The twelve-minute hiatus of Explorer 1
by Stuart Harris
|The lack of tracking stations meant that the science team had to wait 90% of an orbit before they knew the bird had really flown. And the agonizing fact was that America’s first satellite was late.|
Early unofficial planning for a US satellite began in 1952–53 at the Redstone Arsenal (on that same Huntsville site) under Wernher von Braun and his chief scientist Ernst Stuhlinger. A civilian team led by William Pickering of JPL and James Van Allen of Johns Hopkins Applied Physics Laboratory were also involved, contributing instrumentation for test flights of the alcohol/LOX/peroxide-fuelled Redstone rocket, the group’s prime task at the time.
On 29 July 1955, President Eisenhower made the public announcement that the USA intended to launch “small unmanned …satellites” to mark the International Geophysical Year (IGY). One day later, a Soviet government official announced that the USSR would do the same. The US armed services were confident, however, that they could go farther and faster than anything the Russians could launch. NASA, of course, didn’t yet exist.
All three services were eager to get in the game. Batting for the Army was von Braun’s team with Jupiter C, a derivative of the Redstone with solid upper stages stacked on top in a spin-stabilized “tub”. The addition of one small additional stage would turn Jupiter into Juno, and make it orbit-capable. Von Braun even announced the development program as “Project Orbiter”. The Air Force had the mighty Atlas-A ICBM, but it was untested at the time. The Naval Research Laboratory in Washington DC had Vanguard, a derivative of the Viking weather-sounding rocket, itself a semi-clone of von Braun’s V-2 that had terrorized London during World War 2. Two of Vanguard’s three stages were liquid-fuelled.
An ad hoc “Committee on Special Capabilities” chaired by Homer Stewart of JPL was formed to arbitrate, and quickly gave the nod to Vanguard. Their primary rationale was that Vanguard was never going to be an ICBM and so, unlike Jupiter and Atlas, its performance could be advertised without giving away military information. Project Orbiter was abruptly cancelled. Jupiter C development would continue to be strictly military, despite the earnest ambition of its designer. James Van Allen and physicist George H. Ludwig started serious design of a cosmic ray detector suitable for installation on a small satellite.
In early 1956, the Redstone group was re-organized as the Army Ballistic Missile Agency (ABMA), and on September 20, they successfully launched Jupiter C from pad 5 at Cape Canaveral to an altitude of over 1,000 kilometers. The rocket was deliberately hobbled with a dummy fourth stage, to prevent it from “accidentally” going into orbit. By December, Vanguard was ready for a suborbital test launch. Three tests were successfully carried out, with satellite launch scheduled for late 1957 (IGY ran from 1st July 1957 through the end of 1958).
Then came the stunner. The Soviet rocket “Sapwood” was in reality the R-7 “Semyorka” ICBM, designed to carry a three-megaton nuclear warhead up to 8,800 kilometers but which instead put the 83.6-kilogram Sputnik into a 215 x 939 kilometer orbit on October 4, 1957. That was followed up on November 3 by Sputnik 2, much heavier and carrying a dog. Eisenhower did his best to belittle the accomplishments, but he and every other adult American knew what it meant: if they could orbit a dog over our territory they could equally well send a nuclear bomb. Five days later von Braun’s phone rang and he was authorized to proceed with Juno all the way to orbit. Then, on Decmber 6, Vanguard’s first attempt at orbit resulted in one of the most spectacular public failures in the history of the Space Age. The 23-meter-tall rocket, looking as precarious as a lead pencil standing on end, rose about a meter off the pad, then settled back down into a vast fireball which not only destroyed the rocket but severely damaged the launch pad. The tabloid press howled “Flopnik”, “Kaputnik”, and “Stayputnik”.
Von Braun’s team went into overdrive, and had Juno 1 ready for launch by late January 1958. Since he’d been secretly planning for orbit all along, the additional design work was not too taxing. Van Allen and Ludwig undoubtedly lost some sleep adapting the experiment package to its new vehicle in the short time von Braun allowed. After two aborts due to high-altitude wind conditions, Explorer 1 (the satellite was not officially named until after launch) went into orbit… or did it?
