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After receiving a full scale ablative coating to protect the craft from the high temperatures associated with high-Mach-number supersonic flight, the X-15A-2 (56-6671) rocket powered research aircraft was then covered with a white sealant coat and mounted with additional external fuel tanks. This ablative coating and sealant would help the #2 aircraft reach the record speed of 4,520 mph (Mach 6.7). Under the lower fin is a dummy ramjet engine. It was intended to use the X-15A-2 for tests of an actual engine but this never happened.