posted 11-28-2017 08:11 AM               

The countdown given by Shepard reached zero. Nothing happened. At this point everyone was expecting the automatic sequencer to fire the three retro-rockets.

Carpenter hit the manual override button. And again – nothing happened. There was a two second delay before the rockets actually fired producing in the cabin a puff of acrid smoke probably caused by a short circuit in the firing mechanism. This delay contributed some 15 to 20 miles to the overshoot on landing.

When the rockets fired in their ripple pattern they did not produce the kick that Carpenter was expecting. This loss of the expected thrust added another 60 miles.

The remaining 170 or so miles came as a result of a misalignment during retrofire. Not in the vital pitch angle but in yaw. When the rockets fired the capsule was slanted about 25 degrees off to the right. As retrofire progressed Carpenter gradually brought this back to zero, but because the rockets did not fire in an absolutely straight line down the flight path they lost effectiveness.

All three elements added up to 250 miles over the expected landing point.

posted 11-28-2017 10:52 PM            

It has always been stated that Carpenter was blamed for three things:

• Using too much fuel to chase Glenn's "fireflies"
  
• Not monitoring his fuel consumption
  
• Firing his retro-rockets too late

posted 11-29-2017 01:44 AM               

First, the fuel management problem. Wolfe talks about Carpenter being unaware "...until the third and last orbit" that he was short of fuel having received warnings from "...several Capcoms" to start conserving during the second orbit. Carpenter himself, barely half way round on his first orbit, is the first to mention that the need to orientate the capsule to meet the requirements of some of his experiments had caused him to use more fuel than was expected.

At the end of this first orbit Carpenter is already discussing ways of conserving his remaining supplies with the ground and by the beginning of his third orbit the message from the ground was to the effect that they were reasonably happy with the measures taken and were content for the flight to continue for its planned duration. Incidentally, it is worth recording at this stage that Glenn on his flight also received a warning about the rate he was using fuel and was told to allow his capsule to drift.

Some of the blame is readily accepted by Carpenter. He himself said after the mission that "...the spectacular novelty of the view from space challenged me to make the most of my opportunity and lured me into an unwise expenditure of fuel early in the flight." Then there were times when the fly-by-wire high thrust units were accidentally activated during some manoeuvres. (Fly-by-wire was the system used when the astronauts operated the manual control system but depleted automatic fuel supplies.) This is translated in the official report as "...pilot deviated from planned procedures."

Set against this must be added the fact that he was given, as part of the flight plan, a greater amount of manual control tasks to perform than previously attempted, some of which necessitated a frequent return to orbital attitudes. For these manoeuvres the automatic system was used consuming more fuel than was strictly necessary. Carpenter again acknowledged this stating that the experimental part of the mission consumed "fuel or time." What he meant by this was that the economical way to proceed was to nudge the capsule towards the desired attitude with a short puff from the control jets and then allow it to drift into position. This took time.

In order to keep up with the flight plan, Carpenter felt it necessary to drive the capsule on the thrusters all the way, making the manoeuvring faster but less efficient in the use of his fuel. Perhaps he was the first astronaut to find that the planners perception of a reasonable work load did not match the feasibility of operational requirements.

posted 01-25-2020 01:28 PM            

Did Cooper have to use any star sightings to line up for reentry?

posted 06-04-2020 11:34 AM               

quote:

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Originally posted by moorouge:
The retros worked as advertised. There was less kick felt by Carpenter because, as described previously, they fired off-line.

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Nice job! One correction though, the misalignment to the orbital track had no effect on the deceleration as felt by Carpenter. The spacecraft itself decelerated per the force applied to the mass. For a never explained reason, the retros imparted 4%(?) less thrust than designed, and contributed to the overshoot. This lack of force is what Carpenter describes.

As you note, the manual fuel quantity read 7% when it was actually zero. Carpenter can't be blamed for believing he had more fuel than he actually had. If he knew his gauge was off by 7%, he would have been even more frugal with his fuel usage.

Carpenter is regularly blamed for being *unable* to determine the correct alignment of the spacecraft in the yaw plane. Not true. Throughout retrofire the spacecraft is moving along its yaw axis and is correctly aligned at the time of retro cutoff.

Too little, too late. But he was not incompetent.

Kraft absolutely shares the blame for the low fuel state. As Kraft himself states, the astronaut was in charge of the spacecraft, but Flight was in charge of the mission. Early in the flight, Carpenter notes that there is a major disagreement between the attitude instruments and the window scribing. Rather than troubleshoot the problem, Carpenter is sent back to his flight plan operations.

If the trouble with the horizon sensor had been confirmed when it was initially discovered, Carpenter would have known the retrofire alignment would require additional time. This error rests squarely on Kraft, not Carpenter.

