Ram Pressure?

[Mk]

How does ram pressure work, is this the thing that affects meteors?

[thepaqster]

hey! the way i understand ram pressure is that it affects anything moving through the atmosphere. it is actually the compression of air in front of an object as it moves (i.e a baseball or a comet). the air in front of the object is compressed until it starts heating up and this is why comets burn up in our atmosphere. many people say that comets burn due to air friction, but the friction actually acounts for a small part of the heat... ram pressure is the main source of heating (i think hehe...) someone plz correct me if im wrong

[Astronuc]

For a description of ramjet engines, which utilize the compression of air through which it is flying, see -

http://www.grc.nasa.gov/WWW/K-12/airplane/ramth.html

The forward motion (kinetic energy) of a ramject causes the air to be compressed in the inlet. It's similar to the effect of a piston pushing on a closed volume of gas.

Objects flying through the atmosphere transfer momentum and energy to the air which is compressed and heated. Again think of rapidly pushing a piston against a closed volume of gas, or even working a bicycle pump very rapidly - the bike pump can get quite hot.

Meteors, like the space shuttle, travel at 1000's of mph (shuttle is coming out of orbit where it is traveling ~17,000) - hypersonic velocities. The air molecules cannot move out the way fast enough (they are constrained by the speed of sound), so they get compressed and very hot (that's why the heat shield on the Mercury, Gemini, Apollo and Space Shuttle).

The kinetic energy of the meteor or returning space craft is converted into thermal energy and pressure of the atmsphere. Meteors, without a thermal shield heat up and vaporize, with little of the initial mass surviving to the surface.

[thepaqster]

oh ya.... umm... thats what i meant... :D

[Mk]

You both gave a very helpful explination, thank you very much. How does the ramjet get going fast enough to utilize ram pressure capabilites for propulsion?

[russ_watters]

There are very few "pure" ramjet craft and most of those are boosted by rockets.

[Astronuc]

How does the ramjet get going fast enough to utilize ram pressure capabilites for propulsion?

Well clearly the thrust provided by the ramjet engine(s) must be sufficient to provide the force necessary to provide the compression in the ramjet, and also the air resistance against the craft which is flying. Remember, at constant speed, the thrust is balanced by the force of the air against the craft (some of the air going into the engines and the rest flowing in opposition at and around the wings, tail and fuselage).

The SR-71 uses a ramjet in a sense. The compressor (diffuser) section actually sits in front of a turbojet. The supersonic flow is slowed to subsonic and a turbo fan compresses the flow before it enters the combustor. It is a phenomenal piece of engineering. Here is somewhate of a technical description somewhere on-line that discusses the engineering behind the SR-71 engines - http://www.airspacemag.com/asm/mag/supp/fm99/oxcart.html.

As for the X-43, it is still under development and at the moment it is boosted by a Pegasus rocket.
http://www.nasa.gov/missions/research/x43-main.html

[pmb_phy]

How does ram pressure work, is this the thing that affects meteors?
Ram pressure is the force per unit area due to a body moving through a medium. Its different than fluid pressure. The former can exist in the absense of the later.

Take a system consisting of a bunch of dust particles all of which are at rest in a given inertial frame of reference S. If a body is at rest in S then the pressure on it will be zero. However if the body moves through the dust then the dust particles will impact on the surface and thus exert a force. The force per unit area is the ram pressure. In such a case the ram pressure is non-zero whereas the fluid pressure is zero.

Pete