Hyperloop is proposed to operate in a tube evacuated to 100 Pascals.Â This is 1/1000 the pressure at sea level.Â Removing 99.9% of the air molecules that would ordinarily slow the vehicle makes pushing it cheap.Â Reducing the amount of friction by replacing wheels with air cushions or magnetic levitation (maglev) brings the energy consumption to unbelievably low numbers.Â Thatâ€™s the good news.Â The engineering challenge is to build a vehicle that can move between the remaining air molecules at .91 Mach (Musk’s estimate).Â Lots of peculiar things happen around objects moving in an airflow at the speed of sound.Â People who saw The Right Stuff remember that Chuck Yeager had a devil of a time controlling the X-1 until it was going faster than sound.Â Those effects are called Mach buffet and Mach tuck.Â Jet fighters today are designed to fly slower than sound (subsonic) or faster than sound (supersonic).Â They just donâ€™t hang around Mach 1.0 (transonic).
The challenge a designer faces is that a vehicle travelling almost the speed of sound will be forcing some air to move faster around the bumps and corners of the vehicle â€“ typically across the top of the wing.Â The speed of the vehicle is limited to a critical Mach (Mcr), where some airflow is moving at the speed of sound.Â The Boeing 747 has a critical Mach about .93.Â Thatâ€™s a big smooth object.
The academic and commercial teams proposing capsule designs have been producing artistsâ€™ renderings daily.Â There is no indication that these are anything more than dream machines â€“ at least until they start doing some wind tunnel testing.Â Weâ€™ve been building supersonic airplanes since 1947, so we have some idea what shapes work.Â The fan in many of the renderings includes a sleek, streamlined cowl.Â Thatâ€™s not the way a fan works at .91 Mach.Â It needs to have a relatively small inlet and a fat section where the fan is.Â The air moving through the fan needs to slow down a little, else it reaches Mach 1 in the fan, and â€¦ I explained that already.
The world land speed record is 1228 km/h (763 mph), set in 1997.Â The ThrustSSC burns 18 l of jet fuel per second.Â Itâ€™s not impossible that we move in twenty years from gas guzzler to electricity-sipper to achieve the same speed.Â We went from first airplane flight to supersonic in 44 years.Â It is a high hurdle, though.Â The airplane developed in the context of two world wars with the best engineering talent in many countries working on it.Â To achieve a similar rate of development of Hyperloop will likely require more than the work of fifty full time engineers or five hundred part-timers.
Investors in Hyperloop need to differentiate between artists renderings, so easy to develop on a pc, from design drawings.Â Likewise, we should be skeptical when we hear claims of a smooth ride after the spokesperson likens air skis to an air hockey table.Â It is almost inescapable that the capsule will generate bluff body noise, which will manifest as sensible vibration and noiseÂ inside the capsule.Â Air skis, if they are part of a solution, will generate edge noise, in the same manner in which an airliner becomes noisier as it is configured for takeoff or landing with lots of parts sticking out.
The takeaway from this discussion is that while the atmosphere in the Hyperloop tube is 1/1000 that outside the tube, all of the engineering challenges of building a .91 Mach vehicle remain.