked at about 29 g (Section 4.4), compared with 7 g to 8 g for an orbital re-entry. Modification may not have been required, given that the April 1965 system performance/design requirements of Corona-J, specified (on page 88) that the cassette assembly with film of its SRV, "shall be capable of withstanding re-entry accelerations up to 35 g's longitudinally, and 5 g's laterally."

The total heating during a sub-orbital re-entry is far less than that of an orbital one, due to its much lower velocity, but the maximum rate of heating is greater. That might have required the H-30A's ablative heat shield to be modified; however, if it was designed for maximum re-entry acceleration of 35 g, then it probably was adequate, since peak heating and maximum acceleration occur at about the same time.

There is no public information on the expected or achieved resolution of the Program 437AP satellite inspector. An indication of what may have been possible is found in the memorandum of October 1, 1965, from the Chief of the NRO's Systems Analysis Staff, in apparent response to a query by William A. Tidwell, Chairman of the Committee on Overhead Intelligence, regarding the use of KH-4 to photograph orbiting satellites.

Of the several possible circumstances of encounter evaluated, the estimated best resolution was 2 ft., from a vantage point 10 NM (19 km) above a near-coplanar target. This was without image motion compensation. Resolution from encounters at a 90 deg cross-track would have been 25 ft., apparently limited by the much greater, uncontrolled, image motion. The summary gave an indication of the technical challenges involved:

In summary, it might be estimated that if a launch window of about five minutes could be obtained, pictures of a Soviet on-orbit satellite could be obtained from a KH-4 to a resolution of a few feet. It should be added that every time the KH-4 camera is turned on, an appreciable fraction of its film is expended. An orbiting satellite might be photographed when ground intelligence was collectable also. Finally, the limited on-orbit camera control flexibility of KH-4 might result in several, if not many, attempts being necessary before success. Ultimately the purpose and value of the photograph should be considered. As a trick, it may be great; the substantive intelligence value may be considerably less. It must be obvious that this cursory examination is insufficient. A detailed plan would be needed, both refining these rough calculations and including eccentricity, before the feasibility, chance of success, etc. can be firmly established.

The following section provides a detailed account of a Program 437AP mission.

Four Program 437AP satellite interceptor vehicles were launched between December 1965 and July 1966. The first flight ended in failure, but it is the most readily studied of the four, because it received by far the greatest attention from Austerman. He provided sufficient detail to accurately reconstruct the circumstances of the launch, the orbital trajectory of the target, and the sub-orbital trajectory of the 437AP vehicle. The results of those analyses are used to provide additional context for the discussion of the mission results.

Austerman provided the following facts about the first flight (p.54, p.57 and p.95), that help to constrain the trajectory of the interceptor.

Table 2: Program 437AP Flight 1

Lift-off time 1965 Dec 07 19:29 MST = 1965 Dec 08 02:29 UTC Launch site Johnston Atoll, Pad II Target Agena with SPADATS Object Number 613 (1963-027A) Time to interception 8.18 min. (491 s) after launch, after 328 s ballistic flight Closest approach to target within 0.56 NM (1 km) of planned 3.2 NM (6 km) Azimuth at interception 153 deg Range at interception 1528 km slant, 1398 km ground Altitude at interception 487 km

The above lift-off date and time disagree by exactly one day with those tabulated in Appendix III of Austerman's report. The above information was determined to be correct, as a result of the analysis discussed in Section 4.2.

Pad II was formally called Launch emplacement 2 (LE2). It and LE1 were used for all Program 437 launches. The following co-ordinates were determined with the aid of the site plan and Google Earth:

Table 3: Program 437 Launch Pads

Latitude Longitude Altitude Launch Pad deg N deg E m Launch emplacement 1 (LE1) 16.72956 -169.53790 -14 Launch emplacement 2 (LE2) 16.73210 -169.53437 -14

The precise time of the interception was determined using the orbital elements of the Agena 1963-027A / 613, which are available in TLE (2-line elements) form in Jonathan McDowell's public archive. The two TLEs that bracket the launch time, were of epoch 3.8 d earlier and 6.0 d later. They agreed to within 1 s on the time of interception. The closer in epoch of the pair was selected for the trajectory analysis:

1 00613U 63027A 65338.26395386 .00001480 27881-8 64242-4 0 591 2 00613 82.3360 136.6698 0028279 146.4922 213.7506 15.21993418135027

The difference between the mission elapsed time of interception (491 s) and the duration of ballistic flight (328 s) was 163 s, just 1 s longer than the nominal time of VECO (vernier engine cut-off) for a 1500 NM (2800 km) Thor IRBM launch. Adding this difference to the time of lift-off yields the start of ballistic flight, at 02:31:24 UTC.

The procedure used to estimate the sub-orbital trajectory of the interceptor is described in the following sub-section.

