Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
Each re-entry vehicle was extensively instrumented, mounting nearly two dozen
breakwire ablation sensors along with pressure and temperature sensors. The latter
were resistance thermometers employing 0.0003-inch tungsten wire, reporting temperatures
to 2000ºF with an accuracy of 25 to 50°F. The phenolic nylon showed
anew that it had the right stuff, for it absorbed heat at the rate of 3,000 BTU per
pound, making it three times as effective as boiling water. A report from GE noted,
“all temperature sensors located on the cylindrical section were at locations too far
below the initial surface to register a temperature rise.” 82
With this, the main effort in re-entry reached completion, and its solution—
ablation—had proved to be relatively simple. The process resembled the charring of
wood. Indeed, Kantrowitz recalls Von Braun suggesting that it was possible to build
a nose cone of lightweight balsa soaked in water and frozen. In Kantrowitz’s words,
“That might be a very reasonable ablator.” 83
Experience with ablation also contrasted in welcome fashion with a strong tendency
of advanced technologies to rely on highly specialized materials. Nuclear
energy used uranium-235, which called for the enormous difficulty of isotope
separation, along with plutonium, which had to be produced in a nuclear reactor
and then be extracted from highly radioactive spent fuel. Solid-state electronics
depended on silicon or germanium, but while silicon was common, either element
demanded refinement to exquisite levels of purity.
Ablation was different. Although wood proved inappropriate, once the basic
concept was in hand the problem became one of choosing the best candidate from
a surprisingly wide variety of possibilities. These generally were commercial plastics
that served as binders, with the main heat resistance being provided by glass
or silica. Quartz also worked well, particularly after being rendered opaque, while
pyrolytic graphite exemplified a new material with novel properties.
The physicist Steven Weinberg, winner of a Nobel Prize, stated that a researcher
never knows how difficult a problem is until the solution is in hand. In 1956 Theodore
von Karman had described re-entry as “perhaps one of the most difficult problems
one can imagine. It is certainly a problem that constitutes a challenge to the
best brains working in these domains of modern aerophysics.” 84 Yet in the end, amid
all the ingenuity of shock tubes and arc tunnels, the fundamental insights derived
from nothing deeper than testing an assortment of candidate materials in the blast
of rocket engines.
50
Nose Cones and Re-entry
1 Vannevar Bush, testimony before the Senate Special Committee on Atomic Energy, December
1945, quoted in Clarke, Profiles, p. 9 and Time, 22 May 1964, p. 25.
2 MacKenzie, Inventing, pp. 113-14. 1,500 feet: Neufeld, Ballistic, pp. 69-102 passim.
3 North American Aviation Report AL-1347, pp. 1-6 (quote, p. 4); Fahrney, History, p. 1291,
footnote 3.
4 North American Aviation Report AL-1347, pp. 38-39; Colonel Roth: Letter, Colonel M. S.
Roth to Power Plant Lab, Air Materiel Command, USAF, 11 February 1948.
5 Author interview, Colonel Edward Hall 29 August 1996, Folder 18649, NASA Historical
Reference Collection, NASA History Division, Washington, D.C. 20546.
6 North American Aviation Report AL-1347, p. 39.
7 Other North American Aviation names of that era included the NAKA, NALAR, and NASTY
battlefield missiles, the Navion private plane, and the NATIV research rocket; see “Thirty Years
of Rocketdyne,” photo, “Armament Rockets,” 1962; Murray, Atwood, pp. 32, 44.
8 Rhodes, Dark Sun, p. 407 (includes quote); Manchester, Caesar, p. 642.
9 “Standard Missile Characteristics: XSM-64 Navaho”; Report AL-1347, op. cit., p. 88; Development
of the Navaho, pp. 30-31; Augenstein, “Rand”; Fahrney, History, pp. 1296-98. General
Putt: Letter, Major General Donald Putt to Commanding General, Air Materiel Command,
USAF, 21 August 1950.
10 Neufeld, Ballistic, pp. 44-50, 68-70.
11 Ibid., pp. 70-79.
12 Development of the Navaho, pp. 40-46; Journal of Guidance and Control,
September-October 1981, pp. 455-57.
13 Neufeld, Ballistic, pp. 73, 77-78 (quote, p. 78).
14 Rhodes, Dark Sun, pp. 495, 499, 505 (quotes, p. 509). Taylor quote: McPhee, Curve, p. 77.
Hiroshima: Rhodes, Atomic, p. 711.
15 Von Karman and Edson, Wind, pp. 300-01; Neufeld, Ballistic, p. 98.
16 Neufeld, Ballistic, pp. 95, 98-99, 102; quote: Chapman, Atlas, p. 73.
17 AIAA Paper 67-838, pp. 12-13; Neufeld, Ballistic, p. 102 (quotes, p. 259).
18 Rhodes, Dark Sun, pp. 482-84, 541-42 (quote, p. 541).
19 Neufeld, Ballistic, pp. 104-06, 117 (quotes, pp. 105, 106); Rhodes, Dark Sun, p. 542.
20 Neufeld, Ballistic, p. 117; Emme, History, p. 151. Dimensions: Ley, Rockets, p. 400.
21 Miller, X-Planes, ch. 7; letter, Smith DeFrance to NACA Headquarters, 26 November 1952.
22 NACA Report 1381 (quotes, pp. 11, 13).
23 Anderson, History, pp. 440-41.
24 Neufeld, Ballistic, p. 79.
25 Time, 13 June 1960, p. 70. 9,000 K: Journal of the Aeronautical Sciences, February 1958, p. 88.
Quote: Rose, “Physical,” p. 1.
26 Rose, “Physical,” p. 1.
27 Hallion, Hypersonic, pp. xxiii, xxvi.
28 Time, 13 June 1960 (includes quote). Arthur Kantrowitz: Author interview, 22 June 2001.
51