The space shuttle needs protection because it is going at orbital speeds (about 17,500 mph). Alan Shepard’s sub-orbital flight needed a heat sheild because he was traveling at about 5,000 mph and going east. Nuclear warheads–same thing. None of these vehicles go straight up.
If NASA launched something straight up so that it just dropped back (from zero mph) in a geo-sychronous way, would it still need protection from heat?
Yes, absolutely.
It’s the friction of the atmosphere impacting the craft at high speed. Even though the craft would hit less of the atmosphere by going in straight rather than at an angle, the total would be sufficient.
This does not apply to a space elevator because of the much lower speed it would move at.
Yes. A vehicle 0.001 miles high could enter without needing a heat shield as well, if it came in slowly and lowered itself down at a walking pace.
On the other hand, there’s a video floating around YouTube of an asteroud much larger than 100 miles hitting at tens of thousands of miles per second, and there’s a white-hot glow on the parts of it passing through the atmosphere, even though the atmosphere comes up to its waist, if that.
It’s all about relative speed, not size.
From only 5.28 feet, it wouldn’t even have to be that gentle.
I’m pretty sure when the OP says “high” he’s referring to the altitude of the craft, not its size.
I suppose theoretically, even a space shuttle could re-enter without a heat shield if it carried enough fuel into orbit to come to a full stop.
Provided it is in freefall (i.e. thrusters aren’t powering it towards the Earth) it will reach terminal velocity which will change as the atmosphere becomes thicker and gravitational pull becomes stronger.
Starting from such a great height your first concern would be the cold, in the thermosphere and mesosphere the temperatures get as low as -100 degrees C. Someone better at maths might be able to work out how fast you would be falling by the time you encounter atmospheric resistance of any significance.
Weird. I totally misread that. I was thinking ‘diameter of craft’ rather than ‘altitude of craft’.
I have to admit, I first pictured a vehicle “100 miles high” as some kind of Death Star, too.
An object of moderate ballistic coefficient dropped from 100 miles altitude with no horizontal velocity will end up doing around Mach 3 at peak speed. This is enough to cause significant aerodynamic heating, but much less of a challenge than reentry from orbit or interplanetary trajectories. So some thought of heating has to be put into the design but it doesn’t need a heavy, expensive heat shield like those on the Shuttle or Soyuz.
You might like to look up sounding rockets, which do this sort of thing regularly. Also SpaceShipOne, which “feathered” its wings to reduce its ballistic coefficient, resulting in a lower top speed and reduced heating.
Oh, yeah–I guess the question could read like that. But, I meant 100 miles altitude, not a hundred mile death star size vehicle.
At that point, it’s no longer a space station - it’s a moon.
I did a little math and I “think” a vehicle would fall about 40 miles within the first two minutes–going about Mach 3 like pericynthion stated above. I don’t know how to figure the terminal velocity though.
So, Mach 3 at 60 miles-- sound to me like a vehicle made of inconel alloy (like the X-15) could probably make it “without a heat sheld”. Considereing that is about how high the x-15 reached and probably a lot faster. Right??
Thanks pericynthion, your answer really helped.
NASA does this all the time, such rockets are called sounding rockets. They typically go up a couple hundred miles and fall straight down (within tens of miles of the launch site).
The outer surface of the payload does heat up during reentry. Not enough to require a special heat shield like the Shuttle, but enough that you want to keep it in mind when you design the payload.
<clenches teeth>not…friction</unclenches teeth>
In fact, you have to worry about heating on the way up, too; most of the heating occurs by max Q alpha (the point of maximum aerodynamic loading during ascent) but because the mechanism for cooling is mostly limited to radiation at that point, it is difficult to expel heat before it transfers through thermal protection system (TPS) and into the structure of the vehicle, reducing structural capability. TPS is therefore sized to reduce heat transfer to the structure with margin to spare to prevent significant structural degradation.
The answer to the o.p. is a qualified yes; you do not need an ablative heat shield to freefall from 100 miles unless you have added significant velocity.
Stranger
How dense does the atmosphere have to be before you can deploy a parachute?
Well, parachutes have been used to in part to land Mars probes, and even at ground level (let alone at 20,000 feet), Mars’s atmospheric pressure is a tiny fraction of Earth’s, so I’d say… not a lot.
Some of the X-15 flights did use ablative heat shields. I assume these were not simple ballistic flights and involved significant acceleration at altitudes with negligible atmosphere.
I did have a picture at one point of an X-15 prepped for such a flight with a sprayed on epoxy resin as the heat shield. The uneven ablation of the heat shields had negative aerodynamic effects, identifying the need for a non-ablative heat shield for any functional orbital space plane.
That’s no moon…
The gravity on Mars is also significantly less than Earth (~0.38 G), so less force is required to slow the craft, plus the Viking lander used vectored thrust to assure a precise soft landing at the designated landing site. The Viking lander used a series of chutes to slow itself from terminal speed at after aerobraking to a safe soft landing speed.
It is more instructive to look at the landing sequence for a terrestrial return vehicle like the Apollo Command Module (PDF). As can be seen from this document, parachute deployment starts with the vehicle moving ~0.7 Mach at 25 kft with ejection of the Apex Cover (which covers the Parachute Deployment System) which is pulled away by a 7.2 ft dia ring-slot parachute, followed by deployment of two mortar-fired drogue chutes at T+1.6 seconds, which buffer and orient the capsule for main chute deployment at 10 kft. The main chutes are deployed by mortar-fired pilot chutes with a rate of descent of ~29 ft/sec. Note this all occurs at subsonic speeds. From the indicated site:
*The astronaut may deploy the drogue parachutes up to 40, 000 feet altitude if flight conditions make it advisable to do so.
In case of high altitude abort command module motions can result in dynamic pressures as high as 204 psf; this precludes manual deployment of the drogue parachutes above 25,000 feet. Pad abort and low or medium altitude abort require parachute deployment at altitudes as low as 3,000 feet at dynamic pressures in the I0 to 100 psf range.*
As can be seen, the driving condition is the maximum dynamic pressure. At higher pressure the force on the parachute and reefing lines is too large, and at transonic speeds the dynamic pressure center in the chute is unstable. So to answer the implied question, no, you can’t just use parachutes to slow to a safe landing speed from a high ballistic trajectory speed; you have to use aerodynamic drag, either in the form of a blunt-arsed drag body, making a series of energy-wasting S-curves as the STS/Shuttle does, or (for suborbital trajectories) performing weathercocking maneuvers like the Scaled Composites SpaceShipOne and SpaceShipTwo vehicles.
Stranger
Yes, I understand many people–including technically trained people in fields outside of aerothermal dynamics–refer to such heating as due to “friction”, but the primary mode of heating due to reentry is from compression of air within or at the confluence of sonic shock waves and subsequent radiation and convection to the skin, not due to “skin friction” from interaction of the viscous atmosphere to the vessel hull at the boundary layer. It is not friction by any continuum mechanics or tribological definition of friction as the area where heat is generated is not adjacent to, and sometimes several centimeters away from the outer mold line of the vessel.
Stranger
