12

I have a basic understanding of how RCS and reaction wheels work, but I can't figure out what "reaction" refers to in their names.

Is it referring to how a craft's attitude "reacts" by changing in response to the use of RCS or reaction wheels? Or is it referring to something else?

JAK Zero
  • 221
  • 1
  • 6

2 Answers2

18

I think there is little doubt that "reaction" refers to the "equal and opposite reaction" of Newton's third law.

First consider "reaction control system". In principle, that can include both thrusters and reaction wheels, but in the Kennedy Space Center website it generally refers to thrusters. And the term "reaction" from Newton's third law is very closely tied to rocketry. For instance, consider the second sentence here:

Rocket thrust is the reaction force produced by expelling particles at high velocity from a nozzle opening.

Now consider reaction wheels. Reaction wheels, like thrusters, do in fact depend on Newton's third law, so, if in fact it is true that "reaction control system" refers to reaction in that sense, it makes since that the "reaction" in "reaction wheel" does too. The fact that reaction wheels, which function based on producing an equal and opposite reaction, are distinguished from gyroscopes, which measure changes in orientation by orthogonal torques, further supports that idea.

It is hard to find a source explicitly saying that is why they are given that name. But the centrality of Newton's third law to rocketry makes it hard to imagine it could come from anywhere else.

Community
  • 1
Mark Foskey
  • 10,971
  • 34
  • 47
  • This is interesting, google ngram reaction wheel .vs. reaction control system suggests "reaction wheel" became regular parlance in the 1830's (steam locomotives took off around then) while "reaction control system" popped up out of nowhere in the late 1950's. Also https://en.wikipedia.org/wiki/Aeolipile & results for "reaction engine" and "reaction turbine". I think that reaction-based systems has a long, solid history in engineering nomenclature. – uhoh Jun 26 '23 at 06:05
  • 1
    your 1830s reaction wheels are turbines, not attitude control systems @uhoh -- but again it's 3rd law reaction – JCRM Jun 26 '23 at 06:10
  • @JCRM no doubt. While folks are in general agreement that we all think the term RCS was coined as a 3rd law refrerence, and some are suggesting the question is off topic and belongs in English Language Usage SE I think it's possible with some work to draw a line to previous usages of "reaction" and with more work find where it was done. We know the decade is probably the 1950s... – uhoh Jun 26 '23 at 06:16
  • 2
    The first acknowledged reaction motor (and referred to as such) was in a boat in 1886. By the 1930/40s, there were references to controllable reaction motors. By the Gemini program, the principle of what became RCS was referred to as reaction torque jets. By the time of Apollo, Marquandt, who designed and built the distinctive 100lb engines, the term RCS was part of the program. – blobbymcblobby Jun 26 '23 at 06:58
  • 1
    My take is that in aerospace the term arose in the X-15 program; that vehicle had dual sets of controls, "aerodynamic" and "reaction" aka "ballistic", each with their own control stick. One set for aeroflight, one for when the vehicle was out of the sensible atmosphere. Since it was introduced so early in US human spaceflight, none of the later programs felt the need to define the term. – Organic Marble Jun 26 '23 at 11:18
6

As Organic Marble mentioned in a comment, "reaction" is used in the same sense as Newton used it: "equal and opposite reaction". A reaction wheel is designed to produce a reaction, just as a propulsion system is designed to produce propulsion.

JCRM
  • 693
  • 1
  • 15
Woody
  • 21,532
  • 56
  • 146
  • 1
    "I agree with Organic Marble" is an opinion and another comment. It will be interesting to see if someone comes up with a reference. A Stack Exchange answer should say more than that, and support its assertions with supporting sources, calculations, or other means. It's just this kind of answer that we close questions which are likely to attract primarily opinion-based answers for in order to prevent them from being posted. Move this to a comment? – uhoh Jun 25 '23 at 06:12
  • 1
    @uhoh ... It would be even better to move the question to an English language usage site. The question is about semantics, not exploration of space. “Reaction” has numerous definitions, so the question is valid from that perspective.. But that’s just an opinion. – Woody Jun 26 '23 at 02:15
  • 2
    ya, but that's just a whataboutism - and a reason to vote to close as off-topic, not a justification for posting an "I agree with a comment" as a Stack Exchange answer. – uhoh Jun 26 '23 at 05:55