|T O P I C R E V I E W|
|Rusty B||The Mercury spacecraft would fire its retro rockets just off the California coast (between California and Hawaii) and land in the Atlantic off of Florida.|
The Gemini spacecraft would fire its retro rockets over Canton Island in the Pacific (near Baker & Howland Is, between Australia and Hawaii) and land in the Atlantic off of Florida.
Why the longer distance between retrofire and landing for Gemini vs Mercury?
Was it because of the heavier spacecraft at retrofire due to the heavier re-entry vehicle plus larger, heavier retro section? Or because of the guided re-entry the Gemini flew (the Gemini re-entry module had a offset center gravity that was used to guide it during re-entry) vs the ballistic re-entry that Mercury flew?
|RichardH||Are you talking about sub-orbital Mercury missions vs. Gemini or orbital Mercury missions vs Gemini? |
Sub-orbital/orbital physics aren't my forte, but I'd guess that an orbital mission would have a higher re-entry speed and thus would require a longer distance to shed off all that delta v before landing. (This is just a guess, not based on any facts whatsoever.)
|Rusty B||Sorry, I should have been clear. I talking about the orbital Mercury missions that landed in the Atlantic. I guess for manned that's Friendship 7 and Aurora 7 vs Gemini 3, 4, 5, 6, 7, 10, 11 & 12.|
The Gemini would be on re-entry from before Hawaii all the way past Florida. The Mercury just from the California coast until past Florida. Why the difference?
|randy||Could the weight of the spacecraft have something to do with it?|
|Beau08||I suspect they might have used the same retro rockets as Mercury to save money. Similar orbit plus heavier vehicle equals longer re-entry time for same splashdown area. Just a guess...|
|Rusty B||I wonder if the Gemini had a thicker heatshield due to the longer re-entry?|
|garymilgrom||Perhaps the longer time in the atmosphere actually decreased the peak temperatures of re-entry. We need an engineer on this.|
|kyra||The two spacecraft used different solid motors, three Thiokol TE-M-316 for Mercury and four Thiokol TE-M385's on Gemini. |
The weight was a consideration for retrofire, as the relatively "fixed" thrust provided by these engines created a new free return trajectory that would have the spacecraft intercept the atmosphere. That was the first calculation.
For upper atmospheric flight, the distance travelled over the Earth involved the banking angle of the heat shield. The plots for various angles and times determined the "long" part of the landing footprint. Banking left and right determined the narrow component of the ellipsoid shaped footprint.
After the banking just prior to the deployment of parachutes that was a standard profile that was effected by winds at various altitudes that would shift the final landing location. This was the final calculation. All three had to be calculated in order.
The advantage Gemini had was the banking portion of reentry was controlled and optimized by computer on some flights.
Originally posted by Beau08:
I suspect they might have used the same retro rockets as Mercury to save money. Similar orbit plus heavier vehicle equals longer re-entry time for same splashdown area. Just a guess...
Just in case Buzz happens to read this post, I know that you mean mass and not weight
|Max Q||Didn't NASA start experimenting with lifting re-entries for reduced-G on entry? Wouldn't that have had an effect on distance covered? |
|moorouge||You might some of the answers you seek in NASA Space Vehicle Design Criteria (Guidance and Control). NASA reference SP-8015.|
On edit - please note change in reference number from 8051 to 8015. 8051 deals with solid rocket design and function. Ooops - sorry for slip.
quote: Thank you Kyra. Did you work on those plots?
Originally posted by kyra:
The plots for various angles and times determined the "long" part of the landing footprint.
|robertsconley||Mercury was a pure ballistic re-entry. While Gemini's center of gravity was offset for a lifting re-entry.|
The result was very different re-entry profile for the two crafts.
Basically to keep on track the astronaut came down in a heads down position.
Then roll right or left to keep their rate of descent less than 50m per second to minimize heating the lower atmosphere. Of course rolling left or right altered their track so it was a bit of back and forth to keep everything right on the line.
You find a handy diagram (copied from an original manual) on page 13 of the manual for my Project Gemini add-on for the Orbiter Space Simulator.
The big thing I am missing in the add-on is the use of the Gemini onboard computer. Once the parameters were punched and the computer set to re-entry mode the needles on the eight ball could be used by the astronauts to keep on track. The general idea is to roll left or right to re-center the needles.
For the documents I got all this stuff from I have this webpage.
|ilbasso||It's also a function of orbital mechanics. To re-enter, you change the perigee of the orbit so that it intersects the Earth. To do that, you fire the retros at the apogee of the orbit.|