Inside The J-2X Doghouse: Engine Control — Open versus Closed Loop

As I was driving to work this morning, I came up over a rise and saw suddenly appear in my windshield, over towards the left on the other side of the road, a police cruiser with a radar gun mounted in the window.  Even before I could think about it, the pressure of my right foot on the accelerator lessened.  I then instinctively looked at the speedometer and found that I was traveling 48 miles per hour on a road for which the speed limit is 45 mph.  Thankfully, the officer apparently forgave me the 3 mph violation and continued to wait where he was for a better opportunity to serve and protect the community. 


What I find interesting about that little episode was the immediate, unthinking response I made in response to seeing the cruiser.  In terms of control systems, that could be called a feedback loop.  My senses received data, my brain rapidly processed that data and then sent a signal to react to the data, and then my calf muscles responded by easing up on the throttle.  The car, in turn, slowed to respond to the lower throttle.  Never mind, of course, that if I’d been really, really speeding all this would have been too late (and I would have arrived at work very angry), there was nevertheless a closed-loop response that is not too dissimilar to what we do for rocket engine control … in some cases.


First, let’s get familiar with a couple of terms –


Open-loop: We typically refer to something as open-loop when we have instrumentation that measures conditions in the engine but the engine itself does not respond to those measurements.


Closed-loop: We typically refer to something as closed-loop when we have instrumentation that measures conditions in the engine and then, potentially, the engine takes action based upon those measurements.


It is based upon those definitions that I would call my response to seeing the cruiser closed-loop since I responded and did something with the data.  Another example would be the more modern systems that are used to monitor and control automobile systems.  It used to be that you had a temperature measurement stuck in the coolant loop.  You could watch the temperature rise, but until the system went kaput and boiled over leaving you stranded alongside the road, there was no active, closed-loop control.  Nowadays, if the computer in my pickup truck sees that the engine temperature is too high, it will take action to try and protect itself.  For example, it will inhibit the use of the air conditioning system since that represents an additional power requirement on the engine.

 So, what do we control on a rocket engine?


One thing that we control is power level.  On the Space Shuttle Main Engine (SSME), power level is controlled in a closed-loop manner.  This means that the main combustion chamber pressure is measured as an indication of thrust level and in response to that measurement a valve is opened or closed to increase or decrease the engine power level.  On the J-2X, power level is controlled in an open-loop manner.  This means that we measure the main combustion chamber pressure but we don’t have any feedback loop where we control a valve to ensure that we’re on target.  Instead, should we happen to be off on power level, we have to physically change an orifice in the engine between tests.  The “feedback loop” is data analysis and a guy with a wrench.  Which approach you choose to take are dependent upon your requirements of performance and affordability. 


Another thing that we control on a rocket engine is the mixture ratio (i.e., the ratio of oxidizer to fuel).  Given that on a rocket you are carrying both your oxidizer and fuel with you in the vehicle, you certainly want to make sure that you consume your propellants in the correct ratio to get the most uumph out of them.  Again, on SSME we control mixture ratio in a closed loop manner.  There is actually a small flowmeter on the SSME and, using the data from that flowmeter (and some associated calculations), we move a valve on the engine to dial in the correct mixture ratio.  It’s a pretty nifty system.  Also again, on the J-2X, we have an open-loop system for mixture ratio just like we have for power level.  We test the engine, look at the results, and, if necessary, make a physical change to the engine in the form of an orifice.


Because of these two areas, power level and mixture ratio, SSME is usually referred to as a “closed-loop engine” and J-2X is usually referred to as an “open-loop engine.”  Now, this terminology is not entirely correct since there are some closed feedback loops within the J-2X control system pertaining to engine health and status diagnostics, but we all know how enduring shorthand designations can be.  Also, engines don’t have to be one or the other.  They can be half-and-half.  The engine used on the Delta IV vehicle, the RS-68, sort of falls in this category. 


How you choose to design your engine control system is driven by your requirements.  Put real simply:  The SSME is all fancy-schmancy because it had extremely tight power level and mixture ratio precision requirements and because it was a reusable engine.  The J-2X is intentionally more simplistic because it has looser precision requirements and because it is expendable (and throwing away orifices is a whole lot cheaper than throwing away valves if your requirements will let you get away with it).  Requirements drive design.


Note that I will save the fun topic of engine diagnostics — and the potential for long philosophical meanderings within that realm — for future posting. 




Let’s end this posting with a fun little exercise.  Above is a simplified schematic of a gas-generator cycle engine kind of like a J-2X.  I have shown in the schematic two orifices #1 and #2 (highlighted in yellow).  With those two orifices, we can calibrate the engine.


