Were the J-2 engines of the third stage of Saturn V throttable? I know that the F-1 for first was not, but what about J-2? I could not find any information about it.

Any reference or info for this?


  • $\begingroup$ The F-1 was only used on the first stage; the J-2 was used on both the second and third stages. $\endgroup$
    – DylanSp
    Commented Feb 5, 2020 at 13:59
  • $\begingroup$ Hi @DylanSp,you're right. my mistake, thanks for the clarification. Do you know anything about its (alleged) throttling capability? $\endgroup$
    – venom
    Commented Feb 5, 2020 at 14:10
  • $\begingroup$ I don't, sorry. I can confirm that the F-1 wasn't (see this answer), not sure about the J-2's. $\endgroup$
    – DylanSp
    Commented Feb 5, 2020 at 14:17
  • 1
    $\begingroup$ Related: space.stackexchange.com/q/27425/195 $\endgroup$ Commented Feb 5, 2020 at 18:30

1 Answer 1


The J-2 was, to use a technical term "kind of throttleable".

The engine incorporated a Propellant Utilization (PU) valve that could change the mixture ratio and thrust during operations.

enter image description here

Picture from the J-2 Fact Sheet (really a whole, fabulous book).

During engine operations propellant sensing devices in the vehicle propellant tanks control the valve gate position for adjusting the oxidizer flow to ensure simultaneous exhaustion of fuel and oxidizer.

An additional function of the PU valve is to provide thrust variations in order to maximize payload. The second stage, for example, operates with the PU valve in the closed position for more than 70 percent of the firing duration. This valve position provides 225,000 pounds of thrust at a 5.5:1 propellant (oxidizer to fuel by weight) mixture ratio. During the latter portion of the flight, the PU valve position is varied to provide simultaneous emptying of the propellant tanks.

The third stage also operates at the high-thrust level for the majority of the burning time in order to realize the high thrust benefits.

The exact time at which the engine will operate with the PU valve closed will vary with individual mission requirements and propellant tanking levels.

When the PU valve is fully open, the mixture ratio is 4.5:1 and the thrust level is 175,000 pounds.

Source Saturn V Press Kit J-2 Engine

So not throttleable in the sense like the SSME where the vehicle sends throttle commands to it "arbitrarily" (which would change the thrust but not the mixture ratio), but the system was designed so that the engine and propellant system could modify its mixture ratio and thrust level operating point to achieve proper propellant depletion.

This chart shows the effect on thrust and Isp of moving the PU valve.

enter image description here

Graph from the Apollo 12 / SA 507 Flight Manual.

Operationally the second stage engines' PU valves were sometimes operated by signals from the booster's Instrument Unit rather than by the propellant sensing system. An example of this operational mode is described in the Apollo 12 Saturn postflight report paragraph 6.5. For details, please refer there, but it describes how the mixture ratio started at "Null", then was commanded to "High" at start +5.6 seconds, then to "Low" at start +324.2 seconds.

The third stage PU system remained in "null" for the duration of the first burn, was commanded to "Low" prior to restart, then to "High" 109.8 seconds into the burn. See paragraph 7.9 in the same document for details.

  • 11
    $\begingroup$ so throttleable like gluing the accelerator to the floor, and choosing between "eco" and "sport" :p $\endgroup$
    – user20636
    Commented Feb 5, 2020 at 15:08
  • $\begingroup$ great, thanks a lot! $\endgroup$
    – venom
    Commented Feb 6, 2020 at 5:37
  • 1
    $\begingroup$ This is why I love this site! Fascinating even though it's purely historical at this point. $\endgroup$
    – PJNoes
    Commented Feb 6, 2020 at 18:32

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