# Was the nature of Explorer 3's orbit intentional?

A number of early spacecraft launched into orbits other than the intended one. For example, Explorer 1's apogee of 2550 km was much higher than intended, and Vostok 1's was too high to decay in the intended timeframe of "less than ten days".

Explorer 3, like Explorer 1, was in an elliptical orbit with an apoapsis above 2500 km. Was this the result of a deliberate decision to permit it to investigate the radiation anomalies Explorer 1 found, or was it another accident?

• – uhoh Jan 1 at 2:20
• Did you find a source for the intended orbit of the Explorers? – asdfex Jan 1 at 10:26
• Do you only ask why the apogee was higher than planned, or do you also ask about why the orbit was elliptical in the first place? – asdfex Jan 1 at 13:14
• @asdfex, I'm interested in knowing if Explorer 3's elliptical orbit was intentional or accidental. – Mark Jan 1 at 19:49

The elliptical orbits of the first satellites were intentional, and the only kind of orbit that could be achieved by these early rockets. Elliptical orbits with a low apogee can be reached by one almost continuous burn of the various stages of a rocket. To reach a circular orbit, it is either necessary to do a circularization burn after one half orbit to raise the perigee to the same height as the apogee or to have one long burn with constant adjustment of attitude and thrust direction. While today's rockets are perfectly capable of this feat, the first ones were not for several reasons:

• All maneuvers where initiated by commands sent from ground control. After half an orbit the satellites were roughly over the Indian Ocean. Sending a command at this point would have required a manned ground station somewhere in this region. These were set up for later missions, but were not available for these first flights.

• All 3 upper stages of the Juno were built from (identical) solid boosters with a $$\Delta v \sim 1500\,\rm{m/s}$$ each and couldn't be controlled or throttled (¹). The circularization burn on the other hand requires a few hundred m/s at most - i.e. it would require yet another, smaller stage - but yet this was too large for a simple cold gas thruster to be viable (these were used for attitude changes after separation of the first stage).

• The upper stages did not have any reaction control system to control their orientation. After the first stage burnt out, the upper stages were pointed in the right direction and spun up to 750 rpm. Everything happening after that was as "simple" as firing the engines and hoping that the rocket still pointed in roughly the right direction. For a cirucularization burn to happen, this attitude would have to be stable for almost one hour (²).

@OrganicMarble found a reference for the intended orbit in Willy Ley's 1957 "Sputnik Edition" of Rockets, Missiles, and Space Travel given as 320x2250km. A difference in the perigee of 300km might seem a lot, but it surely wasn't very far off in terms of rocket performance. Note that changing the apogee in a elliptical orbit doesn't need much propulsion, e.g. the difference between 2550km and 2250km is just a $$\Delta v\sim50\,\rm {m/s}$$ or roughly 1% of the total performance of the upper stages.

Comparing the early American and Russian orbits, they were very similar - Sputnik was in a comparable 215x939km orbit. Gargarins orbit was lower and less eccentric, but this was due to the fact that the requirements were different: Gargarins orbit was planned to decay quickly, while the other satellites were intended to stay for a longer time in orbit to provide useful data.

(¹) They operated for an incredible short time of 6 seconds only, resulting in acceleration exceeding 25g.

(²) And as a matter of fact, it wasn't: The upper stages started to tumble very soon because the dynamics of rotating bodies were not fully understood at this time.

• Willy Ley's 1957 "Sputnik Edition" of Rockets, Missiles, and Space Travel gives the intended Explorer 1 orbit as roughly 200 x 1400 miles. (320 x 2250 km) p. 339, 1957 3rd revised printing. – Organic Marble Jan 2 at 18:46
• Downvoted for the first paragraph: you certainly can reach a circular orbit with a single continuous burn. The key is to time it so that you hit the orbital velocity for your desired circular orbit right when you hit apoapsis. (You can't reach a circular orbit with a single impulse burn, but launches from Earth's surface take far too long to be approximated as impulse burns.) – Mark Jan 2 at 19:56
• @asdfex, have you watched any of the Falcon 9 launches? They sometimes reach orbit after apoapsis. The patched-conics model in which apoapsis is 45 minutes away from launch only applies to impulsive burns (eg. a space gun). On a purely ballistic trajectory, it takes a little over four minutes to reach an altitude of 320 km, so any launch with a burn time longer than that can theoretically reach circular orbit in a single burn. – Mark Jan 2 at 22:38
• Then, let's change this to something like "with modern guidance techniques and a long burn (~ 8 minutes in case of Falcon 9) it would be possible, but not feasible at all at that time" ? – asdfex Jan 2 at 23:25