Carpenter himself takes the blame for maneuvering to investigate the "fireflies" which delayed his alignment for retrofire. If his attitude instruments had functioned correctly, he had more than enough time to correctly align the spacecraft.

posted 06-18-2020 05:02 AM            

An impulse of 128 m/s was given, while only ~70 m/s was required to actually enter the atmosphere (and therefore land safely).

I imagine the over-budgeting was to take faulty rockets (in case one of the three rockets not firing), inaccurate retro attitude (as on Aurora 7), or other effects into consideration.

On Mercury, airplanes and recovery crews were located all over the Earth, ensuring that an astronaut could be recovered at any possible landing point on the globe within 24 hours, so where the astronaut were to land was strictly speaking not a big problem. The number one priority was simply to get him back alive.

And just to append an answer to the original question, both Shepard and Grissom manually controlled the attitude during the retrosequence (page 39 in summary of MR-4, NTRS document 19640056774). From what I understand, this was to test the systems before orbital flight. As the Redstone missions were suborbital, the retrofiring were strictly speaking not needed, but performed as a test of what the orbital astronauts might experience on their flights.

The cited document also gives permissible deviations in attitude during retrofiring (page 41), giving values of +- 12.5 deg in pitch (from normal -34 deg aim), and +-30 deg in both yaw and roll (both 0 deg aim).

posted 06-22-2020 08:59 AM               

quote:

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Originally posted by Explorer1:
Do you know how or why John Glenn ran out of attitude control fuel? During the orbital phase of his flight.

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posted 06-27-2020 11:43 AM            

quote:

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Originally posted by asbjos:
I imagine the over-budgeting was to take faulty rockets (in case one of the three rockets not firing), inaccurate retro attitude (as on Aurora 7), or other effects into consideration.

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posted 06-27-2020 12:36 PM            

I was talking about the general power of the Mercury retropack, which was stronger than necessary.

On the flight of Aurora 7, the spacecraft was not properly aligned for the retrofiring, so it landed off the target. This is already described in the first reply in this thread, by 'moorouge'.

During Project Mercury, no retrorockets stopped inadvertently during firing.

posted 07-02-2020 10:39 AM               

quote:

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Originally posted by moorouge:
Then there were times when the fly-by-wire high thrust units were accidentally activated during some manoeuvres. (Fly-by-wire was the system used when the astronauts operated the manual control system but depleted automatic fuel supplies.)

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• Pitch
  Two variable 0 to 24 lb thruster (manual)
  Two fixed 24 lb thruster (automatic)
  Two fixed 1 lb thruster (automatic)

• Yaw
  Two variable 0 to 24 lb thruster (manual)
  Two fixed 24 lb thruster (automatic)
  Two fixed 1 lb thruster (automatic)

• Roll
  Two variable 0 to 6 lb thruster (manual)
  Two fixed 6 lb thruster (automatic)
  Two fixed 1 lb thruster (automatic)

The twelve automatic thrusters were attached to the automatic fuel tank.

Manual Mode

You pull the manual knob out and the joystick is now connected to the variable thrusters on all axis.

As for the automatic axis normally they are under the control of the relay logic built into the capsule. There were three modes controlled by the ASCS switch

1. Normal which uses the horizon scanner, gryos and predefined values to keep the capsule in retro attitude.

2. Aux Damp which uses the gyros to null any rotation the capsule was experiencing

3. Fly by Wire which engages the 12 fixed thrusters to the joystick. Push the joystick out a little and the 1 lb thruster will fire, push out more then the 1 lb thruster will shut off and the 24 lb thruster will fire.

Finally there was rate stabilization control system or RSCS which was enabled by flipping the mode switch from Auto to Rate Comd. What this do was use the gyros, joystick, and manual thrusters to control the spacecraft's rate of rotation in all axis.

How much rotation depended on how far the joystick was pushed for that axis. At maximum extension the relay circuits was set to 10 degrees per second.

What happen with Carpenter is that he pulled the manual knob. And flipped the ASCS switch from ASCS to Fly-By-Wire.

So whenever he maneuvered he not only had the variable manual thrusters connected to the manual fuel tanks activated but the automatic thrusters connected to the automatic fuel tanks as well. This would give you a lot of control authority but at a cost of high fuel consumption.

The other problem which all astronauts experienced with the mercury capsule. That in ASCS mode you only have fixed thrusters. So keeping the capsule in retro attitude was not a precise operation.

Instead the opposing thruster would fire and the capsule would not stop but reverse direction slightly. Eventually it will reach the other side of its limits and the other thruster would fire and the capsule would drift back to the other side.

The capsule would go back and forth like this in all three axis if the automatic mode was left on. Thus consuming fuel.

Last when using Fly-by-wire you would hear a multiple bangs every time you extend the joystick. One from the 1 lb thruster being turned on and the off. Another from the 24 thrusters being turned on and off if you fully extend the joystick.

For precision operation during retrofire the astronaut would uses the mode switch to RSCS to null rotation rates using the manual thrusters. And the ASCS switch to Fly-By-Wire to control the capsule.