Did NASA use metric or English units for Project Mercury?
For example, did they measure delta-v in meters/second or in feet/second?
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The Mercury spacecraft main instrument panel from Project Mercury Indoctrination, May 1959 (Source: NASA. Click for full size)
They used statute miles as a measure of distance in schematics, diagrams and other vectors in the documentation, for example this Proceedings of a Conference on Results of the first U.S. manned suborbital space flight from June 6, 1961:
First U.S. manned suborbital space flight - pictorial presentation of the overall flight (Source: Public archive (in PDF format))
And even went at length to provide their personnel travelling abroad (in this case Mexico) with unit conversion tables from the imperial to the metric system of units in NASA's Introduction to project Mercury and site handbook:
Page extract from the Introduction to project Mercury and site handbook, notes on personnel travels to Mexico (metric system of units)
Imperial units of measure persisted throughout the Mercury program. This is the mission timeline for the Mercury-Redstone 4, launched on July 21, 1961:
T+ Time Event Description --------------------------------------------------------------------------------------- T+00:00:00 Liftoff Mercury-Redstone lifts off, onboard clock starts. T+00:00:16 Pitch Program Redstone pitches over 2 deg/s from 90 deg to 45 deg. T+00:00:40 End Pitch Program Redstone reaches 45 deg pitch. T+00:01:24 Max Q Maximum dynamic pressure ~575 lbf/ft² (28 kPa). T+00:02:20 BECO Redstone engine shutdown - Booster Engine Cutoff. Velocity 5,200 mph (2.3 km/s) T+00:02:22 Tower Jettison Escape Tower Jettison, no longer needed. T+00:02:24 Craft Separation Posigrade rockets fire for 1 s giving 15 ft/s (4.6 m/s) separation. T+00:02:35 Turnaround Maneuver Spacecraft (ASCS Automatic Stabilization and Control System) system rotates spacecraft 180 deg, to heat shield forward attitude. Nose is pitched down 34 deg to retro fire position. T+00:05:00 Apogee Apogee of about 115 mi (185 km) reached at 150 mi (240 km) downrange from launch site. T+00:05:15 Retrofire Three retro rockets fire for 10 s each. They are started at 5 s intervals, firing overlaps. Delta-V of 550 ft/s (170 m/s) is taken off forward velocity. T+00:05:45 Retract Periscope Periscope is automatically retracted in preparation for reentry. T+00:06:15 Retro Pack Jettison One minute after retrofire retro pack is jettisoned, leaving heat shield clear. T+00:06:20 Retro Attitude Man. (ASCS) orients spacecraft in 34 deg nose down pitch, 0 deg roll, 0 deg yaw. T+00:07:15 .05 g Maneuver (ASCS) detects beginning of reentry and rolls spacecraft at 10 deg/s to stabilize spacecraft during reentry. T+00:09:38 Drogue Para. Deploy Drogue parachute deployed at 22,000 ft (6.7 km) slowing descent to 365 ft/s (111 m/s) and stabilizing spacecraft. T+00:09:45 Snorkel Deploy Fresh air snorkel deploys at 20,000 ft (6.1 km). (ECS) switches to emergency oxygen rate to cool cabin. T+00:10:15 Main Para. Deploy Main parachute deploys at 10,000 ft (3.0 km). Descent rate slows to 30 ft/s (9.1 m/s) T+00:10:20 Landing Bag Deploy Landing Bag Deploys, dropping heat shield down 4 ft (1.2 m). T+00:10:20 Fuel Dump Remaining hydrogen peroxide fuel automatically dumped. T+00:15:30 Splashdown Spacecraft lands in water about 300 mi (480 km) downrange from launch site. T+00:15:30 Rescue Aids Deploy Rescue aid package deployed. The package includes green dye marker, recovery radio beacon and whip antenna.
Units are also converted into metric system for convenience, original mission communications transcript is available here (in PDF format).