The upper stage of the Juno 1 rocket was spun up to a high speed prior to launch so it would be spin stabilized and not require a guidance system. After the upper stage separated from the 1st stage it coasted to apoapsis, oriented itself to point at the horizon with air jets, and then ignited to put the Explorer satellite in orbit.

How did the upper stage orient itself to point at the horizon while it was spinning? Wouldn't the spinning upper stage be very resistant to changes in its orientation? Were the air jets fired in such a way as to take advantage of the gyroscopic nature of the spinning upper stage to make orienting it easier?

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    $\begingroup$ Not a full answer, but some data: The Explorer satellite had a diameter of only 16 cm and a weight of 13 kg. The rocket had 180 cm diameter and a launch weight of 40,000 kg. The momentum of the spinning mass was rather low compared to the whole assembly. $\endgroup$
    – asdfex
    Dec 25, 2017 at 17:37
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    $\begingroup$ Looks like the total rotating section had a weight of about 500 kg and a diameter of 90 cm (1020 pounds and 2.9 feet). As a side fact, the upper stages had a top acceleration of close to 30 g - must have been a sturdy beast. $\endgroup$
    – asdfex
    Dec 26, 2017 at 1:16

1 Answer 1


This answer addresses the part of the question: "How did the upper stage orient itself to point at the horizon while it was spinning? Wouldn't the spinning upper stage be very resistant to changes in its orientation?".

This manoeuvre is called precession of the spin axis. You are right that the spin stabilisation does exactly that, it makes the spin axis more stable to small perturbations, however that doesn't mean that its not viable to do such a manoeuvre. Off the top of my head I couldn't say how much more it would cost in propellant though obviously it needs careful phasing from the thruster driving the precession so that it only fires at the right time.


This is a typical precession manoeuvre (not necessarily that of Explorer 1, works like this, picture this concept:

i) a radially mounted thruster that does not point through the Centre of Mass and is firing continually - there will be no precession,

then imagine

ii) that it is synchronised only to fire when pointing in one direction - now there will be a turning force (though the reaction has to be consistent with gyroscopic behaviour, my memory is rusty on this). As it will only fire for a small part of the spin period the manoeuvre will take longer than for a non spinning body.

EDIT 1/]

Note that the Juno 1 / Explorer 1 mission was also famous because of a misunderstanding about the nature of spin stabilisation at the time. The answer to this question Explorer 1 describes that point in more detail (I just found and added some relevant material) as the satellite tumbled because of energy transfer due to flexible modes.


I accept the foregoing stops short of saying what actually was the design for Explorer 1. An obvious query would be what combination of sensors and control loops was used for the Explorer 1 mission. What I have been trying to describe is the physics of the problem from a blank sheet of paper perspective.

  • $\begingroup$ Were the thrusters spinning, or were they attached below the ball-bearing? $\endgroup$
    – Thomas
    Dec 26, 2017 at 23:30
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    $\begingroup$ Is precessing the spin axis via a long series of very carefully phased/synchronized firings an educated guess, or do you know for sure that this is indeed what Juno 1 was designed and equipped to do? Is it possible to add a supporting link for this? $\endgroup$
    – uhoh
    Dec 26, 2017 at 23:38
  • $\begingroup$ The first part is what the manoeuvre is called. The second bit as you put it "by a long series ..." is a guess, though I can't immediately see how else you'd do it, thanks for picking up on that, slight edit introduced. $\endgroup$
    – Puffin
    Dec 27, 2017 at 1:30
  • $\begingroup$ @Puffin yep I understand the principle, but if the spacecrat were intended to rotate at 750 RPM (that's 12.5 revolutions per second) as stated in your answer to the linked question, that would have to be some pretty rapid and tightly-controlled pulsing, somehow phased/synchronized to an inertial frame, like a Sun sensor for example, in order to pull this off. That would be some pretty amazing technology to have matured in 1958! So is this purely a hypothesis, or is there some evidence that this is how it really was supposed to work? $\endgroup$
    – uhoh
    Dec 27, 2017 at 5:02
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    $\begingroup$ @uhoh The thrusters were mounted to the instrument section which was not rotating. See history.nasa.gov/sputnik/jupsketch.jpg Only the "tub" was rotating $\endgroup$
    – asdfex
    Dec 27, 2017 at 10:32

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