A sub-orbital interceptor trajectory that closely agrees with the known facts of the mission has been estimated using GMAT R2014a (General Mission Analysis Tool), "developed by a team of NASA, private industry, public, and private contributors."

The analysis was performed using GMAT's Dormand-Prince 78 numerical integrator, with a 90 degree, 90 order gravity field, and the MSISE90 atmosphere model.

The starting point of the propagation was VECO, determined earlier to have occurred at 02:31:24 UTC. The precise range from pad LE2, and altitude at VECO are not available; however, the nominal values for a 1500 NM (2800 km) Thor IRBM launch were approximately 154 km down range, and 126 km altitude. The position vector was estimated on the assumption that the launch azimuth would have been about the same as the 153 deg at intercept, which would have placed the vehicle 126 km above 15.49603 N, 168.88412 W.

No information is available on the ballistic properties of the Program 437AP payload. Since the trajectory of greatest interest occurred well above the dense layers of the atmosphere, nominal values of C d (co-efficient of drag) and A/m (area to mass ratio) were used.

The initial estimate of the velocity vector was based on the nominal 4.42 km/s VECO velocity of a standard Thor IRBM. Rough Cartesian components were estimated on the assumption that they would parallel the vector from the launch site to the point of VECO. These estimates were refined through a process of trial and error, involving numerous GMAT runs, until a trajectory was obtained that closely matched the time and position of the interception reported earlier. The agreement is to within 2 s of time, and a few kilometres miss-distance. The geocentric velocity at VECO was found to have been 4.88 km/s. That is about 0.46 km/s greater than the standard IRBM. It is speculated that the increase may have been due to a net decrease in payload mass. The theoretical ground range to impact of 2,991 km is not much greater than typical for Thor, and within its known range of performance. The resulting GMAT script is available here.

A subset of the GMAT propagated state vectors at almost exactly 5 s intervals was extracted and entered into an Excel spreadsheet designed to generate ephemerides, including geodetic ground track co-ordinates and altitude above geoid, and topocentric co-ordinates from user-specified locations. The Excel file contains ephemeris sheets for the interceptor and the target. Columns 1-13 contain the computed ephemerides. Changing the observer's co-ordinates near the top of col 15 causes the topocentric portion of the ephemeris to be re-computed. Columns 20-26 contain the GMAT-computed state vectors. Columns 27-29 contain derived velocity and acceleration information. Columns 31-34 are used to format the trajectory for use in kml files. VBA macro, kml_placemark, reads this data to generate a file of place marks, ready to be inserted into a kml file. (The macro includes a number of hard-wired values that need to be edited for each case, e.g. input data sheet, path to output file, and object description.)

Users of Google Earth, may wish to download the 3D trajectory kml files of the interceptor and the target, which were used to generate the following graphics.

Figure 8 (below) depicts the trajectory of the target Agena (blue and white line) and the interceptor (red and white line). The trajectory of the interceptor begins with VECO, about 154 km downrange of Johnston Island. Much like the sport of skeet shooting, the interceptor was aimed at a point far down range, where it would meet its orbital target. The interceptor's average velocity was about half that of the target, so it was given a head start of more than 1300 km. As the interceptor neared its apogee of 490 km, the target caught up and overtook it, resulting in a close encounter that lasted several seconds.

After the interception, the orbiting target continued to move downrange, but the sub-orbital satellite inspector could only fall back to Earth, re-entering the atmosphere just 6.5 minutes later.

Fig. 8. East-facing profile of interception.

Figure 9 (below) depicts the trajectory of the target Agena (blue and white line) and the interceptor (red and white line) as seen looking north. The objects are moving toward the viewer. The crossing angle at interception is evident in this view. It was 23.9 deg, well within the maximum allowed 45 deg for acceptable relative velocity. After the interception, the satellite inspector re-entered. Any fragments that survived the heat of re-entry fell into the Pacific Ocean. The target remained in orbit.

Fig. 9. North-facing view of interception.

Figure 10 (below) depicts an overhead view of the trajectory of the target Agena (blue and white line) and the interceptor (red and white line). The alternating coloured line segments of both trajectories span 5 s of flight. Those of the target are noticeably longer, due to its much greater velocity.

Fig. 10. Overhead view of interception.

Figure 11 (below) depicts the final six minutes of the satellite inspector's descent toward impact. The near vertical final drop of about 20 km is due to the loss of all horizontal velocity due to atmospheric drag. This vertical descent occurs with meteors and sub-orbital, as well as orbital vehicles. The heat of re-entry would have subsided, so that the object would no longer have been incandescent. In the case of meteorites (meteors that reach the Earth), this final, non-luminous descent is called dark flight.

Fig. 11. East-facing view of final descent of interceptor.

Most of the public information about the mission is due to Austerman (pp.57-58). Table 4 summarizes the key events, augmented by information from the present trajectory analysis.

