Is non-evacuated tube maglev launch possible? [duplicate]

Question

I heard about evacuated tube transportation technology and it's application to launch cargo or even shuttles to space. Example of such project would be StarTram.

The fact that acceleration phase to gain escape velocity happens in evacuated tube let's us cut the air resistance.

My question is, would it be possible to accelerate a space shuttle (or any object) using maglev technology without vacuum tube? What would be the impact of such object on it's surroundings when it would gain such high velocity so close to the ground?

Is anyone familiar with vehicles that gain high speeds (not necessarily escape velocity, but at least supersonic) close to the ground?

What I imagined was a shuttle that would take off as ordinary airplane, then navigate into the series of concrete blocks strongly attached to the ground. Each of those blocks would emit strong magnetic field, creating a maglev-like linear acceleration. The pilot (or maybe autopilot) would have to maintain the flight trajectory above blocks. After the last block, it would reach high enough velocity and then the pilot could alter the flight level to exit the Earth's atmosphere.

Of course, such launch facility would require to be build in unpopulated areas, near the equator, and it would be hundreds of kilometers long...

Here is the list of engineering problems I would like you to comment:

1. The impact of very high speed (supersonic or even escape velocity) of a shuttle close to ground on it's surroundings.
2. Would this be possible to maintain stable flight with this velocity on such low altitude? Could this be safe?
3. How much higher than escape velocity should be the speed of that shuttle so that it could reach low Earth orbit without loosing escape velocity because of all air resistance. We can assume the shuttle would have aerodynamics similar to recently operated by the NASA.

Answer 1

Supersonic at ground level has beeen done once, by the Thrust SSC land speed record car. At supersonic speeds, a shockwave forms on the nose of the car or air/spacecraft. If you're close to the ground, this shockwave will reflect off the ground and potentially cause trouble. You'd also need to consider the noise level for the neighbours.

A lot of the technological challenges are similar to those of a space gun, see this question. The major limitation will be air resistance: if you accellerate to a significant fraction of orbital speed at ground level, you're losing enormous amounts of energy through drag.

Using an aircraft as you describe, taking off under its own power and then descending onto the magnetic track, is even more difficult. Maglev trains hover a few cm above the track. The higher they go, the smaller the magnetic force. Imagine trying to fly an aircraft at a constant altitude of 5 cm over a concrete track.

Answer 2

While it is theoretically possible, it is most likely impractical.

Specific concerns include the bowshock, and the resultant sonic boom. If using also a tube, the air itself will also generate a considerable force profile and tube-exit turbulent flow.

To use a non-evacuated tube, you're needing to generate supersonic airflow - this can be done, but requires a massive amount of moving air from high pressure reservoirs. (Functionally, any rifle does exactly this - generating the supersonic gas wave by detonation of gunpowder.) The effects of the rush of supersonic, or for launch to orbit, hypersonic, air out of the barrel will create a massive exit turbulence, and by venturi effect, drag surface air into the flow. It will also be the loudest cannon yet. To generate this supersonic flow, air will have to be added to the tube to prevent back pressure; at that point, the maglev may as well overpressure and make use of the pressurization, by massive pressure additions behind the vehicle timed to hit just as it passes. If the air in the tube is already moving at high speed (certain wind tunnels can produce over 400 kph), the in-tube drag can be reduced, but there will be a sudden deceleration when the non-moving air is encountered upon exit.

Note that supersonic flight, especially in the hypersonic region, is a high drag configuration.

The pressure wave from the barrel exit will, as previously noted, transfer a lot of kinetic energy to the exterior air, resulting in an upward flow, which creates a surface low, which draws air in from around. This will pull surface materials towards the exit point. This is a multiple hazard situation - it's a hazard to anyone or anything near the exit point, and it's a hazard for repeat use because the exit port must be kept clear of debris.

The bowshock only exists when the projectile is faster than the medium. All objects moving through fluids generate bowshock - but it's only a significant noise situation when the speed is a high fraction of the speed of sound in that fluid, relative to the fluid itself. (A bullet doing 400 m/s down a 200 m/s airflow isn't generating a 400 m/s bowshock, but a 200 m/s one.) Since the bowshock is a major portion of aerodynamic and hydrodynamic drag, the bowshock is going to vary widely based upon the nature of the air being moved through.

The bowshock will sound like an explosion (and is, mechanically, nearly identical to one at range), as the transition to supersonic speed happens. The air, since it cannot accelerate quickly enough through normal kinetic transfer, converts the movement energy from directional vector to vibration. We hear vibrations, and this generates the large sound.

In an enclosed tube, the boom becomes relevant upon exit (the tube itself must be constructed to survive it. In an open system, the entire path of the rail must be capable of withstanding the noise and outward air rush. This makes the unenclosed supersonic system a major environmental quality issue for noise pollution, in addition to the previously mentioned venturi effect and ground drag.

Further, the post-vehicle shockwave can generate a low pressure zone that itself causes a second boom - few aircraft hit velocities high enough for the human ear to notice this second, weaker, cavitation, but in water, cavitation causes bubble formation and demonstrates the principle - the noise of a high speed submarine is both the bow shock and the collapse of the partial vacuum behind it. A launch system will have speeds that generate a much longer cavitation event than self-propelled aircraft do. This is going to be perceived as a longer boom, or even a second boom (certain high powered rifles generate a separate cavitation sonic event).

So, in the end, there is the turbulent airflow, the disruption of surrounding landscape, and the noise pollution, plus much increased drag.

Answer 3

Maglev launch without tubes is possible, although not in the fashion you are envisioning. Look at Launch loops--not that I would want one anywhere nearby--the whole thing falls apart if the power goes off.