I’d had this notion that a two-step reentry might have the advantage of lower g-forces; but with the problems given above wouldn’t be practical. Thanks.
Anyone know why it took so long to come out of radio blackout during re-entry?
According to Gene Kranz (the “failure is not an option” flight director), the blackout was longer than usual because the reentry was at a shallower angle than usual.
[hijack] I heard Jim Lowel give a corporate talk a few years ago. Fascinating. He also said that when they went around the moon, the other guys were busy taking photos. Jim said, “guys, could use a little help here.” “Nah, we trust you, and we’ll never have another chance to see the moon so you deal with it.” [/hijack]
At what point do they dump the big orbital engine? And once they’re down to the capsule, do they have any maneuvering capability left at all (f’rinstance, pitch/yaw/roll to orient the capsule for re-entry)? I think you could do that with gyros and not need engine thrust, correct? Did the capsule have gyros that could do that?
Very late, they are running on internal battery power and an internal oxygen tank once the service module is gone. Apollo 11 separated about ten minutes out from start of re-entry.
Yes, the command module has pairs of dual thrusters to provide pitch, yaw and roll. The capsule doesn’t just plummet, since it is a lifting body, it is flown, and the pilot has control of this. It had some rather limited cross-range capability too.
This image
shows the thrusters quite well, the ones labelled 27, and 28 are some of the set.
Given they tested unmanned capsules you might assume correctly. The entire guidance and nav system was resident in the command module. (Indeed that was one bit that was reused between missions, being stripped out of a used capsule to be installed in a later one.)
THAT’S a “lifting body”? I thought the Shuttle was a lifting body, not the Apollo capsule.
So how big of a dent would it have made if an Apollo command module would have crunched onto the deck of the Iwo Jima?
The STS Orbiter (the NASA “Shuttle”) is a compound delta wing vehicle, not a lifting body. The Apollo CM is a lifting body at high speeds, althogh at subsonic speed the amount of lift it generates is negligible, hence the need for parachute descent.
Stranger
it is less an issue of damage to the ship than hazard to the occupants of the capsule due to a hard landing.
Stranger
I’m not sure I’m getting this. Is a brick lifting body if it plunges into an atmosphere and is decelerated by atmospheric friction?
Let me ask the question better. What about, say, a meteor? Is it possible for one to hit the earth’s atmosphere at a very shallow angle and skip off like a stone on a pond? If that happens would it be considered a lifting body?
No. A lifting body is something whose main shape provides lift as it goes through the atmosphere. An airplane is not a lifting body because its body does not provide any lift. Only the wings do.
What is probably confusing you is that we tend to think of capsules as just dropping straight down until the chutes open. That’s not actually the way the Apollo capsules worked, though. The Apollo capsules had their center of mass offset from the capsule’s geometric center. This made the capsule fall at a bit of an angle, which when combined with the capsule’s shape resulted in it giving the capsule a bit of lift. The capsule actually couldn’t fall straight down. If it wanted to go straight down it had to spin so that it would fall in a corkscrew pattern. The crew steered the capsule simply by using the thrusters to point it in the direction that they wanted it to go.
The Apollo capsules didn’t exactly glide like the space shuttle, but they didn’t drop like rocks either.
Meteors skip off of the earth’s atmosphere fairly often. Here’s a cool picture of one doing exactly that:
A meteor wouldn’t be a lifting body unless its shape generated lift, and most meteor shapes aren’t very aerodynamic.
More info about lifting bodies here:
Ok, this is clearer to me. Except…
If a meteor’s shape generates no lift, why would it skip off the atmosphere?
Speed. It’s going too fast, and in the wrong direction, to fall to Earth.
thanks
I sometimes wondered about that. The heat shields I have seen at the Smithsonian appear to be in remarkably good shape. Especially considering how they work-as ablative shields. It now seems likely that it would have been better to design the shuttle with more such shielding-especially along the leading edges-and simply replace the shields every flight or couple. Of course it would have not been in the spirit of “reusable” to have designed a component that was one-time use. But it seems likely it would have been a better choice.
To elaborate on my answer…ever skip stones on the water? Flat ones work best, but it’s possible to skip irregular or round shapes, too. Just give them a little more oomph and don’t expect airfoil perfection.
I think the “skip off the atmosphere” idea is a simplification that confuses more than it reveals.
Remember that unlike the surface of a lake, the atmosphere doesn’t have a hard boundary.
A meteor could easily be on a path which grazes the upper atmosphere, but which doesn’t intersect the rocky part of Earth. The result would be a visible fireball but no impact with the surface.
A properly designed & controlled reentry vehicle could generate an upward force from its interaction with the upper atmosphere, and use that to increase its altitude while decreasing its speed. Coupled with a grazing trajectory and the curvature of the earth, entering & then leaving the upper atmosphere is quite doable. But “skipping” is a lousy descroption of why it happens.
A randomly shaped, randomly tumbling rock might generate some lift briefly, but not with any material net impact on the trajectory.
IOW, IMHO “skpping off the atmosphere” is an analogy, not an explanation. Don’t try to carry the analogy very far.
As applied to Apollo…
As engineer_comp_geek said so well, the descent is far from vertical. To get from 100-ish miles up to parachutes deployed at 2 or 3 miles up they travel about 1/3rd of the way around the world. In other words, ~8000 miles downrange for ~100 miles descent = 80 to 1 descent angle = ~1.5%.
