Why would Gemini-B have needed 6 retro-rockets when Gemini used only 4?

[John Charles]

According to best available info, the Gemini-B to be used on MOL missions was to have 6 of the Star-13E (a/k/a TE-M-85) solid-fuel retrorockets for use during launch aborts on the pad and early in powered flight to separate the capsule from an exploding Titan IIIM booster enough to provide adequate crew separation to allow the pilots to use their ejection seats safely.

But here is the interesting part: all 6 retrorockets were also to be used for de-orbit at the end of the mission, although a 1968 Bellcom consultant report says a minimum of 5 would have been sufficient.

The NASA Gemini missions used only 4 of those same retrorockets for de-orbiting.  It does not appear that Gemini-B could have de-selected 2 retros for routine re-entries.

The planned Gemini-B/MOL orbit was no higher than the Gemini orbits, and probably lower, so additional thrust was not required due to altitude.

The Gemini-B heat shield was qualified using approximately the same trajectory and heat load that was used to qualify the Gemini heat shield, so no high-energy re-entry was baselined for Gemini-B.

The Gemini-B would not have been significantly heavier than Gemini at de-orbit (except for the mass of the 2 extra retros). NRO info declassified last year shows Gemini-B possibly carrying up to 4 film spools, which I assume weighed about 80 pounds each (based on CORONA data), for a mass just over 5% heavier than Gemini at retrofire.

I don't think the atmosphere profile over the poles (during Gemini-B's re-entry arc) is so much less dense that 50% more thrust would have been required at de-orbit--would it?

If all 6 motors really were intended for routine use, were they required for a different purpose, perhaps cross-range capability after de-orbit?

Are there any other possibilities I missed?

This is another of those questions that I hope is answered in future declassification data drops.

[Jim]

It would have little affect on cross range.  I bet it is weight and attitude.  Sun sync was a possibility

[John Charles]

Yes, it was a sun synchronous orbit, maybe 96 degree inclination. But that shouldn't require 50% more thrust for deorbit. Neither does altitude or weight as far as I can tell.

[Antilope7724]

The 4 retro Gemini must have had somewhat anemic retro performance. The Mercury capsules fired 3 retros at the California coast to land in the Atlantic. The Gemini missions fired their retros over Canton Is in the Pacific to land in the Atlantic. The 6 retros must have been planned to add more performance and a shorter re-entry time? Maybe the Titan II didn't have enough performance margin for the weight of 2 extra retros but the Titan III did?

According to http://www.braeunig.us/

Mercury orbital mass (with retros) 1,355 kg  (1,118 kg plus 237 kg)

Gemini Reentry and Retro module combined mass 2,573kg (1,982kg plus 591kg)

[the_other_Doug]

Mercury used a ballistic re-entry trajectory, while Gemini used a lifting profile.  The result of which was that Gemini flew downrange a lot further after hitting the atmosphere than Mercury did.  Thus the different distance covered downrange from retrofire to splashdown.

[MegabytePhreak]

It sounds like they needed all 6 Star-13E's for aborts, so they were carrying them to orbit anyways (Unless they added a way to dump some on the way up, which would only make sense if they could do so when it still gave a substantial performance boost). One they are up there, why not use them? Seems preferable to having live rockets on the re-entering adapter module, which could expand the potential debris impact zone.

[Antilope7724]

Mercury used a ballistic re-entry trajectory, while Gemini used a lifting profile.  The result of which was that Gemini flew downrange a lot further after hitting the atmosphere than Mercury did.  Thus the different distance covered downrange from retrofire to splashdown.

If that were the case and the Gemini, through a computer failure, was forced to do a ballistic reentry, wouldn't it land in Kansas? 

The Gemini was somewhat steerable in reentry, but I don't think that extended the reentry by 4,500 miles (Canton Is to California distance). The Gemini probably had a longer time between retrofire and entry interface than the Mercury did. Whether the Soyuz does lifting profile or a ballistic reentry, it lands in pretty much the same location, not thousands of miles short.

[the_other_Doug]

Mercury used a ballistic re-entry trajectory, while Gemini used a lifting profile.  The result of which was that Gemini flew downrange a lot further after hitting the atmosphere than Mercury did.  Thus the different distance covered downrange from retrofire to splashdown.

If that were the case and the Gemini, through a computer failure, was forced to do a ballistic reentry, wouldn't it land in Kansas? 

The Gemini was somewhat steerable in reentry, but I don't think that extended the reentry by 4,500 miles (Canton Is to California distance). Whether the Soyuz does lifting profile or a ballistic reentry, it lands in pretty much the same location, not thousands of miles short.

First, I'm pretty certain you're overstating the distance between retrofire and splashdown on Gemini.  Second, a Soyuz that has to downmode to a ballistic entry lands hundreds of miles short of the planned landing point, not in "pretty much the same location."

