Shuttle Re-entry Speed?

[mgidm86]

This is something I've been wondering but have been afraid to ask (I just thought I should KNOW this). Someone asked me this and I couldn't answer it entirely.

There is a minimum velocity to escape the Earth's atmosphere. Is there also a minimum re-entry speed? I was asked why the shuttle has to re-enter the atmosphere so fast.

My answer was that it would take too much fuel to slow the shuttle down from it's orbital speed (what, a paltry 1-2 million pounds?!). But is there a minimum re-entry speed for an object? My somewhat-ejukated guess is that there is not.

I searched for this and didn't really find an answer, although I found an interesting theory that on long future spaceflights, other planets could be used to lower the ships velocity as it nears Earth, similar to the way they can also increase it. I have no idea if that's possible or not.

Anyone have the answer? Or maybe some wild schemes to slow re-entry?

I was thinking that a gigantic net could catch the shuttle, or maybe have an orbiting or lunar gas/petrol station (full serve) and...

 

[patnray]

Orbital velocity at shuttle altitudes (about 150 miles) is approx 18,000 mph. Interestingly, the closer to Earth an object is, the faster it has to go to stay in orbit (from Keppler's law, orbiting bodies sweep out equal areas in equal times).

Consider that it took a solid rocket booster and two large tanks of fuel to get the shuttle moving that fast (and lift it that high). Thus it would take a large amount of fuel to slow it down again.

And if you did reduce it's speed to 0, it would drop towards the earth, accellerating at 32 ft/sec/sec with no air friction to limit it's speed. By the time it had fallen to about 50 miles (the outer reaches of the atmosphere) it would have acquired enough speed to cause significant heating.

It is interesting that to leave orbit the shuttle only has to reduce it's speed by about a hundred feet per second. Just enough to start a slow spiral towards the earth.

 

[CurtC]

And the minimum velocity is to escape the earth's gravity well, not the atmosphere. You could have a rocket that went up at a snail's pace until it reached the vacuum of space. The problem with this is that it would take a *huge* amount of fuel. The most efficient way to get stuff up high is to accelerate it as quickly as you can.

The escape velocity of the earth at the surface is around 25,000 mph, but the shuttle goes only around 17000 mph. That's because it's not all the way out of earth's gravity. In fact, the gravity at the shuttle orbit distance is something like 90% of what it is here at the surface.

 

[Crossbow]

Strictly speaking, there is no minimum re-entry speed provided the space craft has the power needed to overcome the gravitional forces that are generated between the planet and the space craft.

This may sound straight-foward, but in the case of the shuttle (or most space craft), it is not.

When the shuttle lands, it does so by gliding in. The shuttle does not have the engines that would be needed to enable it to make a powered descent; and if it was a powered descent, as opposed to an unpowered descent, then one would have much more control over the rate of descent. In other words, the shuttle could land more like a convential aircraft as opposed to 1.3 million pound glider.

I hope this helps!

 

[mgidm86]

Yeah it helps Thanks for all the info! So I had the right idea I guess on some of that. I figured there was no minimum re-entry velocity, but as Crossbow pointed out (I think it's kinda what you meant) it would be like a stalled airplane freefalling. It would need at least some forward momentum just to fly and not drop like a stone. Unless it fell pointing toward earth maybe.

Looks like I need to brush up on this subject more than I thought. I've recently been fascinated by studying light and ftl travel, the paradoxes, relativity, just general near light travel and the ideas of what would occur at such speeds.

I've found some great resources. Studying relativity and such is kind of like trying to remember someones name, and having it right at "the tip of your tongue", but not quite able to recall it. What I mean is, just when I think I'm understanding a complex aspect of it and my minds' comprehension is being stretched to the limit, it collapses back... like "Damn, I almost had it!"

Like trying to teach my mind to comprehend things it wasn't meant to (so-called "beyond comprehension"). BTW, I started the FTL stuff at a site called www.physicsguy.com. There's a GREAT series on FTL and special relativity, written in "advanced laymans" terms I'd say.

Thanks again for the info everyone I love this stuff!

 

[xouper]

Crossbow: When the shuttle lands, it does so by gliding in. The shuttle does not have the engines that would be needed to enable it to make a powered descent; and if it was a powered descent, as opposed to an unpowered descent, then one would have much more control over the rate of descent. In other words, the shuttle could land more like a convential aircraft as opposed to 1.3 million pound glider.

Pardon my failure to get your point, so I guess I'm asking for clarification. The shuttle lands just like any other airplane, just faster. Put a Boeing 767 cruising at 41,000 feet and shut off the engines and it can land just fine (e.g. the Gimli Glider). So I guess I didn't get your point. Can you help me out here? Thanks.

 

[shanek]

Orbital velocity at shuttle altitudes (about 150 miles) is approx 18,000 mph. Interestingly, the closer to Earth an object is, the faster it has to go to stay in orbit (from Keppler's law, orbiting bodies sweep out equal areas in equal times).

Actually, I think you're thinking of Kepler's Third Law, which states that the period of an orbit squared is proportional to the cube of its average distance. (Sweeping out equal areas in equal times is Kepler's Second Law.)

 

[Crossbow]

Pardon my failure to get your point, so I guess I'm asking for clarification. The shuttle lands just like any other airplane, just faster. Put a Boeing 767 cruising at 41,000 feet and shut off the engines and it can land just fine (e.g. the Gimli Glider). So I guess I didn't get your point. Can you help me out here? Thanks.

To: xouper

True, the shuttle lands pretty much like any other airplane with the exception of being the worlds largest glider.

Ideally, when one is landing an airplane, the plane should be close enough to runway, and high enough off of the ground so that the plane could be landed if the engine(s) were to fail. As a result, most planes are designed to land with their engines at idle.

However it is possible when one is about to land that there may be a problem with the weather, the runway, conflicting traffic, or something else that would force the plane to delay, expedite, or abort the planned landing.

For a powered airplane, that is no problem. If the landing needs to be delayed for some reason, then hit the throttle, gain some altitude, and circle the airfield until it is time to land. If the landing needs to be made somewhere else, then just hit the throttle and head off. If the landing needs to be made sooner rather than later, then just hit the throttle and travel faster to make the time. If the plane comes in a bit low, then just hit the throttle to climb to the desired altitude. And so on.

In other words, a powered approach provides one with many options.

However, the shuttle makes an unpowered approach, so there are far fewer landing options and the parameters of the landing (i.e. approach speed, rate of descent, glide slope, etc.) cannot be modified by simply hitting the throttle.

I hope this helps!

 

[Crossbow]

Actually, I think you're thinking of Kepler's Third Law, which states that the period of an orbit squared is proportional to the cube of its average distance. (Sweeping out equal areas in equal times is Kepler's Second Law.)

To: shanek

Not to sound terribly fussy, but Keplers Second Law is applicable in the situation that pantray discussed.

Think about comets. They have an extremely ellipitical orbit that can have a period lasting decades, centuries, or even millenia. When they are at there greatest distance from the sun, they travel very slowly for a given time period (so the length of the displacement arc is very short). However, when they get near the sun, they are travelling much, much faster, so the length of the displacement arc is very long during this same time period.

But in either case, as you pointed out, the area swept out will be the same.

Thanks much and I hope this helps!