Table 4: Program 437AP Flight 1 Mission Events

Time Range Altitude UTC km km Event 02:28:44 0 0 Lift-off 02:31:24 154 126 VECO - start of ballistic flight 02:36:50 1397 488 Closest approach to Agena target 02:40:00 2098 406 Radio contact with H-30A RV expected by Surface Recovery Unit 02:40:20 2173 384 Passed below horizon of Johnston Island 02:43:00 2801 126 H-30A RV last heard by Surface Recovery Unit 02:43:25 2905 71 Onset of significant re-entry heating 02:43:45 2975 31 Peak deceleration (approx. 29 g); peak heating 02:45:00 2992 14 "Contrails and smoke" spotted by surface and aerial recovery units 02:48:45 2991 0 Theoretical impact at 7.49 S, 157.71 W (without parachute)

Austerman wrote that the mission had been nominal through the interception of the Agena, which occurred shortly before 02:37 UTC. The satellite inspector missed its programmed stand-off distance from the target of 3.2 NM, by 0.56 NM. That would put the closest approach between about 5 and 7 km. Soon after the interception, the film should have been spooled into the H-30A RV (re-entry vehicle), cut, and the film door sealed; however, this failed to occur, and mission control at Johnston Island received no indication that the RV had separated.

The Surface Recovery Unit, located near the planned impact point nearly 3,000 km SE of Johnston Island, was expected to pick up the telemetry of the H-30A RV at 02:40 UTC, shortly before it passed below the horizon of Johnston Island. Apparently, the signal was received, but was lost at about 02:43 UTC, by which time the RV would have descended to 126 km. About 25 s later, it was down to 71 km, and beginning to experience significant re-entry heating. After another 15 s, it was down to 31 km, where it reached the maximum rate of deceleration, estimated by the trajectory analysis at about 29 g. This was also the point of peak heating, where anything not protected by a heat shield would break up and disintegrate.

At 02:45 UTC, the surface and aerial recovery units spotted "contrails and smoke," which presumably were the dust left by the disintegrated booster stage and the non-recoverable section of the payload, with its cameras. Since this was a daytime re-entry, the trails would have been readily visible, several tens of kilometres overhead. Had the H-30A RV survived, then it would have been down to about 14 km, probably on its main parachute (details of the planned recovery sequence are unknown), but it was never recovered. It initially appeared that the recovery parachute had failed to deploy properly and the water impact destroyed the capsule, but a subsequent data analysis by Space Systems Division found the root cause:

...the malfunction was caused by a momentary short-circuit in the in-flight disconnect cable between the payload and recovery vehicles. After squib firing to provide inflight electrical disconnection, the recovery vehicle cable terminal flew back against its own harness, causing cable damage. The momentary short circuit caused the recovery vehicle programmer to reset, which precluded physical separation of the payload and recovery vehicles. (Austerman, pp.57-58)

Corrective actions were quickly taken to prevent a repeat of the failure of Flight 1. Flight 2 followed on 1966 Jan 18 UTC, reportedly successfully targeting an orbiting Agena. Film was recovered, but there is no information on image quality. (Austerman, p.58)

Flight 3 was launched on 1966 Mar 12 UTC. All "research and development feasibility demonstration objectives," reportedly were met, but there is no information on image quality. (Austerman, p.58)

Flight 4 was conducted at the request of NASA, in an effort to learn why its OAO-1 (Orbiting Astronomical Observatory) had failed soon after it reached orbit. The scheduled launch occurred on 1966 Jul 02 UTC, but an electrical short circuit sent the satellite inspector off-course, preventing its camera from acquiring the target, resulting in a photo of "the void of space." (Austerman, p.58)

Despite the mixed record of success, there was support for Program 437AP. In March 1966, the Commander in Chief of the Continental Air Defense Command requested a launch in April against a Soviet satellite. This was rejected by the Joint Chiefs of Staff and the USIB (United States Intelligence Board). The USIB recommended against launches from Johnston Island, because President Johnson had publicly identified it as an ASAT base, which somehow would have caused the Soviets to realize that one of its satellites had been targeted by a 437AP launch. The USAF terminated the program on November 30, 1966. (Austerman, pp.59-60)

Much remains to be learned about the design and operation of the Program 437AP satellite inspector vehicle. In Section 3, the attempt to reconcile Austerman's description of it with the launch photo and the known features of Corona, is somewhat speculative, especially regarding the possible incorporation of modified parts of the G.E. Mark II re-entry vehicle. The details of how the imaging was to be accomplished are unknown. How was the target to be acquired by the sensors, i.e. the cameras? How were the three cameras intended to be used? Did the vehicle perform as expected? How good were the images obtained on Flights 2 and 3? This report concentrated on the 437AP vehicle, but on the political and policy front, any role the NRO may have had in this unique program is of great interest.

I wish to acknowledge the assistance of Vicente-Juan Ballester Olmos, Dwayne Day, Jonathan McDowell and Allen Thomson.

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