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One can often spot that liquid apogee engines are 440 N and attitude control systems are 22 N.

Is there a reason why the thrusts are proportional? If the engines are scaled for reducing development cost and time, are there any engineering advantages to have them proportional? Or more specifically, why is a 22N and 440N engine quite common?

Some citations:

Stationkeeping class 22 N engines fabricated from iridium-coated rhenium have demonstrated steady state specific impulses 20 to 25 seconds higher than niobium chambers. Ir-Re apogee class 440 N engines are expected to deliver an additional 10 to 15 seconds

The 440N thrust bi-propellant Liquid Apogee Motor(LAM) is used in INSAT-2 series of spacecraft for orbit raising and the 22N thrust bi-propellant engines are used for attitude control and station keeping.

The subsystem consists of one 490-N (110-lb) apogee thruster and twelve 22-N (5-lb) attitude and orbit control thrusters, using liquid bipropellants

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    $\begingroup$ While the values 440 and 22 in SI-units sound arbitrary, they translate to a nice and round 100 lbf and 5 lbf in some ancient system of measurement units. Most likely some company started to make thrusters with these values and others followed to have some compatibility. $\endgroup$ – asdfex Oct 10 '18 at 12:59
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    $\begingroup$ @asdfex That's a perfectly good answer. $\endgroup$ – Russell Borogove Oct 10 '18 at 13:05
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As noted by @asdfex, 440N and 22N are convenient round numbers in imperial units: 100 lb-f and 5 lb-f.

The exact thrust values for small spacecraft maneuvering thrusters aren't usually critical to designs of those craft; if the thruster is a little larger or smaller, maneuvers will just take a little less or a little more time. Thus standardized, known-reliable commodity thrusters with round-number performance classifications are often used; the exact thrust values may differ quite a bit from the class figure.

Your references generally call the 440Ns "apogee" or "orbit raising" thrusters -- used to get a satellite into its final orbit after a launcher puts it in a transfer orbit -- and the 22Ns "stationkeeping" thrusters -- used to keep it there. Orbital insertion has to be done over a relatively short timeframe to be efficient, hence the larger thruster, but attitude control and orbital correction can be done at leisure. The masses of most orbital satellites are about 2-5 tons, in which range 440N and 22N thrusters are appropriately sized for insertion and stationkeeping.

(The Apollo spacecraft, being 10-20 times heavier than a typical satellite, needed 440N thrusters for attitude control!)

The R-4D bipropellant thruster is probably the origin of the common "440N class" thrusters. It was originally developed by Marquardt for the 1964 Lunar Orbiter, adopted for both the Apollo service module and lunar module, and has been widely used on many satellites and spacecraft since; Aerojet Rocketdyne now owns the design. It is in fact a 490N (110 lb-f) engine; I suspect the Lunar Orbiter project specified a 100 lb-f engine and Marquardt overdelivered.

The GOES apogee thruster is almost certainly an R-4D; the document you link describes a 164:1 expansion ratio nozzle which is one of three standard options for that unit.

The ISRO LAM looks at least superficially like a copy of the R-4D, which makes good economic sense if you intend to fly a lot of them.

The R-6 was derived from a design intended the US military's Advent satellite around 1959; this "22N class" thruster was historically available in a 33N version.

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    $\begingroup$ That's much more an answer than my 3-line comment above :). Rocketdyne has a nice overview of their different engines, including a intermediate 110N version and a large 4000N one: rocket.com/propulsion-systems/bipropellant-rockets $\endgroup$ – asdfex Oct 10 '18 at 16:53
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    $\begingroup$ Similarly, fairings opening when heating would reach the four digit number 1135 W/m^2 is possibly because in Imperial units, that's 0.1 BTU per square foot. $\endgroup$ – uhoh Oct 10 '18 at 17:20
  • $\begingroup$ Haha! This turned out to be really anticlimactic, for I was not expecting an unit based historical answer! Thanks for the answer.. $\endgroup$ – karthikeyan Oct 10 '18 at 18:03
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    $\begingroup$ @uhoh Interesting. What can be expected at a heat flux of 0.1 BTU/sqft? $\endgroup$ – karthikeyan Oct 10 '18 at 18:04
  • $\begingroup$ @karthikeyan Oh I think that the answer at the link explains that. If not, it's best comment there. Here I made the comment only to note of another post here with a similar "anticlimactic" result (this time to me) where a funky number in SI units is revealed to be a one-digit figure in older imperial units. $\endgroup$ – uhoh Oct 10 '18 at 18:20

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