It would use a three-stage Navaho-derived launch vehicle to achieve a single orbit with an apogee of 120 km and a perigee of 75 km.
In the aftermath of Sputnik 2, the Air Force quietly asked its leading contractors for "unsolicited" proposals for manned spacecraft that could be quickly executed and beat the Russians in putting a man in orbit. Harrison Storms of North American conceived of a bold move to get an American into space as quickly as possible, in order to beat the Russians in the next obvious step of the space race. North American had a warehouse full of partially-completed G-38 boosters for the just-canceled Navaho missile program. Storms threw together a proposal to cluster them four of them in order to launch an orbital version of the company's X-15 manned rocketplane. He took the proposal to the Air Research and Development Command (ARDC) at Wright Field in November 1957.
Storms' X-15B was a 'stripped' X-15A with an empty mass of 4500 kg. The launch vehicle consisted of 4 x G-26 Navaho booster stages plus the X-15B's own XLR-99 engine. These would allow the X-15B to achieve a single orbit with an apogee of 120 km and a perigee of 75 km. Due to the low perigee and aerodynamics of the X-15, no retrorocket was required, although the X-15's restartable engine could be used if necessary. Using its cross range capability of about 800 to 1,000 km, the X-15 would ditch in the Gulf of Mexico. The heat shield would consist of beryllium oxide and Rene 41 alloy. The pilot would eject and land by parachute, with the aircraft being lost. The spacecraft had a ballistic coefficient (W/CdA) of 250 kg per square meter. It was expected that a first manned orbital flight could be achieved 30 months after a go-ahead at a cost of $ 120 million.
The general in charge of ARDC found it interesting but said there was no official requirement to orbit a man in space. But the political pressure to do something in response to Sputnik mounted, and a secret conference was held at on 29-31 January 1958 at Wright field. Eleven aircraft and missile firms outlined for the Air Force and NACA observers the various classified proposals for a manned satellite vehicle that they had submitted during November and December 1957.
The ARDC boiled down the 11 proposals to the three that had the best chance of quick realization - the X-15B, acceleration of the nascent program for the X-20 Dynasoar winged space glider, or one of the simple ballistic capsule designs, boosted by an existing launch vehicle. On 27 February they took these straight to Curtis LeMay, head of the Strategic Air Command, who's main comment was, "Where's the bomb bay?" Nevertheless, he instructed ARDC to select one of the concepts and submit a detailed plan for an Air Force man-in-space program as soon as possible.
On 10-12 March ARDC held a conference at its Ballistic Missile Division (BMD) in Los Angeles of more than 80 rocket, aircraft, and human-factors specialists. The objective was to reach agreement on a plan to get a man-in-space - soonest - in accordance with LeMay's orders. The BMD itself had its sights set on Project Lunex, a long term plan to establish an Air Force base on the moon before 1970. Unfortunately for Storms, the consensus at the conference was that the "quick and dirty" approach would consist of a simple ballistic capsule using parachutes for a water landing, weighing around 1300 kg. This would be 1.8 m in diameter and 2.4 m long. The capsule would be completely automated - the human-factors people felt there was no certainty that a pilot could function under the stresses of space flight. This last point seemed to rule out the piloted X-15B approach.
ARDC continued on the Manned-in-Space-Soonest project into August 1958, and in June Storms had even been told he would receive the contract for the manned spacecraft. But meanwhile, President Eisenhower and the Congress had created a new civilian Agency to take on the Soviet spaceflight challenge - NASA. And Max Faget, the lead spacecraft designer at NASA, was one of the originators of the ballistic capsule concept. The USAF budget for the initial manned spacecraft was transferred to NASA, and with it evaporated Storms' expected contract, and the X-15B. The contract for what NASA renamed project Mercury would go to McDonnell.
This was not quite the end of the orbital X-15. It was known that North American later proposed use of four Titan I booster stages in place of the Navaho boosters.
The notes of NACA engineer Clarence A. Syvertson from this meeting indicate that four Navaho G-26 boosters would be used. The biography of Harrison Storms indicates that G-38 boosters were proposed. A quick calculation showed that four G-26 boosters could not get the X-15 into orbit; G-38 boosters just about could. So in this case physics and the memory of Storms trump the contemporaneous notes. It was likely in any case that the boosters proposed were derived from, but not identical to the G-26 or G-38 boosters.
A drawing also emerged of an X-15 atop a single G-26 booster. It was likely that a program of slow build-up to orbital speeds, using Navaho surplus assets, was proposed.
Crew Size: 1. Spacecraft delta v: 2,450 m/s (8,030 ft/sec).
Gross mass: 13,500 kg (29,700 lb).
Unfuelled mass: 4,500 kg (9,900 lb).
Height: 15.00 m (49.00 ft).
Span: 6.80 m (22.30 ft).
Thrust: 262.45 kN (59,000 lbf).
Specific impulse: 276 s.