Scenario:  Power level too low, i.e., measured main combustion chamber pressure too low.

·         Solution:  Increase the size of orifice #1.

·         Explanation:  By increasing the size of orifice #1, I will deliver more oxidizer to the gas generator.  This will deliver more power to both turbines thereby increasing how much propellant gets pumped into the engine.  More propellants pumped in equals more thrust and greater overall power level.


Scenario: Power level too high, i.e., measured main combustion chamber pressure too high.

·         Solution:  Decrease the size of orifice #1.

·         Explanation:  The exact opposite of the previous scenario.


Scenario:  Mixture ratio too low, i.e., the flow of oxidizer is too low in proportion to the flow of fuel, as measured by the test facility.

·         Solution:  Decrease the size of orifice #2 and decrease size of orifice #1.

·         Explanation:  By decreasing the size of orifice #2, I decrease the amount of flow that is diverted around the oxidizer turbopump turbine.  I therefore increase the flow through the turbine thereby increasing pumping power of the oxidizer side.  So I increase oxidizer flow to the engine.  However, by increasing oxidizer flow to the engine and doing nothing else, I’ve probably messed up my overall engine power level so I’ve got to back down a little bit by decreasing the size of orifice #1.


Scenario:  Mixture ratio too high, i.e., the flow of oxidizer is too high in proportion to the flow of fuel, as measured by the test facility.

·         Solution:  Increase the size of orifice #2 and increase size of orifice #1.

·         Explanation:  The opposite of the rationale for the scenario immediately above.


See, being a rocket scientist isn’t that difficult, really.  Now you too can calibrate an open-loop rocket engine.



P.S., I read in the paper this morning that NASCAR racer Kyle Busch had his civilian driver’s license revoked for 45 days for doing 128 mph in a 45 mph zone.  Well, at least I wasn’t going that fast when I came over the rise this morning and saw the police cruiser.  Then again, I wasn’t driving a $400,000 Lexus LFA sports car like Mr. Busch was…





12 thoughts on “Inside The J-2X Doghouse: Engine Control — Open versus Closed Loop”

  1. Thanks for the informative post. So AJ2003 aborted test was a scenario #2 case? With such a ‘simple’ engine how is propellant utilization implemented if the mixture ratio is fixed like this? Or is it done by carefully loading the correct amount of propellants?

    Good Luck to all with future tests!

  2. Your question about propellant utilization is quite perceptive. And, yes, the most basic means (basic, but not necessarily simple or easy) of ensuring effective usage of the propellants on board goes back to making sure that you’ve loaded the correct proportion of propellants to start with. That’s accomplished by doing what it called a propellant inventory that accounts for every drop of propellant loaded, boiled off, used to start the engines, used during flight, and left remaining after engine shutdown. No matter what else you do, that is the most basic and effective propellant utilization approach. But, you could consider doing more.

    Note, however, that even with a more complex engine like the SSME that has closed-loop control on mixture ratio there never was an actual provision to alter mixture ratio in flight. It was theoretically possible, and it was an initial capability built into the engine, but it was never used. Why? In part it comes back to the question of how a control loop works. If you are going to make decisions about how the engine mixture ratio has to change in flight to accommodate the remaining propellant on board, then you need data. In other words, you need a means of estimating how much propellant you’ve got left. That is not easy. It is possible, but not easy. So then the question comes down to a trade study of implementing a measurement system into the tanks such that you can estimate your propellant quantities in flight and accepting the inherent errors any such system would have, as compared to the potential benefits of such a system. Typically, the proposed gains don’t outweigh the downsides of greater complexity, more cost, potential inaccuracy, and possibly added weight or failure modes on the vehicle.

    But let’s say that you can measure what you’ve got left in the tanks comfortably. Do you necessarily need to dial into an exact mixture ratio on the engine? No, not really. What if you had two mixture ratio settings: one higher, one lower? If the mixture ratio what you had left always fell between these two points, then you could use a combination of time at high mixture ratio and time at low mixture ratio to integrate into any mixture ratio you wanted. So even with a relatively simple engine-side control system, you could accomplish a larger vehicle-level closed-loop control system for propellant utilization. In fact, the Apollo Program using original J-2 used such an approach.

    But back to the original question regarding simple versus more-complex engines and propellant utilization. What a more-complex engine like the SSME gives you in terms of its control points of power level and mixture is greater precision around the calibrated set points. If you combine this precision with a good propellant inventory, you can get very, very good at propellant utilization with no vehicle-level feedback loop necessary. This is how the Space Shuttle flew and because there were so many missions and so much practice, they had the whole thing evolved to nearly an art form.

