Apollo Re-entry another exposé from Aulis

[Bob B.]

What I will say is that the Kraft quote is not seemingly found outside the Popular Mechanics article and isn't attributed there so we have no real idea of context.

One thing that conspiracy theorists repeated do wrong is that they take comments made in popular publications, interviews, etc. and try to glean some scientific or engineering significance out of it.  Remarks made to general audiences often have to be taken with a grain of salt.  They may be dumbed down answers intended to convey a general idea while actually being flawed in some technical aspects.

A good example of this is a description that I've heard Jim Lovell use repeatedly to describe Apollo 13's reentry.  He has said that if Apollo's entry angle were too shallow, the spacecraft would "skip off the atmosphere like a stone skipping off water."   This is an incorrect description.   If the spacecraft comes in too shallow, it will not descend deep enough into the atmosphere to generate the drag needed to slow it down enough to keep it from exiting.  The spacecraft's altitude increases after its closest approach simply because that is what its orbital trajectory naturally does.  It is not because the spacecraft "skips off the atmosphere."  This doesn't mean that Lovell doesn't understand the technical realities of the situation, or that he is intentionally being deceptive.  He is just trying to get the audience to understand a basic idea by using an analogy that a layman can identify with.

These types of descriptions create apparent conflicts when the conspricists start quote mining.  They'll compare one person's generalized and somewhat inaccurate description to anyone person's generalized and somewhat inaccurate description.  The conspiracists then cry foul when the two different experts chose to use different language to provide a dumbed down answer to a technically complex subject.  The conspiracists don't understand the subject well enough to recognize that two experts are just using different terms to describe the same thing.

If someone wants to perform a serious scientific or engineering analysis of some part of Apollo, they need to refer to NASA's technical reports.  They shouldn't go quote mining from Popular Mechanics.

[smartcooky]

The next big thing that Bennett complains about is that the Apollo missions apparently never performed a double-dip reentry because the Apollo records she's looking at lists only one acceleration peak.  The records she's looking at lists the maximum g.  Only one of the two peaks can be the maxiumum, so that is the one that is listed.  Just because they don't list the smaller second peak doesn't mean it didn't happen.

When I played golf last Saturday, my longest drive (according to GPS) was 261m. I also hit other drives of about 244m and 237m, but they were not the longest, so they didn't get recorded. 

Apollo 10 Mission Report, Supplement 10 - Entry Postflight Analysis

Toward the end of the article Bennett complains about why there are "different entry interface speeds" listed.  Entry velocity is the velocity at entry interface, i.e. an altitude of 400,000 feet.  Maximum velocity is just that, the maximum.  They are not the same thing.  The spacecraft continued to speed up after crossing entry interface until the air became dense enough that drag started to slow it down.

Would this be because gravity is still strong enough to accelerate the capsule despite the drag after entry interface, until the drag becomes great enough to overcome gravity?

I guess this is also why, if the re-entry angle is too shallow, the drag never builds up enough to prevent the acceleration and so the capsule "skips off" like a stone on a pond.

[Echnaton]

If the angle is too shallow, the capsule doesn't slow down enough for gravity to pull it from its orbit but continues on a new orbit.  Since the capsule is not at escape velocity, it will fly out quite some distance, because it just fell all the way from the moon.  The orbit will depend on how much velocity was shed in the atmosphere and will have a perigee that once again intersects the atmosphere.    However, the capsule doesn't have the resources to keep the astronauts alive for that time so it is only academic how far it actually goes.   

[Bob B.]

Would this be because gravity is still strong enough to accelerate the capsule despite the drag after entry interface, until the drag becomes great enough to overcome gravity?

Yes.  The perigee of the return orbit (the closest distance to Earth) is set within Earth's atmosphere.  For example, the perigee of Apollo 11's trajectory was at an altitude of about 39 km.  If there were no atmosphere, the spacecraft would continue to speed up until reaching its maximum velocity at perigee.  After that it would start to move farther away from Earth and would begin to slow down.  Entry interface is defined as an altitude of 400,000 feet (121.92 km).  At that altitude the air is very thin.  Although the spacecraft starts to experience a small amount of drag at that altitude, it is still speeding up as moves toward perigee.  As it drops lower in altitude, the air gets thicker and the drag increases.  After a short time the drag increases to the point that it becomes the dominate force and the spacecraft starts to slow down.

I guess this is also why, if the re-entry angle is too shallow, the drag never builds up enough to prevent the acceleration and so the capsule "skips off" like a stone on a pond.

Having a shallow entry angle is just another way of saying that the perigee altitude is higher.  Anytime the spacecraft passes through the atmosphere it will lose velocity, but if it stays higher up in the thinner air, the drag will be less and the spacecraft will retain more of its initial velocity.  If it retains enough velocity, its apogee will remain above the atmosphere and the spacecraft will exit and complete another orbit until it reenters the atmosphere as it approaches its next perigee.

In some cases the new apogee remains just above the entry interface altitude.  In this case the spacecraft will rise above entry interface, quickly pass through apogee, and then drop right back into the atmosphere.  This is the scenario that I believe the term "skip reentry" is referring to.
 

[ka9q]

But the Apollo CM was a lifting body; it had an off-center c.g. so that the blunt end was not perpendicular to the airflow. This generated lift, which could be directed by rolling the spacecraft. If you didn't happen to need any lift at the moment, the spacecraft simply went into a continuous roll to cancel it out.

The same method was used to target Curiosity on Mars with the required accuracy.

Was this lift insufficient by itself to take the CM back out of the atmosphere?
 

[Bob B.]

Was this lift insufficient by itself to take the CM back out of the atmosphere?

It was never the plan to lift the CM back out of the atmosphere.  Reentry consisted of several preprogrammed phases with varying amounts of lift.  It might have even flown with negative lift at times.  Apollo 11, for example, descended to an altitude of 55.5 km before it began to lift itself up.  It then ascended to 67 km before starting its final descent.  We see that it didn't come anywhere close to leaving the atmosphere, but that was by design.

Several years ago I did a simulation of Apollo 11's reentry to see if I how closely I could replicate the results.  If I alter that simulation so that I'm getting maximum upward lift the entire time, the CM does exit the atmosphere.  If my simulation is to be trusted, it appears that the CM might have been able to lift itself out of the atmosphere if that's what was wanted.

[gwiz]

Does anybody know why the max speeds were not quoted after Apollo 12?¿

The person who compiled the table couldn't find them?

Do you mean they didn't have a maximum speed? 

Someone must have gone through documents from each mission to compile that table.  It could well be that different missions reported the facts in different ways, and for the later missions maximum speed was only mentioned in some document that the compiler didn't find.

[JayUtah]

We see that it didn't come anywhere close to leaving the atmosphere, but that was by design.

The ascent was meant to provide a "rest" period for the heat shield.  In some of the descent profiles the heat load was more than nominal, so the CM ascended for a bit to let the shield cool slightly.

[Luke Pemberton]

The ascent was meant to provide a "rest" period for the heat shield.  In some of the descent profiles the heat load was more than nominal, so the CM ascended for a bit to let the shield cool slightly.

I recall in another post you were initially 'bemused' by the re-entry profiles. My memory does not help me here, but you explained that the altitude increased. However, I think you said this was something to do with the cooridnate effect of the descent path relative to the Earth's curvature. I'm probably wrong.

In any case, seems we have learned from Bob and your post again. I love this forum 

Apollo believers 456 823

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[Count Zero]

Iirc, the Soviet Zond 6 circumlunar mission in November of 1968 used a skip re-entry:

The 7K-L1 then made the first successful double skip trajectory, dipping into the earth's atmosphere over Antarctica, slowing from 11 km/sec to suborbital velocity, then skipping back out into space before making a final re-entry onto Soviet territory. The landing point was only 16 km from the pad from which it had been launched toward the moon.

Encyclopedia Astronautica

This was actually an unmanned precursor to a manned circumlunar flight, planned for early December, 1968 that would have beaten Apollo 8 to the Moon.  The Soviets announnced that Zond 6 was a complete success, and it looked like the Moon Race was neck-and-neck coming down to the wire.  The December launch window came and went, and the Soviets did not launch.  For decades the West did not know why.

It turned-out that a gasket failed and Zond 6 depressurized several hours before re-entry, depressurizing the capsule and killing the animals on board; then - after flying the skip re-entry perfectly - the parachute failed and the capsule crashed.  The Soviets wanted a truly successful unmanned test before risking a cosmonaut.  The December launch was delayed until January 20, 1969, when it was launched unmanned.  The second stage failed and the capsule landed (safely) in Mongolia.  A month later, the first flight of the massive N1 rocket ended in destruction just over a minute into the flight.  Though they kept trying, that was pretty much "it" for the Soviet manned moon program.

[Bob B.]

I was just reading parts of the following article and found something interesting.

Apollo Experience Report - Mission Planning for Apollo Entry

There is a point in the reentry trajectory that is called "skipout".  It is the point in the ascending part of the trajectory where the acelleration decreases to 0.2 g.  It is also the transition point between two different phases in the entry guidance logic.  Skipout is shown in Figure 3 and is described in the accompaning section on Entry Guidance.

Although Apollo did not perform a "skip reentry" as it is often defined, it did execute what is defined in the entry guidance logic as "skipout" followed by "second entry".

This helps to explain the terms used by Chris Kraft in his statement quoted below.  He is simply describing the skip maneuver as defined above.  The only thing that might be considered misleading is that he implies Apollo skipped 'out of the atmosphere'.  I consider that just a minor mistatement.  Apollo skipped up into a higher and thinner part of the atmosphere.

"Because the velocity is so high, if you tried to come in directly, the heat-shield requirements would be too great. So what we did was get them into the atmosphere, skip it out to kill off some of the velocity, and then bring it back in again. That made the total heat pulse on the heat shield of the spacecraft considerably lower."  (P. Mech, 2009)

[Bob B.]

Iirc, the Soviet Zond 6 circumlunar mission in November of 1968 used a skip re-entry

Zond 5 was supposed to perform a skip reentry but couldn't because of a guidance system failure.  Instead it perform a direct ballistic reentry, pulled upward of 20 g, splashed down at its backup target area in the Indian Ocean, and was successfully recovered.  The animals onboard survived.

[JayUtah]

I recall in another post you were initially 'bemused' by the re-entry profiles. My memory does not help me here, but you explained that the altitude increased. However, I think you said this was something to do with the cooridnate effect of the descent path relative to the Earth's curvature. I'm probably wrong.

Yes, the effect that bemused me has been discussed in this thread.  If the x-axis -- downrange distance -- is rendered as usual in a linear cartesian plane, the trajectory appears to include an ascent that is more pronounced than what actually occurs.  However, if the trajectory is geodetically rendered properly as an orbit, it is a fairly uninteresting orbit with an atmospheric perturbation.  It looks like an orbit should.

At the end of the ascent you find a relatively pure parabola representing a ballistic trajectory.

[ka9q]

"Because the velocity is so high, if you tried to come in directly, the heat-shield requirements would be too great. So what we did was get them into the atmosphere, skip it out to kill off some of the velocity, and then bring it back in again. That made the total heat pulse on the heat shield of the spacecraft considerably lower."  (P. Mech, 2009)

Modulating the brakes, as it were.

So cooling the heatshield was the only reason? What about limiting acceleration?

[Bob B.]

So cooling the heatshield was the only reason? What about limiting acceleration?

Yes, limiting acceleration was also part of it.  The steepest entry angle was defined as the flight path angle for which the maximum load factor would not exceed 12g.  Landing site targeting was also a big factor in determing the amount of loft for each reentry (for longer ranges the CM was lofted higher).