In a lecture at the University of Iowa marking James Van Allen’s 90th birthday, George Ludwig reminisced:
Sixty miles up, 156 seconds after takeoff, the first stage burned itself out. The three upper stages with the satellite payload separated from the booster and zoomed upward, spinning in their tub-shaped assembly in free-coasting, unpropelled flight, toward the apex. Ernst Stuhlinger had built a special “apex predictor” to determine the instant that the assembly would reach the top of its trajectory, at which instant the remaining three rocket stages had to be fired…
[It] sent a radio signal to the speeding missile to fire the second stage. Off went the first cluster of scaled Sergeants, which quickly boosted the speed by thousands of miles per hour. Seconds later, the next cluster of rockets ignited, pushing the final-stage rocket, with its satellite, ever closer to that critical orbital velocity. Then the single rocket in the final stage ignited. Its thrust drove the 18.13-pound payload over the 18,000 mile per hour mark. Post-launch analysis revealed that his timing was impeccable, and Ernst has been known by his colleagues ever since as “the man with the golden finger”.
Then came a long waiting period. The rockets had fired, but was the instrument in orbit? The new satellite had to complete the major portion of a full orbit before that could be determined with any certainty… The time of expected signal acquisition came amid great expectation and excitement, but passed with the disappointing absence of any signal at any of the ground stations. During the next few minutes, we all waited with growing fear that the rocket or instrument might have failed.
AOS (Acquisition of Signal) at Goldstone was due at 12:30 am EST. The expectant scientists were made to wait until 12:42 for the announcement “Gold has it!” This is a cruel three minutes later than would be predicted by the theoretical equation for T, the orbital period:
T = 2πsqrt(Asmaj3/µ) sec, where Asmaj = semi-major axis of orbit, km; µ = the standard gravitational constant = 398,600 km3 sec-2 for planet Earth>
Data: Planned orbit: 352 x 1,600 km, Actual orbit: 357 x 2,547 km, Diameter of Earth: 12,750 km
so, planned Asmaj = (352 + 12750 + 1600)/2 = 7351 km; actual Asmaj = (357 + 12750 + 2547)/2 = 7827 km
planned T = 104.5 min ; actual T = 114.8 min
|The more significant point, surely, is that manufacturing tolerances of solid rockets fifty years ago were not what they are today, and some slop was to be expected.|
Since Cape Canaveral and Goldstone are separated by 36°, with the satellite going the “long way” around, Explorer 1 would have completed almost exactly 90% of its first orbit as it appeared over Goldstone, leading to a theoretical delay over the predicted time of just 9 minutes and 16 seconds. Perhaps even less, since Goldstone would not necessarily have to wait until Explorer 1 was directly overhead. On the other hand, the first part of that orbit was during Juno’s acceleration phase and not all at orbital velocity.
Van Allen was clearly not inclined for a post mortem on the point. He told the Des Moines Sunday Register: “There’d been just a slight error in our quick estimate of the satellite’s initial speed and period of revolution.”
Each of the “Baby Sergeant” class solid rocket motors—in clusters of eleven, three and one to form the three upper stages—burned over 20 kilograms of fuel, a blend of polysulfide-aluminum and ammonium perchlorate. One superficially plausible explanation for the over-enthusiasm of Juno’s propulsion was that the single rocket of the fourth stage was loaded with JPL-532A fuel, a slightly more efficient fuel than the T17-E2 in the lower stages. However, that was a known fact and would certainly have been allowed for in velocity calculations. The more significant point, surely, is that manufacturing tolerances of solid rockets fifty years ago were not what they are today, and some slop was to be expected.
Orbital velocity is a direct function of altitude, and can be calculated using this equation:
V = sqrt(µ(2/R - 1/Asmaj)) where R = distance from center of Earth, km
at insertion, actual R = 12750/2 + 357 = 6732 km, other variables as above
The actual orbital insertion velocity works out to 8.215 km/sec, compared with the planned velocity of 8.018 km/sec—an excess of just 2.46%. (8.215 km/sec is 18,484 mph or 27,110 ft/sec)
That notorious miscalculator, Richard Hoagland, has recently written an essay in three extensive parts, and held forth on the radio show “Coast to Coast AM” for four solid hours, expounding the theory that the excess velocity was more than could possibly be explained by overperformance of the solid rocket upper stages. In his written material he claims the overperformance was 17%; on the radio he said 30%—and he uses that pseudo-fact as a springboard for one of his wildly speculative theories, in this case invoking a “still classified non-Newtonian discovery.” Having told us the discovery is still classified, he feels able to “reveal” the dirty details. It’s an anti-gravity effect—“Von Braun’s Secret.”
It’s not hard to see how he strayed so far from the truth. The calculations in his written material are quite amazingly erroneous. He uses a wrong version of the Tsiolkovsky equation to calculate the velocity boost of a rocket stage; inserts a wrong value into that wrong equation, and lumps all three stages together in one calculation instead of calculating stage by stage independently. Unbelievably, he even omits to derive a logarithm that the equation requires.
Will he find a publisher ignorant enough to publish his erroneous calculations in book form? Probably. It’s happened before.