Thread by @divert_thruster

1) 500 follower special: Let's talk about wind tunnel testing for reentry vehicles. Pictured: unnamed RV in NASA Langley's high-temperature structures tunnel. Test section diameter: 8 feet.

Photo credit: NASA

2) The flight regimes for strategic RVs are incredibly harsh. In 30 - 45 seconds, the RV reaches speeds up to 7km/sec and ~200g acceleration loads. For reference, an AMRAAM reaches around ~1.5km/sec.

Photo credit: DTIC

3) Hypersonic velocities are unattainable thru turbine-driven facilities. Instead, hypersonic tunnels store pressurized gas and blow them through expansion nozzles for short duration. The gas is then collected in vacuum chambers, like these at NASA Langley.

Photo credit: NASA

4) Expansion nozzles are key, allowing the gas to accelerate faster than Mach 1. Most of them look very similar to rocket nozzles; they can produce different flow characteristics depending on profile. Contoured nozzles are generally preferred.

Photo credit: NASA

5) As gas expands through the nozzle, its temperature drops dramatically—often to the point where it instantly liquefies. So testing gas must be preheated by thousands of degrees, typically with a plasma arc.

Photo credit: JHU APL

6) These capabilities are critical to reducing RV dispersion error. Of these variables TRW identified in a 1974 report, lift-related phenomena (especially uneven boundary layer transition) was the biggest contributor to inaccuracy.

Photo credit: DTIC

7) General layout of a hypersonic test facility. NSWCDD Tunnel 9 was used to test MK4, MK5, and AMaRV reentry vehicles. Curiously, the Navy never tested the MK500 MaRV there, as the tunnel came online in 1977.

Photo credit: Naval Surface Warfare Center, Dahlgren Division

8) More discussion of the boundary layer transition problem for RV accuracy. AEDC Tunnel B and NSWCDD Tunnel 9 appeared to be typically used. Asymmetric BLT causes weird AoA dispersions

Photo credit: BMO, MIT

9) The Navy first tested the MK4 RV at tunnel 9 in 1979. According to Dahlgren Division, the tunnel needed to be upgraded with a gas-injection system when flight test showed that ablation offgassing could be affecting MK4's trajectory.

Photo credit: NSWCDD

10) The Polaris MK-1 and MK-2 RVs were tested in tunnels originally captured from Nazi Germany. Tunnels 1 and 2 were first built in Peenemünde, moved to Koche after U.S. bombings, and finally brought to White Oak, Maryland in 1946.

Photo credit: NSWCDD

11) Given these characteristics, tunnels can be easily identified through satellite imagery. See the visible vacuum tanks, pressure lines, and gas tanks at India's Vikram Sarabhai Space Centre and Orbital's GASL shock tube (not an arc-jet).

Photo credit: Maxar Techs., Apple

12) Presence of power generation (for preheaters) or heat exchangers (for cooling) may also be useful signatures. But smaller facilities may be harder to spot. The JF-12 tunnel was reportedly used to test China's hypersonic vehicles, but it appears to be a covered facility.

13) These facilities could be shock-tubes, which operate for extremely short durations and are used for some subscale aerodynamic tests. Pictured is a shock-tube setup for the Indian Space Research Organization.

Photo credit: NASA, ISRO

14) If I had funding, it'd be interesting to run a tunnel-hunting effort to track foreign RV development. Large arc-jet tunnels are very useful for RV testing. Pictured: suspected shock-tunnel facility at Iran's Imam Hossein University.

Photo credit: Google/Maxar