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Author
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Topic: Mercury and Gemini retrofire to landing distance
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Rusty B
Member Posts: 239
From: Sacramento, CA
Registered: Oct 2004
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posted 05-13-2012 07:10 AM
   
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
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posted 05-13-2012 08:25 AM
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
Member Posts: 239
From: Sacramento, CA
Registered: Oct 2004
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posted 05-13-2012 10:29 AM
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
Member Posts: 2231
From: West Jordan, Utah USA
Registered: Dec 1999
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posted 05-13-2012 11:11 AM
Could the weight of the spacecraft have something to do with it? |
Beau08
Member Posts: 159
From: Peoria, AZ United States
Registered: Aug 2011
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posted 05-13-2012 11:55 AM
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
Member Posts: 239
From: Sacramento, CA
Registered: Oct 2004
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posted 05-13-2012 12:09 PM
I wonder if the Gemini had a thicker heatshield due to the longer re-entry? |
garymilgrom
Member Posts: 1966
From: Atlanta, GA
Registered: Feb 2007
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posted 05-13-2012 03:22 PM
Perhaps the longer time in the atmosphere actually decreased the peak temperatures of re-entry. We need an engineer on this. |
kyra
Member Posts: 583
From: Louisville CO US
Registered: Aug 2003
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posted 05-13-2012 07:55 PM
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. |
MCroft04
Member Posts: 1647
From: Smithfield, Me, USA
Registered: Mar 2005
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posted 05-13-2012 08:19 PM
quote:
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
Member Posts: 399
From: Whyalla South Australia
Registered: Mar 2007
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posted 05-15-2012 06:49 AM
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
Member Posts: 2458
From: U.K.
Registered: Jul 2009
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posted 05-15-2012 08:13 AM
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. |
garymilgrom
Member Posts: 1966
From: Atlanta, GA
Registered: Feb 2007
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posted 05-15-2012 08:39 AM
quote:
Originally posted by kyra:
The plots for various angles and times determined the "long" part of the landing footprint.
Thank you Kyra. Did you work on those plots? |
robertsconley
Member Posts: 59
From: Meadville, PA
Registered: Jun 2005
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posted 05-29-2012 12:37 PM
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
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 05-29-2012 02:30 PM
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. |
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