Expert: Steve Nelson - 7/21/2009

Question
QUESTION: Sir,
I'm just an old guy (69)  browsing and came across your answer to this question regarding velocity needed to escape  the Earth's gravitational pull. Sorry to waste your time, but I would really like to know why a regular passenger jet that is capable of flying 12 - 14 hours, flying at a 45 degree angle, would not break through Earth's gravitational pull. It just seems that if a plane can fly 12,000 miles straight, at a 45 degree angle, after 10 hours, it should be in space, assuming there is breathing equipment available for the pilot.
This is from your answer of 6/5/2008. I would be very grateful If you could dumb down your answer a bit so I have a chance of understanding.
Thank you and I apologize if my question is really bad.

ANSWER: A jet does indeed have the massive specific impulse one needs to reach orbit, with a couple of big caveats which make spaceflight absolutely impossible.

1)  Their 12-14 hour flight time depends on pretty much level flight after they reach a cruising altitude in the 10's of thousands of feet (only a few miles up).  They can't continuously fly at a 45 degree angle for that long.

2)  The radius of the Earth is a bit under 4000 miles.  Flying 12,000 miles at even 45 degrees would put them far enough away from the Earth's surface that an angle measurement of 45 degrees is no longer really meaningful...and they can't fly straight up (which is what they'd be doing at that point).

3)  To overcome the Earth's gravity, you really do need to achieve a velocity that makes jets look like snails.  They have a maximum speed, and it's about 3.5% of typical orbital speeds.  To make them go 30 times faster like they'd need to reach orbit or escape gravity, you'd have to give them 900 times the energy.  This is the energy you'd have to use pushing against gravity to overcome its potential energy.  That would reduce their flight time to about a minute, which is way less than the space shuttle engines can burn for full-out (because the shuttle's mass is mostly fuel, has to be to reach orbit).

4)  This is the big one:  they need air to fly!  NOT just for the wings (again, they can't fly straight up) but for the jet engines as well.  The service ceiling on a 747 is only 43,000 ft for a good reason, you're down to almost 10% of an atmosphere at that altitude.  The pressure of air falls off exponentially after that, and that's only 0.2% of the radius of the Earth in altitude (you'd need to go 500 times higher to get one Earth radius away from the surface, where the gravity is still pretty strong (1/4 of surface gravity).  Jets are fundamentally trapped within the thin skin of atmosphere that covers our planet by their design.  They use massive amounts of air to provide thrust, oxidizer and mass which rockets must carry with them all the way up (making them really heavy).

In short, jets can plot along efficiently in the nice atmosphere with all that external air they get to use to combine with fuel for thrust, just fine.  Escape Earth's gravity?  No way.

---------- FOLLOW-UP ----------

QUESTION: Dear Mr. Nelson,
Thank you very much for taking the time to answer my question. I fully understand your explanation and greatly appreciate you taking the time for me. I don't know where I'm supposed to go to rate your reply but it's definitely 5 stars.
Thanks,

Answer
Thank you.  Due to people who would rate a question badly just because they didn't want the truth or who would lash out if I'd gone away for the weekend (this is a volunteer thing we do) and didn't answer their questions fast enough, I've turned off rating of my answers.  My ego will live without praise, and if people don't like my answers they can find other experts.  But it's good to know once in a while they help out.