Question about suborbital flight and ballistic missiles

[Armed Update]

Question on suborbital flight and ballistic missiles.

When they are in space, past 100 km above the surface of Earth where there is little air, when it's in the ballistic arc, past apogee does it slow down when it drops or speeds up due to gravitational acceleration without drag? I Am talking about the velocity heading to earth.

 

[Drakkith]

does it slow down when it drops or speeds up due to gravitational acceleration without drag? I Am talking about the velocity heading to earth.

The radial velocity (the part of the velocity pointed towards/away from the Earth) increases as it falls back to Earth.

 

[Armed Update]

So if the launch vehicles final boost is Mach 20 what decrease usually would it have at apogee? By the time it its almost about to reach the atmosphere does it going to Mach 20 again?

 

[Drakkith]

Well, the vehicle is not going Mach 20 in a radial direction. Most of that is in a tangential direction. The radial velocity upon reentering the atmosphere is probably fairly small compared to the tangential velocity, but I really don't know an accurate number.

 

[Armed Update]

What is the difference between radial and tangential?

 

[Drakkith]

Radial heads directly away from a point, like how the spokes on a bicycle wheel head directly away from the hub.

Tangential is basically the direction perpendicular to any of these spokes. Put simply, if we look at a spoke that runs straight up away from the hub, then the tangential direction is basically the directions straight left or right (or out and away, or any direction that runs perpendicular to the spoke).

See this image for a visual.

In short, radial is the up/down direction while tangential is the left/right direction if you're looking at the trajectory in 2d (like on a map or a graph).

 

[berkeman]

So if the launch vehicles final boost is Mach 20 what decrease usually would it have at apogee? By the time it its almost about to reach the atmosphere does it going to Mach 20 again?

Wikipedia gives a pretty good idea of the flight envelope of an ICBM...

https://en.wikipedia.org/wiki/Intercontinental_ballistic_missile

Flight phases
The following flight phases can be distinguished:

• boost phase: 3 to 5 minutes; it is shorter for a solid-fuel rocket than for a liquid-propellant rocket; depending on the trajectory chosen, typical burnout speed is 4 km/s (2.5 mi/s), up to 7.8 km/s (4.8 mi/s); altitude at the end of this phase is typically 150 to 400 km (93 to 249 mi).
• midcourse phase: approx. 25 minutes—sub-orbital spaceflight with a flightpath being a part of an ellipse with a vertical major axis; the apogee (halfway through the midcourse phase) is at an altitude of approximately 1,200 km (750 mi); the semi-major axis is between 3,186 and 6,372 km (1,980 and 3,959 mi); the projection of the flightpath on the Earth's surface is close to a great circle, slightly displaced due to Earth rotation during the time of flight; the missile may release several independent warheads and penetration aids, such as metallic-coated balloons, aluminum chaff, and full-scale warhead decoys.
• reentry/terminal phase (starting at an altitude of 100 km, 62 mi): 2 minutes – impact is at a speed of up to 7 km/s (4.3 mi/s) (for early ICBMs less than 1 km/s (0.62 mi/s)); see also maneuverable reentry vehicle.

ICBMs usually use the trajectory which optimizes range for a given amount of payload (the minimum-energy trajectory); an alternative is a depressed trajectory, which allows less payload, shorter flight time, and has a much lower apogee.[20]

 

[Janus]

Wikipedia gives a pretty good idea of the flight envelope of an ICBM...

https://en.wikipedia.org/wiki/Intercontinental_ballistic_missile

Then using the 1200 km apogee and the 6372 km semi-major axis, we get a velocity of ~6.5 km/s at apogee.

 

[Armed Update]

My question is are ascent speeds greater than descent speeds(before reentry) in a space enviornment. I am just talking about midcourse phase.

 

[berkeman]

My question is are ascent speeds greater than descent speeds(before reentry) in a space enviornment. I am just talking about midcourse phase.

Now that you understand the radial and tangential components of vector velocity, and given that air resistance is very low in near-Earth space, what do you think happens to the speed of the projectile?