Relative Speed and Striking Speed
Name: Myra S.
Date: June 2003
I do not understand how the foam insulation that broke off the fuel tank
could strike the shuttle wing at a speed of 500 mph as reported. The debris would
have essentially the same speed as the shuttle when it broke off, and I know the
shuttle was accelerating, but how can the difference over such a short distance
amount to a speed difference of 500 mph? What was the acceleration at the moment?
Very fine question. This was also my question. One can assume that the speed difference
between the time at which the foam broke off and the time at which it struck the left wing
is ~ 0. However, after some thought, and after watching the high speed videos (Courtesy of
NASA TV), it is conceivable that the foam might accelerate to a relative speed (relative to
the orbiter) of a few hundred MPH. "How", you ask? I am no NASA engineer. So I cannot
tell you the exact speed of the orbiter when the suspected fatal debris hit the wing.
But I will venture a guess at > 5000 MPH. The shuttle experiences most of its
acceleration in the first minute or two while the SRB's are still 'catapulting' the
entire vehicle into orbit. After the SRB's are spent and break away, the three main
engines burn for ~ 6-7 minutes or so to do the rest of the work. My point is that just
because it seemed as if the damage occurred toward the beginning of the flight does not
mean that the orbiter was going relatively slow.
Furthermore, you are viewing images on your TV screen that scale down a mammoth sized
space vehicle into a ~ 25 - 32" diagonal screen. So you really do lose some perspective
on the true magnitude of what is going on. This type of foam insulation might conceivably
have a density = a fraction that of water. This means that its air drag coefficient would
be ridiculously HUGE. As the foam dislodged from the vehicle's external tank, it probably
had time enough to travel ~ 100 feet or so to accelerate (actually decelerate as viewed
from someone on the ground) and build up enough relative velocity due to this massive
deceleration to strike the orbiter at a relatively small speed of a meager 500 MPH.
I believe in the accuracy of the relative speed figure of 500 MPH. After all, they are
NASA, they put people on the Moon and got them back to Earth safely. Furthermore, it is
just a believable figure.
Here is the question to ask. How is it that something so NON-DENSE as foam, could cause
any damage or aid in dislodging of any heat shield tiles? The very physical property of
the foam that gives rise to its massive relative speed increase, would cause that same
foam to do less damage because of its lowered density
( ie less momentum / kinetic energy ).
I just wish that the orbiter got hit by micrometeorites or something beyond our control.
It just sickens me to think that it is possible that we lost seven human lives, very
intelligent and brave souls, just to learn a few relatively petty details of the shuttles
possible failure mechanisms.
Go to link #1 for more details and a decent quality report (dated April 2003) on the shuttle
Go to link #2 for a great link to remember all astronauts/cosmonauts/space travelers.
They have paid the ultimate price to help mankind gain experience and to ultimately make
manned space flight a safer reality.
Quote from URL #1
"Investigators said the National Imagery and Mapping Agency has further enhanced a video
that shows the left wing impact during the ascent. Tetrault said the team now believes a
two-pound piece of foam approximately 24 inches by 15 inches by 5 inches, traveling about
430 mph, struck the left wing somewhere in a two-foot-wide "footprint" centered on the
lower side of carbon fiber panel number 6. The footprint includes portions of two carrier
The shuttle needs to have a certain velocity in order to escape the earth's gravitational
pull, which is on the order of 25,000 miles per hour. So, if a small chunk of foam falls
off, there is no longer thrust behind it to accelerate it (or maintain constant velocity)
and there is now wind resistance and gravity slowing it down. I do not the exact value of
the acceleration at that time, but I can imagine that the velocity estimated is pretty
close when things are traveling at 5,000 mph or better.
Christopher Murphy, P.E.
I do not know how NASA arrived at the speed at impact for several reasons. I have not read
what their frame of reference is. Presumably, it is either with respect to the center of
mass or the space shuttle, which would be about the same. In addition, it is not clear
whether the piece of foam was accelerated by a rush of gases from rockets nearer the top
of the shuttle and/or the hydrodynamic speed of the air that passes over the shuttle
generated by the ignition of the rocket engines.
A related aspect is that is not talked about in the press reports that I have seen is the
fact that a piece of foam, even "squishy" foam can act like a brick at high speeds. The
impact is of such short duration that the foam has no time to deform mechanically. An
analogy is diving. If you jump off a 3 meter diving board -- it is fun -- but if you
jump out of a plane at 3000 meters. The water behaves essentially like a hard surface.
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Update: June 2012