    With a less complex engine like the J-2X, where you don’t have a closed-loop control system for mixture ratio, you get less precision around your calibration point. So, you can either build in a propellant utilization system at the vehicle-level, as discussed, or you can build your vehicle with enough margin to accommodate that lack of precision. Choices, choices, choices.

    Thus, once again, it comes down to requirements and decisions made across the vehicle. There is almost never exactly and only one way to solve a problem and make something work. The SSME and the J-2X, with their similarities and differences, are but puzzle pieces to a larger solution of making something work. Some would say, for example, that we’re taking a step backwards, technologically speaking, in going from SSME to J-2X. But how is that possible when they both fulfill their requirements? The point is NOT to build the most neato engine. Trust me, if that was ever the point, boy could we go to town! No, the point is to fulfill the mission. The engine contributes to fulfilling the mission by fulfilling the assigned and allocated requirements, including, sometimes, such messy things like affordability, safety, operability, etc.

    Hopefully, that long-winded answer provided some more info.

  3. Could you explain this two-or-more-ratio thing a bit more for the hard-of-engineering?

    To my simple mind, if you have an H2/O2 engine, then surely you will only ever want to have one ideal ratio: 2 molecules of H2 to every one of O2, resulting in perfect combustion and maximum energy release. If you stray away from this, either you will have an excess of the fuel or an excess of oxidiser being sent out of the exhaust. This seems to me to be inherently inefficient. Am I missing something?

  4. Common misconception.

    One would intuitively think that the most efficient way to use propellant in a rocket is to extract every bit of the chemical energy from the combustion of those propellants. And, one would intuitively think that the greatest, most efficient extraction comes with a stoichiometric mixture ratio. It is at this point where you have the perfect combination of molecules so that in the case of a hydrogen/oxygen rocket all that you would produce is pure steam. Your two hydrogen molecules combust with one oxygen molecule to form two water molecules a la:

    2H2 + O2 ==> 2H2O

    This is the natural intuitive notion. This notion is, however, wrong.

    Go all of the way back to the discussion about what makes a rocket a rocket and how it works via the rocket equation. The point of a rocket is to throw the exhaust away from the vehicle with as much momentum as possible, with momentum being the product of mass and velocity. It turns out that while you might get more chemical energy extraction at the stoichiometric point, you do not get maximum generation of momentum of the exhaust. Why? Because lighter molecules like, for example hydrogen, are easier to throw faster than fat and heavy water molecules. So, you can either make really, really hot, heavy, and slower water molecules, or you can go fuel rich and make a cooler but lighter and faster-moving mixture of water molecules and hydrogen molecules. What you actually want in a rocket is this:

    (2+x)H2 + O2 ==> 2H2O + xH2

    In terms of mixture ratio (which is a ratio of mass rather than molecules), the stoichiometric point is 8.0. Where does the best performance happen for a hydrogen/oxygen rocket engine happen? In terms of specific impulse it happens somewhere in the vicinity of a mixture ratio about 3.5 to 4.0 (the exact value depends upon your assumptions regarding completeness of the reaction and stuff like that). Yep, we want that much excess, un-combusted hydrogen in the exhaust. A whole lot. It’s not intuitive, but it’s the truth.

    However (there are always lots of “howevers” when discussing engineering decision making), hydrogen is very light, i.e., not dense, so it takes up a lot of space. Taking up a lot of space means big tanks to hold it. Big tanks mean heavy vehicle. So, while an engine might get its best specific impulse at something like a mixture ratio of 3.5, the overall vehicle performance is better when you go to a lower performing engine mixture ratio of, say, 5.0 to 6.0 (still fuel rich but not as much) so that you can have a smaller hydrogen tank.

    The SSME nominally runs at mixture ratio of about 6.0. The J-2X nominally runs at a mixture ratio of about 5.5. Different usages, different stages, different decisions design points, but these kinds of values are the historical norm.

    Also, one more thing about mixture ratio. If you get too close to stoichiometric for whatever reason, you generally can’t handle it since the combustion products are far too hot for most metals to survive, even when actively cooled. Thankfully this isn’t an issue since you’re best performance naturally comes at lower, cooler, fuel-rich mixture ratios.

  5. Thank you for keeping this blog up. I was totally unaware that the SSME PU system was never used. I assumed that on STS-114 when we lost a injector plug which put a hole in a engine collant tube caused a large hydrogen leak that the LOX low level cut off we got was due to the PU system burning more LOX. However if the PU system was not active the low performance due to the H2 leak drove a longer burn than planned thus we gotlow on LOX.(Since a nominal MECO was based on a velocity cutoff).