Gemini IV did have a computer failure, and did have to do a rolling ballistic re-entry.  The computer failed several hours prior to retrofire, however, and so the retrofire timing was altered to account for the lack of lift.  They did retrofire later than nominal because they knew they were going to downmode to a ballistic entry well before retrofire.

On Gemini 3, their entry trajectory calculations left out the rotation of the Earth below them, believe it or not.  The initial entry trajectory, even with the planned amount of lift, was going to land extremely short (reports vary between 250 and 500 miles short, depending on whose numbers you believe), so Grissom had to take manual control to maximize the lift available from the Gemini capsule, and managed to fly out all but about 80 miles of the error.  So, Gemini had the ability to do a lifting trajectory even without the computer controlling.  They just chose the rolling, ballistic trajectory the one time the computer failed as a safety measure.

Also, not until the last of the Gemini flights did a crew commander actually allow the computer to control the entry trajectory directly, "hands-off".  On earlier flights, the computer would flash its recommendation for spacecraft roll (the attitude that determined the lift vector), the pilot would call out the designated angle, and the CDR would manually roll to that attitude with the hand controller.  There were actually back-up charts and checklists that gave recommended attitudes vs. times in order to fly a nominal entry.  The computer re-calculated from the nominal plan based on its inertial tracking of the trajectory, to trim out dispersions.  But a successful, if not pin-point, landing could be achieved using a lifting trajectory without the computer.

Besides, Gemini flew on fairly low inclination orbits.  If it landed way short, it was more likely to land in Mexico than Kansas...

[Antilope7724]

First, I'm pretty certain you're overstating the distance between retrofire and splashdown on Gemini. 

http://news.google.com/newspapers?id=DZ8tAAAAIBAJ&sjid=uZ8FAAAAIBAJ&pg=5360%2C3656086

The Montreal Gazette - July 22, 1966

Gemini 10's Astronauts Back Safe And Sound

7th paragraph down

"Gemini 10's blazing return began at 4:30 p.m. as the Astronauts fired their
retro-rockets over Canton Island in the western Pacific, near the end of their
43rd trip around the globe...."

http://news.google.com/newspapers?id=EudVAAAAIBAJ&sjid=UOEDAAAAIBAJ&pg=6596%2C4471272

Eugene Register-Guard - Jul 22, 1966

Astronauts Begin Review of Successful Gemini 10 flight

3rd Column over - to the right

"Young and Collins climaxed their brilliant flight late Thursday by steering Gemini 10 to a near perfect landing in the Western Atlantic Ocean about 550 miles east southeast of Cake Kennedy. The spacecraft splashed down only 7.5 miles from the Guadalcanal..."


http://news.google.com/newspapers?id=FAEhAAAAIBAJ&sjid=QnYFAAAAIBAJ&pg=3077%2C1159222

Gemini 9 - 2nd column over, 3rd paragraph down

"The fiery descent started 160 miles above the Pacific near the international dateline when the craft's four retrorockets fired with a jolt at 9:26 a.m. edt. "Four good retros!" Cernan shouted to the Canton Island tracking station"


http://news.google.com/newspapers?id=Tk0aAAAAIBAJ&sjid=2icEAAAAIBAJ&pg=7284%2C4455744

Gemini 11 - 3rd column over, 3rd paragraph down

"However, he counted off the rockets as they ignited at five second intervals near Canton Island in the Pacific."



http://news.google.com/newspapers?id=UuYcAAAAIBAJ&sjid=CpcEAAAAIBAJ&pg=7152%2C1397867

Gemini 12 - 5th paragraph down

"The four retro-rockets, each generating 2,500 pounds of braking thrust, fired on the rear of a backward-flying Gemini 12 at 1:47 p.m. (EST) as the space chariot flashed 173 miles above the western Pacific Ocean near Canton Island."

[Jim]

I wouldn't quote newspapers for that info.  Need NASA docs.

[WM68]

I wouldn't quote newspapers for that info.  Need NASA docs.

Like from MSC-G-R-66-7 page 4-38.

[John Charles]

Okay to quote newspapers in this case, I think. The reporters were themselves quoting NASA, not making it up. I have tried to find NASA sources on the geographic location of Gemini at retro fire, and it is surprisingly difficult. Even mission reports don't include it. So thanks for any such reports, official or press.

[John Charles]

WM68, great! Confirms the news accounts, too. Thanks for that. What is the document called?

[Jim]

A comparison of entry g loads between mercury and Gemini should show less for Gemini since it had a shallower flight path

[WM68]

WM68, great! Confirms the news accounts, too. Thanks for that. What is the document called?

