Wednesday, September 28, 2016

Orbital Speed


Have you ever wondered why a spacecraft could not just slow down on re-entry to just a few miles per hour using rocket boosters? Would it negate the need for a heat shield?

In other words, is it possible for a spacecraft to control its reentry in such a way that it avoids the atmospheric compression and thus would not require the expensive (and relatively fragile) heat shield on the outside?

Well, the reason it's hard to get to orbit isn't that space is high up.

It's hard to get to orbit because you have to go so fast.

Space isn't like the pic to the left.

Space is about 100 kilometers away. That's pretty far away — I wouldn't want to climb a ladder to get there — but it isn't impossible. If you're in Sacramento, Seattle, Canberra, Kolkata, Hyderabad, Phnom Penh, Cairo, Beijing, central Japan, central Sri Lanka, or Portland, space is closer than the sea.

Getting to space is easy. It's not, like, something you could do in your car, but it's not a huge challenge. You could get a person to space with a small sounding rocket the size of a telephone pole. The X-15 aircraft reached space just by going fast and then steering up. [You will go to space today, and then you will quickly come back.

But getting to space is the easy part. The problem is staying there.

Gravity in low Earth orbit is almost as strong as gravity on the surface. The Space Station hasn't escaped Earth's gravity at all; it's experiencing about 90% the pull that we feel on the surface.

To avoid falling back into the atmosphere, you have to go sideways really, really fast.

The speed you need to stay in orbit is about 8 kilometers per second. Only a fraction of a rocket's energy is used to lift up out of the atmosphere; the vast majority of it is used to gain orbital (sideways) speed.


This leads us to the central problem of getting into orbit: Reaching orbital speed takes much more fuel than reaching orbital height. Getting a ship up to 8 km/s takes a lot of booster rockets. Reaching orbital speed is hard enough; reaching to orbital speed while carrying enough fuel to slow back down would be completely impractical.

These outrageous fuel requirements are why every spacecraft entering an atmosphere has braked using a heat shield instead of rockets — slamming into the air is the most practical way to slow down.

How fast is 8 km/s, anyway?

I think the reason for a lot of confusion about these issues is that when astronauts are in orbit, it doesn't seem like they're moving that fast; they look like they're drifting slowly over a blue marble.

But 8 km/s is blisteringly fast. When you look at the sky near sunset, you can sometimes see the ISS go past... and then, 90 minutes later, see it go past again. In those 90 minutes, it's circled the entire world.

The ISS moves so quickly that if you fired a rifle bullet from one end of a football field, the International Space Station could cross the length of the field before the bullet traveled 10 yards.

Let's imagine what it would look like if you were speed-walking across the Earth's surface at 8 km/s.

To get a better sense of the pace at which you're traveling, let's use the beat of a song to mark the passage of time. suppose you started playing the 1988 song by The Proclaimers, I'm Gonna Be (500 Miles). That song is about 131.9 beats per minute, so imagine that with every beat of the song, you move forward more than two miles.

In the time it took to sing the first line of the chorus, you could walk from the Statue of Liberty all the way to the Bronx: [You'd be moving at about 15 subway stops per second.]

It would take you about two lines of the chorus (16 beats of the song) to cross the English Channel between London and France.

The song's length leads to an odd coincidence. The interval between the start and the end of I'm Gonna Be is 3 minutes and 30 seconds, and the ISS is moving is 7.66 km/s.

This means that if an astronaut on the ISS listens to I'm Gonna Be, in the time between the first beat of the song and the final lines... [Just to burn up in the atmosphere above your door.]

...they will have traveled just about exactly 1,000 miles.

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