I figure this is simple. When the shuttle is braking for re-entry, why not slow down to about 35mph rather than 20,000mph? And then just sort of drop back into the atmosphere mainly due to gravity? (the 35mph would be relative to the earth's ground speed assuming the atmosphere above it would be traveling about the same speed)
In speaking about this with someone else - they offered the following insight:
Considering the laws of thermodynamics and the conservation of energy, in order to slow the down the craft to 35mph it would take the amount of time required from the point of main engine firing at which time the craft is reversed and starts entering the upper atmosphere. Further, the craft is required to remove all of the energy that was injected into the craft from the main engines at the two boosters at the time of initial orbit. Say they were traveling roughly 17,000+mph, if you attempted to remove the energy from the craft at 1mph (for example), it would take slightly less than 17,000 hours. (1.94 years). Remove the energy in sums of 1,000mph and your craft will land 17 hours after initiating re-entry.
Keep in mind that it requires extensive amounts of additional stored fuel to use as an alternative to slowing down the shuttle vs. using atmospheric friction.
Instead, the shuttle is designed to withstand this heat energy in a more timely fashion.
Thanks. I hadn't really realized the amount of fuel it would take to do that but others were saying it had to travel that fast to "break thru" the atmosphere .. which I thought was nutty and the reason for my 35mph rather than 10kmph
I think it can be more simply understood this way. During take-off, the shuttle has to accomplish a few main goals. 1) escape the atmosphere, 2) align for orbit. The speed gained is the result of the efforts to achieve the above goals. Since it took a couple of SRBs (solid rocket boosters) during launch, that are now de-attached... there is no source to reduce speed. The speed reduction is a natural process when re-entering atmosphere (since friction is now a factor). It is actually a clever method of breaking, but I think it could use some tweaking.
.. that's not quite right. The huge rocket boosters used at liftoff are designed with one main goal in mind: provide enough thrust to reach escape velocity to counter the earth's gravitational pull, not break out of the atmosphere. In fact where the shuttle normally orbits still could not technically be considered "outter space", nor even "out of the atmosphere" for that matter since there IS a "micro" atmosphere there.. it is not fore quite some distance that Earth's atmosphere thins to finally "nothing".
As for the velocity - the velocity is determined mathematically in order to achieve "orbit". Orbit is the balanced activity of travelling past a gravitating mass in space at just the right speed such that your forward velocity which, in the absence of gravity, would be sufficient to increase distance from the body by the same amount that the body's gravitational pull moves the body toward it in that same period of time. The net result is that, though you have forward velocity, your distance from the body neither increases, nor decreases: wallah: you are in orbit. The relationships in this math involve elapsed time, distances and arc velocities. Simply put, the closer your distance to the body, the faster your velocity will have to be to counter its pull. Were there no atmosphere on Earth and if its surface were perfectly spherical, you could technically orbit it a meter above the ground - but you'd have to be travelling at a buhzillion miles per hour in order to do so.
So, at the altitude that the shuttle orbits, they have to maintain a certain velocity in order to stay in orbit - any faster and they would begin pulling away from the earth and eventually fly off into space, any slower and they would come spiralling down in an involuntary re-entry. So with different types of satellites and such things at varying altitudes in orbit around the planet, the shuttle must be able to precisely control its forward velocity to within a window of so many thousands of miles per hour.
As such, the rockets that deliver the shuttle into orbit not only have to achieve escape velocity, the minimum speed needed to break free of the Earth's gravitational pull in a "more or less" vertical ascent from the center of gravity of the planet, but they also must achieve a certain velocity in a tangental trajectory to the curvature of the earth in order to achieve orbit. Again, this velocity is determined by the target orbital altitude.
Furthermore, as I explained that if the velocity were too little you would come spiralling down to Earth - this is exactly what the shuttle does for re-entry. When they are ready to break orbit and return to Earth, they simply fire retro-thrusters against the direction of momentum which slows them down ever-so-slightly - slightly enough for the Earth's gravity to begin their spiral descent into the atmosphere. The shuttle crew is then able to use aerodynamics to orient the craft, nose-up, to face the belly of the craft into the most intenst blast furnace generated by atmospheric friction at 20 some-odd thousand miles per hour.
Im sure if NASA had a better way they would do it that way
The same Creator who names the stars also knows the names of the seven souls we mourn today. The crew of the shuttle Columbia did not return safely to Earth; yet we can pray that all are safely home. -G.W. Bush
02-27-2003 04:55 PM