Yes, it’s far from a linear descent. The lower & slower they are in the atmosphere the steeper the descent. But a lot of it happens at a pretty flat angle. Wherein the relatively small lift & cross range forces the capsule can generate are a meaningful size compared to the descent angle.
A meteor or other ballistic object produces aeroelastic resistance in the atmosphere purely by compression; that is, it compresses the air in front of it which pushes opposite of the direction of motion. As many meteors enter the atmosphere at a high velocity and rather shallow entry angle, it doesn’t take much force to push them “around” the upper atmosphere and back into space, having never lost enough energy to be captured by the Earth. It is this pressurization of air, BTW, that causes the heating (ram or compression heating) on a re-entry body, not skin friction as commonly stated.
A lifting body produces lift via differential mass flow; that is, that due to its geometric shape and aspect with respect to the direction of motion (airfoil shape and angle of attack, respectively) the difference in airflow between the top and bottom of the shape. This difference in flow causes a pressure differential that produces lift. This is actually an overly simplified description as hypersonic lifting bodies like the Apollo capsule produce lift via complex shock wave interactions, but this explanation will suffice for the discussion at hand. Lifting bodies like the HL-10, X-24, X-38, and X-43 produce lift purely by body shape (i.e. the body is its own airfoil) and has horizontal control surfaces only for mediating lift. The vertical fixed stabilizers and movable stabilators do not provide lift.
A delta wing body like the STS Shuttle Orbiter Vehicle (OV) produces lift via a large wing surface, which is necessary to obtain lift at transonic and subsonic speeds to permit it to land in a controlled fashion at subsonic speed. (Lifting bodies like the HL-10 come down much faster and require greater drag retardation or rollout distance than the Shuttle.) This also creates long leading edges which heat considerably during re-entry, necessitating the temperature-resistant but fragile reinforced carbon-carbon thermal protection at the leading edges. There are also hybrids like the X-37 which is primarily a lifting body but also has a small delta wing structure for a modest amount of lift and enhanced cross-range at low supersonic and subsonic speeds.
The ablative shielding used on the Apollo CM was compact, but still fairy heavy, and would have been prohibitive from both a overall payload weight standpoint and mass distribution. (It isn’t just a single dumb piece but actually a couple of layers of aluminum honeycomb and a high temperature plenum onto which the ablative material was laid up.) The use of RCC panels on the leading edges and thermal tiles and blankets on the underside (which are actually glued to a felt liner that is basically stapled to the wind structure) was the only realistic passive thermal protection available then or now, although active cooling systems that would pump coolant through the edges to carry away heat or create a thin film layer to protect wing structure have been proposed.
It is also worth nothing that the different shapes of the vehicles play a large roll in controlling heating. The blunt-arsed CM creates a large shockwave that stands off up to several inches away from the heat shield and serves both to push airflow around the outer mold line of the capsule (preventing convective heat transfer) and provide a thick thermal mass of entrapped air (forming a radiative boundary). Heating to the shield is actually primarily radiation on the near side of that boundary, and can actually be accurately modeled and controlled. On the Shuttle OV, however, the “sharp” leading edges form shockwaves that stand very close to the wing fronts and become much hotter; the residual heated air is then distributed under the wing instead of being pushed completely away as with the Apollo CM. By virtue of having the large wing surface for horizontal landing the Shuttle OV requires far more elaborate thermal protection.
In retrospect, the Shuttle OV design was far more complicated in nearly every way than Apollo, and the supposed cost savings and short turnaround times were never realized in operation. To be fair, much of the complexity, like the massive wing size and dimensions of the payload bay, were predicated on Department of Defense requirements for once-around polar orbit cross-range and surveillance satellite payloads that were never actually used. It should also be noted that the Shuttle was originally intended to be an interim vehicle that would be evolved into a second generation spaceplane design implementing greater performance and reusability, a plan that was progressively crushed under the Nixon, Carter, and Reagan administrations by budget cuts and short-sighted thinking. But those who decry a return to capsule designs instead of spaceplanes are missing the point that capsules are robust, reliable, and inexpensive to build and test, while spaceplanes are delicate, complex, and largely unnecessary given the current state of ascent to orbit capability.
One proposal for the STS, the Chrysler Aerospace SERV, actually used a massively upscaled capsule design to attain single stage to orbit (SSTO) capability and powered landing via aerobraking and air-breathing jet engines, but was too different from the STS RFP concept to garner serious consideration. It is a shame that this concept was not and still not taken seriously as it may provide a much more assured path to true reusability and substantial reduction in payload-to-orbit costs.
Stranger
I can’t tell if this thread made me stoopider or smarter. :eek:
I just watched A13 for the 100,000th time last night. When my son asked why they didn’t burn up at re-entry, I shrugged and said maybe they were moving too fast, but he’d have to become an astrophysicist to figure it out. Then he asked how they were able to get back to Earth when they’d shed most of their ship already and I went :eek:.
I decided I’d go on a private ship with my first $30m and let the smart people figure it out. 
On the lift-drag ratio continum you have pure ballastic reentry, semi-ballistic (like Apollo), lifting bodies and winged spaceplanes. In addition, between semi-ballistic and lifting bodies is a relatively obscure configuration called Biconic, which has been tested but so far not used operationally.