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Lord Jubjub
2007-Oct-20, 02:24 AM
What is the minimum orbital altitude that a craft could attain and not be ripped to shreds by the atmosphere? I am assuming that the craft has the propellant necessary to obtain orbital speeds regardless.

I would imagine that the troposphere (the bottom seven miles) would be too thick, but could something orbit in the stratosphere? Of course, friction would slow it down, but would it destroy the craft?

Ronald Brak
2007-Oct-20, 04:03 AM
Sputnik 1 was put into an orbit from 215 km at closest approach to 939 km and burnt up after six months. Lower than that and you'll probably see a significant decrease in orbital time. Of course mass to volume ratio will have an effect as air resistance will have more of an effect on an aluminium balloon than a lead cannonball.

Romanus
2007-Oct-20, 04:08 AM
Though I'm no expert, I'm pretty sure nothing could orbit in the stratosphere--even if our putative spacecraft could avoid getting incinerated, drag alone would slow it and bring it down. Even well above the bulk of the atmosphere, orbits are short-lived and unstable below ~200-250 km.

Jens
2007-Oct-22, 01:18 AM
The OP was assuming that it has sufficient propellant, so the drag isn't an issue. It's a question of whether there is material that could withstand the heating that you would experience at various altitudes.

neilzero
2007-Oct-22, 06:45 AM
A circular orbit at perhaps 60 miles altitude is likely possible for a few minutes. It would need lots of fuel, because of the air resistance. It would also need lots of coolent to avoid melting the outer surface. One possibility is sintered metal powder though which water can pass becoming steam at the outer surface. A possible application is a hyper-sonic airplane which would deliver a pay load to a bolo = tether that tumbles end over end in LEO = low Earth orbit. The aiplane would be somewhat less than orbital speed (18,000 miles per hour) as the bolo's tip speed would subtract from the orbital speed of the weight center of the bolo tether.
Some sucessful experiments indicate that a powerful laser can reduce the air resistance, allowing higher speeds. If this suceeds, perhaps orbital speed at an altitude of 50 miles is possible, but very costly. Neil

Ronald Brak
2007-Oct-22, 07:32 AM
The OP was assuming that it has sufficient propellant, so the drag isn't an issue. It's a question of whether there is material that could withstand the heating that you would experience at various altitudes.

I assumed that the OP meant reach orbital velocity and then the satellite would be unpowered. Where do you draw the line between orbiting and powered flight that happens to be at orbital speed? If someone called me up on the phone and told me they were in earth orbit while their rockets were expending tons of propellent per minute I wouldn't belive her.

cjl
2007-Oct-22, 09:07 AM
I believe the question was with regards to holding orbital velocity while not burning up with current materials technology.

joema
2007-Oct-22, 06:32 PM
What is the minimum orbital altitude that a craft could attain and not be ripped to shreds by the atmosphere? I am assuming that the craft has the propellant necessary to obtain orbital speeds regardless...
I assume you mean propellant necessary to MAINTAIN orbital speed. If there was insufficient propellant to OBTAIN orbital speed, the question has no meaning -- you wouldn't be in orbit (nor at orbital speed), so the question about "ripped to shreds" in a low orbit would be meaningless, since you'd never get there in the 1st place.

Nonetheless, we can broaden the possible meaning and consider several possibilities:

For a coasting, non-propelled satellite, the lowest circular orbit with a lifetime of several hours is around 85-90 nautical miles (157-167 km). The later Apollo missions used an earth parking orbit of about 93 nautical miles (172 km).

At that altitude, the residual atmospheric drag will cause the orbit to decay within a few hours to a day. The drag isn't sufficient to damage a satellite, but it will quickly deorbit and reenter.

For a propulsive satellite, it can go lower but it then becomes a question of power, propellant, drag and heating.

There are two sub-options in the propulsive category:

(1) A regular non-aerodynamic satellite with significant propulsive capability. E.g, the KH-11/12 optical spy satellites are about 4.5 meters in diameter, 15 meters long, and weigh about 18 tons (36,000 pounds, or 16,300 kg). About 14,000
pounds of this is maneuvering propellant. In theory they could maneuver to a low circular orbit and maintain this against minimal atmospheric drag using propulsion. Or they could dip down in an elliptical orbit to a low perigee, compensating for drag with propulsion.

Such satellites obviously aren't streamlined or designed to withstand great atmospheric friction. However some can dip down to a 75 miles perigee, maybe lower: http://www.afa.org/magazine/june2003/0603kh7.asp

"Entry interface" -- the altitude where major atmospheric effects are first felt -- is about 400,000 feet (68 nm, 121 km). The above KH-7 recon sat dipped down close to entry interface. The KH-11/12 likely have enough propellant for this, whether they ever do is unknown. I doubt it.

In theory a regular non-aerodynamic satellite with sufficient propulsion could orbit slightly above entry interface altitude, say 70-75 nm. The propulsion requirement would be large, there's no operational incentive to do this, but it could probably be done.

(2) A satellite thermally and structurally designed for operations within the upper atmosphere. I don't know if any exist (besides the space shuttle). As you descend below entry interface (400k ft), drag would increase very rapidly, requiring huge propulsive capability to maintain orbit and equally advanced thermal protection.

In theory a scramjet-powered vehicle with active cooling could "orbit" below entry interface for a sustained period - say, 30-min to an hour. It would not be a satellite in the normal sense, but something like the X-30: http://en.wikipedia.org/wiki/X-30

Larry Jacks
2007-Oct-22, 09:27 PM
You wouldn't want to dip a billion dollar spy satellite into the atmosphere. The chances of error are just too great. Once reason those satellites cost so much is that they have really, really expensive optics that allow them to image the surface with very high resolution from a safe altitude. The amount of propellant allows for a long operational lifetime and a lot of operational flexibility.

joema
2007-Oct-23, 12:16 AM
You wouldn't want to dip a billion dollar spy satellite into the atmosphere. The chances of error are just too great...
You obviously wouldn't want a perigee below entry interface -- it would likely damage the satellite.

As you said the likely reason for large propellant reserves on current recon sats is operational flexibility.

However earlier recon sats such as the KH-7 "Close Look" used this propulsive ability to periodically descend to a very low perigee for improved resolution on a single pass. They used a restartable Agena-D stage to achieve this -- similar to that used on several Gemini flights: http://en.wikipedia.org/wiki/Agena_Target_Vehicle

Whether current recon sats similarly maneuver, nobody knows -- I tend to doubt it.

However the OP was speculative, asking about theoretical capability, not actual operational practice. The capability exists to orbit just above entry interface for a limited period, using propulsion to compensate for atmospheric drag.

For a satellite not streamlined and structurally and thermally protected, the lowest possible limit would (very roughly) be around entry interface, about 400,000 ft.

Going even lower is possible, but the vehicle would have to be specifically designed for that, and would be less a satellite and more a hypersonic ultra-high altitude aircraft.

mugaliens
2007-Oct-24, 06:20 AM
Sputnik 1 was put into an orbit from 215 km at closest approach to 939 km and burnt up after six months. Lower than that and you'll probably see a significant decrease in orbital time. Of course mass to volume ratio will have an effect as air resistance will have more of an effect on an aluminium balloon than a lead cannonball.

True.

Reminds me of an experiment a friend of mine tried on the highway with a bowling ball.

Don't ask.

At least he had sense enough to try it around 4 am instead of during rush hour traffic.

Kaptain K
2007-Oct-24, 03:03 PM
Don't ask.
Gimme a break! :mad: