Artemis 1 going to the Moon (launched Nov 16)

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In summary, the Artemis 1 rocket is on track for a launch August 29, 12:33 UTC (08:33 local time) or in the two hours afterwards. Backup launch windows are daily from September 2 to September 6.
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  • #177
mfb said:
Huh?
I guess you had to be there.

That document is very interesting, And amusing. However, it assumes the reader knows more than I do. Why the "no thrust" requirement for example? I can see a "not very much thrust" requirement for sure. Oh, I don't know how much GPS would help: R is the worst measured direction, and height above ground might be more useful. And nanoseconds? It is tough to time the rocket engines to better than microseconds.

However, I think I agree with the primary point: consequences for a missed maneuver, for whatever reason, are severe.
 
  • #178
Where would the thrust come from? The capsule has attitude control, but the big engines and fuel tanks have been discarded.

GPS can measure your altitude with an uncertainty of a few meters without issues, probably less than a meter because there is no thicker atmosphere to consider. The flight time of radio signals exchanged with the capsule or reflected by the capsule can be measured with nanosecond precision as additional signal, similar to the concept of GPS.
Velocity measurements were somewhat good (~m/s) even during the Apollo era because Doppler shifts are easy to measure, but position uncertainty was often hundreds of meters and of course the live calculations were far less detailed. Here is an old document on tracking.
To make conditions extremely bad for the ground system, assume that contact was lost with the spacecraft when it departed the Moon. Only 8 hr before entry into the Earth's atmosphere, contact, and thus tracking, was restored. Assuming that one ground station and one ship can track the spacecraft at one sample per minute, the entry velocity error is approximately 1-1.5 m/s and the position error is approximately 1-2 km.
 
  • #179
mfb said:
Where would the thrust come from? The capsule has attitude control
Exactly. That's why I am surprised the requirement was zero, not less than some amount.
 
  • #180
mfb said:
Huh?

A skip-reentry needs a much better control over the trajectory. If you are off by a bit you don't land 100 km away, you might land 1000km away or even on the other side of the planet - with a dead crew because the capsule alone has very limited life support. That's fine today where the capsules have GPS and nanosecond radar timing measurements and can calculate their trajectories in milliseconds, but Apollo didn't have that level of control.

Here is a 1966 discussion.

That 1966 discussion is interesting. It talks about "uncontrolled skips" and a very intentional "lob".

And there's another item I found interesting in that link related to reentry heating:
For heating, the general rule is that the higher the deceleration, the higher the peak heating rate and the lower the total heat.

But back to the "skip"...
The Apollo missions were carried on the ABC TV network with Jules Bergman reporting.
397px-Jules_bergman_1973[1].jpg

Every phase of these mission took hours, if not days - and they preempted a lot of normally scheduled programming. This led to folks like Jules Bergman filling air time with lots and lots of technical discussion - with most of the material provided by NASA.
Making it to the correct reentry corridor to avoid atmospheric skipping was discussed in detail - especially for Apollo 13.

But from the article linked to by @mfb , a form of skipping was apparently standard practice.
Phase 4: Ballistic Lob (KEPL)
Atmospheric exit and reentry are defined as those states at which the sensed acceleration falls below and then exceeds approximately 0.2g. In the time between these conditions no control is exercised save that of roll attitude hold with pitch maintained for proper aerodynamic trim.
 
  • #181
Vanadium 50 said:
Exactly. That's why I am surprised the requirement was zero, not less than some amount.
Attitude control thrusters give you something like centimeter per second corrections. Good enough to change the orientation of your spacecraft, but not enough to have a useful impact on your trajectory. Close enough to zero to just use zero.
.Scott said:
But from the article linked to by @mfb , a form of skipping was apparently standard practice.
That was a proposal but not the trajectory the Apollo capsules used.
 
  • #182
mfb said:
That [Ballistic Lob] was a proposal but not the trajectory the Apollo capsules used.
Per the document you linked to, this was used for AS-202 - which was part of the Apollo program and included an CM-011 Apollo capsule (but without the crew couches).

Here is an excerpt from the NASA page describing the mission (my emphasis):
One hour and 11 minutes after liftoff, the CM separated from the SM and turned its heat shield in the direction of flight to prepare for reentry. At an altitude of 400,000 feet, or about 75 miles, the capsule encountered the first traces of the Earth’s atmosphere at a velocity of 19,440 miles per hour. The CM’s guidance system steered it through a double-skip reentry, first descending to an altitude of about 40 miles, then using the capsule’s lift capability to rise back to nearly 50 miles before continuing the final descent. This reduced physical loads on the capsule. The heat shield reached a temperature of about 1,500 degrees Celsius while the cabin interior never exceeded 21 degrees Celsius, or 70 degrees Fahrenheit.

However (also from the NASA link):
The splashdown point was 235 miles short of the targeted area, later determined to be caused by the CM’s lower-than-predicted lift-to-drag ratio. It took the prime recovery ship, the U.S.S. Hornet (CV-12), eight and a half hours to reach the capsule and execute the retrieval.

That was not enough to discourage NASA from using the skip.
SA-501 was an uncrewed mission that included a fully configured Apollo.

The quote below is from the NASA Apollo 4 web page :
At an altitude of 76 miles, while traveling at 24,974 miles per hour, the Apollo 4 Command Module encountered the first tendrils of Earth’s upper atmosphere, its heat shield absorbing the heat of reentry, reaching a temperature of 5,000 degrees Fahrenheit while the cabin temperature remained comfortable enough for a crew. After dipping down to an altitude of 35 miles, the spacecraft used its aerodynamic lift to briefly skip back out of the atmosphere, reaching a height of 45 miles before continuing the descent. This double-skip reentry reduced deceleration and heat loads on the spacecraft.
... and with better result:
Within 20 minutes of splashdown, U.S. Navy frogmen had attached a flotation collar around the spacecraft. After the Bennington pulled alongside the capsule, sailors hoisted it aboard, along with the spacecraft’s apex cover that protected the parachutes during flight and one of the three main parachutes. The entire recovery operation lasted about two hours.
The manned Apollo launches started with Apollo 7. So far, I have found no detailed descriptions of those reentries.
 
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  • #183
 
  • #184
Hydrolox burns so cleanly that you can hardly notice the center core engines running at full thrust between the solid rocket boosters with their extremely bright exhaust.
 
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