    When is the next J2X test?

    Thanks again.


  6. @Shuttleguy

    Hold on, I did not say that the propellant utilization system for Shuttle was not used. The propellant utilization system with regards to the SSME was, effectively, maintaining a very tight operational tolerance on mixture ratio. That, combined with extensive history and detailed propellant inventories provided a vehicle-level propellant utilization system.

    What was not ever used was the capability of the SSME to make real-time, in-run mixture ratio changes. So, a propellant utilization system at the vehicle level using an active feedback loop was never used.

    In the case that you cite, where we blew out a lox post pin and damaged the nozzle, the low liquid oxygen level later in flight was actually caused by the closed-loop power level control system on the SSME. Let me explain.

    The SSME controls mixture ratio by measuring liquid hydrogen flow with an on-engine flowmeter. The liquid oxidizer flow is not measured but is calculated using an engine-specific calibrated equation and power level. When the hydrogen leak occurred, it was downstream of the flowmeter. There is no way for the engine to measure a leak. But with hydrogen going overboard, the engine power level would tend to fall, i.e., stuff going overboard is not making it to the chamber and combusting. So the engine, with its closed-loop control on power level says to itself, “I need more power.” How does it do this? By increasing liquid oxygen flow into the engine.

    Thus, in the effort to maintain the proper power level, the engine responded to the hydrogen leak by going off mixture ratio. Now, the engine thought that it was still on mixture ratio, but that’s because it couldn’t take into account the leak. That is why at the end of the mission you found that more liquid oxygen had been consumed than expected.

    So a closed-loop system has extreme precision when everything is working well, but might have some drawbacks in anomalous situations. Again, and I keep saying this in different ways, there are always many, many design choices and consequences of those choices. Perfect solutions that cover all situations just don’t exist.

  7. Mr. Greene,

    Thank you very much for your time to give such detailed informative replies to questions posted here, including mine. 🙂

  8. Regarding the orifice #1 and #2 from your analogy, if you’re trying to maintain your nominal mixture ratio of 5.5, won’t orifice #1 and #2 eventually become a matched set based on the mission parameters?
    eg: We have 2 tons of food to send to the ISS, so we need X amount of hydrogen, Y amount of oxygen, and orifice A and B.
    Next time we have a 4 ton satellite to put into orbit, so we’ll need R amount of hydrogen, Q amount of oxygen, and orifice C and D.

    Or are there mission variables that require different “non-nominal” mixture ratios (to affect acceleration, etc)?

    Thanks again for all the time you’ve put into this blog and answering our questions 🙂

  9. @Aaron

    It’s actually far more simple than that. What we want is to calibrate every engine to exactly the same place. We want exactly 100% power level and we want exactly 5.5 mixture ratio. At the engine level, we don’t even have to take into consideration mission objectives in terms of engine calibration targets. We just want the engine to always be calibrated to the same power when it’s delivered and flown.

    Sounds easy, right? If you make the engines exactly the same, then the orifices necessary to calibrate to exactly the same point will be, well, exactly the same. But the fly in the ointment is that notion of “exactly the same.” That does not truly exist.

    You would think that if we used the same drawings, the same machines, the same tolerances, even the same machinists and technicians that every engine would come out the same. Boy do I wish that that was the case. It isn’t. Oh, they’re very, very close, but you have to remember that we’re dealing here with power densities that are unbelievable. Even building the hardware to the tightest tolerances still means that you have tolerances. And a lot of parts all stacked up, each with tolerances to consider, means that every engine is ever-so-slightly a unique beast. The smallest differences with such extraordinary performance demands mean that each engine will perform slightly differently. Thus, every engine has to be individually calibrated to within specific margins for mixture ratio and thrust level.

    Over time, assuming that we build more and more engines and get more and more experience with fabricating, assembling, and testing the engines, we will likely get much better at reducing this “uniqueness” of the engines. But given the typical production rates for engines programs in the past, this could take many years.

  10. Actually, increasing the size of a bypass orifice to decrease the O2 level is not intuitive at all. You’d think the gas generator would generate just enough gas to not need a bypass & the mixture would be controlled by an orifice in line with the turbopump.

  11. @guest regarding the bypass: The gas generator produces just enough high-energy gas to power the fuel turbopump, but too much for the oxidizer turbopump. That’s typically the issue with LO2/LH2 engines. Liquid hydrogen takes a lot of energy to pump because the density is so low. So, you use all of the GG gas in the fuel turbine and only a portion of that flow in the oxidizer turbine.

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