Tony_Buckland@mtsg.ubc.ca (04/19/91)
From: Tony_Buckland@mtsg.ubc.ca A recent posting about a mercifully unrealized plan for a hypersonic nuclear-engined cruise missile reminded me of a long-time puzzlement. Fiction and some fact literature talks about the ground and shipboard damage that can be caused by supersonic or hypersonic aircraft at low altitudes. But where is all the energy coming from? The plane only has the chemical energy in its fuel tanks to both complete its mission and do all this desired or undesired damage. Is this energy really sufficient to leave a trail of destruction across hundreds or thousands of km of the Earth's surface?
deichman@cod.nosc.mil (Shane D. Deichman) (04/22/91)
From: deichman@cod.nosc.mil (Shane D. Deichman) >From: Tony_Buckland@mtsg.ubc.ca >Fiction and some fact literature talks about the ground and shipboard >damage that can be caused by supersonic or hypersonic aircraft at low >altitudes. But where is all the energy coming from? The plane only >has the chemical energy in its fuel tanks to both complete its mission >and do all this desired or undesired damage. Is this energy really >sufficient to leave a trail of destruction across hundreds or thousands >of km of the Earth's surface? It really doesn't take a lot of pressure to wreak havoc on normal standing structures. A brick wall can only withstand peak overpressures of about five psi. (A one-megaton bomb at an altitude of one mile will create such overpressures at seven to eight miles away from ground zero! Remember that such intensities are inversely proportional to the SQUARE of the distance.) Any supersonic vehicle will create a shockwave like this: \ \ \ ============> / / / where the angle of the shockwave to the horizontal is equal to 45 degrees divided by the mach number. Since the vehicle is travelling at a velocity greater than or equal to the speed of sound, the sound energy will accumulate along the leading edge of the shockwave (rather than dissipate into the atmosphere as with subsonic aircraft) creating high overpressures. This is colloquially known as the "sonic boom" -- when the shock wave passes a certain point on the ground, all of that sound energy is heard at once. So, for a low-flying aircraft, all of that energy is building up on the shockwave. A low-flying supersonic aircraft, then, can cause considerable damage by overflying such objects as ships, buildings, even other aircraft! Even peak overpressures as low a two or three psi can make a wooden build- ing collapse onto itself.... -shane
henry@zoo.toronto.edu (Henry Spencer) (04/23/91)
From: henry@zoo.toronto.edu (Henry Spencer) >From: Tony_Buckland@mtsg.ubc.ca >Fiction and some fact literature talks about the ground and shipboard >damage that can be caused by supersonic or hypersonic aircraft at low >altitudes. But where is all the energy coming from? ... [Don't start paragraph's with From, please! It fools the digesting software into thinking its a mail header. --CDR] [F]rom the aircraft's engines. Those shock waves represent huge amounts of drag; the engines have to push both the aircraft and the shock waves through the air. >... Is this energy really >sufficient to leave a trail of destruction across hundreds or thousands >of km of the Earth's surface? If it's a sufficiently powerful engine, yes. The Pluto cruise missile (would have) had a one-gigawatt reactor driving it. Conventional jet engines run out of fuel very quickly at max power, and consequently conventional jet aircraft spend little or no time at supersonic speeds (let alone Pluto's Mach 3!) at low altitude. -- And the bean-counter replied, | Henry Spencer @ U of Toronto Zoology "beans are more important". | henry@zoo.toronto.edu utzoo!henry