NASA MSC-G-R-66-7 Gemini program mission report: Gemini X

[Antilope7724]

I've also read news reports that Gemini 3 and Gemini 4 used their Orbital Maneuvering System (OMS) engines to lower their orbital perigee to 50 miles prior to firing the retros at about 90 or 100 miles altitude, before the low perigee point was reached. News reports said this was done to assure reentry even if some of the solid retros failed.

[the_other_Doug]

I've also read news reports that Gemini 3 and Gemini 4 used their Orbital Maneuvering System (OMS) engines to lower their orbit perigee to 50 miles prior to firing the retros at about 90 or 100 miles altitude before the low perigee point was reached. News reports said this was done to assure reentry even if some of the solid retros failed.

Very true, and this was a major bone of contention with flight planners and crews alike.  It required the sequestration of a fair percentage of the OAMS fuel (and that's the correct acronym, it stood for Orbital Attitude and Maneuvering System -- OMS was the acronym for the Shuttle's main maneuvering engines).  That limited the amount of fuel that could be used for things like orbit changes and rendezvous.

After the retros worked fine on two missions, the guys who were insisting that every Gemini mission had to reserve enough OAMS fuel to basically perform retrofire without needing the solid retros were finally overruled, and Gemini began to depend entirely on the solids to perform their deorbit burns.

Finally, in re the location of the retrofire burns, all spacecraft with lift begin their descents farther uprange than those which depend on ballistic trajectories.  The Shuttle used to perform retrofire something like two-thirds of the way around the world from their landing site.  And, as I said, Soyuz that have downmoded to ballistic entries do land hundreds of miles short of their targets.  A lifting trajectory is by definition a more extended trajectory, especially if all of the lift capability is used to extend the downrange component.  Shuttle and Apollo mostly wobbled back and forth a lot (the infamous S-turns) to manage that lift; each had more lift than desired for the nominal entry trajectories and spent a lot of their entry profiles taking out the lift vectors by swerving back and forth to minimize it.

[John Charles]

Thanks to all for the responses.

I will wager that MegaBytePhreak was closest. Gemini-B would have needed all 6 Star-13E motors for launch abort, so they were available for de-orbiting too. Probably safer not to introduce circuitry to deselect 2 of them for de-orbiting, which would have been another failure path during the launch abort. Also better not to have unexploded ordnance in the retro module burning up near the reentry module during entry. And no good way to jettison them without introducing risky complications. So just go ahead and fire them to de-orbit.

NASA's Gemini routinely used about a 21-degree nose down pitch attitude during retrofire, so by trigonometry the total delta-v capability from the standard 4 solid motors of 97.5 m/sec (320 ft/sec) provided 91.2 m/sec (299 ft/sec) in the direction of flight and about 35 m/sec (115 ft/sec) downward. Assuming Gemini-B would have wanted the same delta-v "aft" and "down" as NASA's Gemini, the capsule could have been yawed left or right by 48 degrees to direct the excess delta-v sideways. This would have provided 102 m/sec (330 feet/sec ) of cross-range delta-v. I estimate this could have shifted the ground track about 83 km (45 n.mi.) to one side.   (Feel free to check my math.) Note that this would have been in addition to Gemini’s cross-range capability of 50 km. (27 n.mi.) from its inherent aerodynamic lift.

Assuming all of these assumptions are correct, maybe Gemini-B recovery planning would have routinely included an 80-km offset to one side or the other.

I hope any upcoming MOL declassifications from the NRO (as Blackstar has hinted) will get down to this level of detail, to confirm or refute this hypothesis.

[Proponent]

By the way, why were Gemini (and I believe Mercury) retrofire burns done in a nose-down attitude?

[John Charles]

I asked a few FIDOs and other orbit-y people, and got a few answers: ensure on-target entry interface, allow visual confirmation of retro attitude. Then I asked Chuck Dieterich, Apollo RETRO officer. Here is what he wrote back:

"Remember Mercury and Gemini used solid retros with unguided cutoff thus any dispersions in delta V would affect the transfer angle from burnout to target point.  (Of course, Mercury had no guidance for burn or entry.) The pitched down attitude provides a negative flight path angle at cutoff. (The burnout flight path angle of a horizontal burn, i.e. Hohmann Transfer, is zero.)  The negative flight path angle at cutoff reduces the transfer angle between the burnout and target point and thus the time that burn dispersions can propagate.  The Apollo guidance controlled the burn, but it was felt a smaller transfer angle was still appropriate and the crew could see the horizon as it lined up with a scribed line of the window.  The Apollo and Gemini entry guidance dealt with most burn dispersions."

As I interpret it, he said: all of the above, but primarily to minimize the time during which inaccuracies in retro-rocket performance could affect the descending trajectory. Then it sort of got to be a habit and lasted until the Shuttle era.