I'm totally hooked on these TED talks.
One of the latest talks given by Regina Dugan the director of DARPA, [link], was as frightening as it was fascinating, but the point of her speech at about the 4:30 mark got me to thinking.

If a glider using only free-fall as its propulsion can do mach 20 then I can imagine that the friction caused by initial re-entry is much of what needs to be overcome to be able to reach speeds hot enough to melt iron.

If the atmospere is the problem then do away with it.
Not entirely, just partially.

Utilizing focused rays to super-heat the atmosphere in front of the craft, like Ayaks, [link], at the point of re-entry to create a partial vacuum collapse in front of the craft entering the atmosphere from space may have the effect of greatly reducing drag enough to enable a sustainable mach 20 for somewhat longer than three minutes.

A relevant analogy would be a continuously erupting depth-charge beneath an oil tanker.
No friction = faster descent.
-- 2 fries shy of a happy meal, Mar 29 2012

http://www.ted.com/...ing_bird_drone.html [2 fries shy of a happy meal, Mar 29 2012]

http://en.wikipedia.org/wiki/Ayaks [2 fries shy of a happy meal, Mar 29 2012]

Aerospike http://en.wikipedia...resistant_aerospike
The mechanical version [spidermother, Mar 29 2012]

External Combustion Aircraft External_20combustion_20aircraft_20engine
See annos from about halfway down for a previous discussion of this topic. [DrBob, Mar 29 2012]

Negative glider Negative_20glider
This idea bats around schemes for speeding freefall. One of my favorites from a HB era with many grand physics schemes and informative discussion. [bungston, Mar 29 2012]

er... well, yes... get down to 10,000 meters before thermal upset, and you've basically re-created Tunguska... what was the point again?
-- lurch, Mar 29 2012


Hmmm cool, thanks. Never heard of Aerospike or Tunguska.

Aerospike looks like it would work better if it could extend relative to speed.
Tunguska could be avoided if the partial-vacuum preceeding the craft collapsed on a curved trajectory allowing the craft to ricochete within a curved collapsing-vacuum tube much as a bullet ricochetes through its own cavitation in water.
This would also decrease the weight of the craft itself because control surfaces would be redundant as the craft could not leave the partial-vacuum column.

I think.
-- 2 fries shy of a happy meal, Mar 29 2012


What was the point again? Fly into space and then come back really, really quickly? How do I slow down before hitting the ground when spacecraft already decelerate at ~9g (sometimes)?
-- DIYMatt, Mar 29 2012


Thank you. I had lost my train of thought there.

The point is to deliver a payload anywhere on the planet from space within sixty minutes.
In Ms./Mrs. Dugan's talk this seemed to be the goal to be attained by whatever means, and un-manned at that, assuming you could not fail.

Of course I assume that said payload would have a humanitarian, rescue mission, or relief effort goal in mind...
...although I've been mistaken about these assumptions in the past, so...

point is that it would work I guess.
-- 2 fries shy of a happy meal, Mar 29 2012


Get it down to 30 minutes and you could own global pizza delivery, from the Azores.
-- RayfordSteele, Mar 29 2012


...trying to determine what would constitute Freefall...est
-- zen_tom, Mar 29 2012


Zen-tom, you schemed this exact idea up in an anno to Negative Glider (linked).

/ A super-heating laser might rarify the air in front of you thinning it out, reducing the pressure against which you are falling - though, it might equally expand the gasses in an explosive way, providing more resistance. — zen_tom, Mar 14 2005 /
-- bungston, Mar 29 2012


//the friction caused by initial re-entry ... to reach speeds hot enough to melt iron// - there isn't "friction" which causes things travelling very fast through the atmosphere to become very hot. Rather, the fast-moving object compresses air in front of it which, following Boyle's law, becomes very hot.
-- hippo, Mar 29 2012


Doh!

The only previous refference I could find when I searched was on the external Combustion Aircraft posting.
Never thought to search the word 'rarify'.
Good scoop sir.
[marked-for-expiry]

//Rather, the fast-moving object compresses air in front of it which, following Boyle's law, becomes very hot//

Ah. I was going by what was said on that TED talk.
Thank you.
Back to the drawing board then. <can be heard muttering 'cauldron Boyle and cauldron Hubble' in the background>
-- 2 fries shy of a happy meal, Mar 29 2012


//Cauldron Boyle and cauldron Hubble// [marked-for-tagline]
-- FlyingToaster, Mar 29 2012


//have a humanitarian[...] goal in mind//
Ah, yes, [8th]'s dictionary indicates humanitarian aid is measured in megatons...
-- lurch, Mar 30 2012


Just a minor point, but the easiest way to deliver payloads is balistic launch from the earth's surface. Since the vehicle never reaches orbital velocity, it doesn't have to decelerate from orbital velocity. ~30 minutes to anywhere on the planet (look up the numbers for ICBM timing if you don't believe me). Admittedly you probably add a few more minutes if you want the payload to touch down gently instead of explosively.
-- MechE, Mar 30 2012


//Since the vehicle never reaches orbital velocity, it doesn't have to decelerate from orbital velocity.//

No, I don't believe you.

The earth has a circumference of approximately 40,000 km. Thus, a speed of 40,000 km/hr would be required to circle it in an hour, or more if you're not doing a surface orbit. The same speed would be required to traverse it halfway in a half hour.

Orbital velocity for Earth is not higher than 29,000 kph for any circular orbit.
-- lurch, Mar 30 2012


I guess they do heat more than I thought, but still. The required delta V for most locations is less than for low earth orbit. Remember that nothing is more than 20,000km away, and most land mass is much less than that, especially if you start from Northern Africa.
-- MechE, Mar 30 2012


//The required delta V for most locations is less than for low earth orbit.// The delta-V for *all* locations is less than for orbit. Otherwise, you'd need to burn again to de-orbit - and you'd no longer be talking about a "ballistic" missile.

However, I really like the idea about basing the missiles in North Africa. I think everybody should do that.
-- lurch, Apr 03 2012


//The delta-V for *all* locations is less than for orbit.//

Not absolutely. A ballistic trajectory is simply an elliptical orbit that happens to re-intersect the earth's surface. Any launch creates such an eliptical orbit. In order to reach a stable circular (ish) orbit, you need to perform an additional (insertion) burn at the correct time.

A ballistic trajectory that passes through a high orbit would have a higher delta V than a lower, circular, orbit. That being said, you're most likely going to aim for fairly low, efficient ballistic trajectories, which typically will be sub-orbital.
-- MechE, Apr 03 2012



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