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The J-2 engine used on the upper stages of the Saturn launchers has a specific impulse in vacuum of 421 seconds. This is substantially lower than that of the smaller RL10, 440-460 seconds depending on the model. The RL10 has a greater nozzle expansion ratio -- 47:1 up to 280:1 depending on the model, compared to 27:1 for J-2.

While the 5 J-2 engines were fairly crowded in the base of the Saturn V second stage, the single engine in the third stage is not; a nozzle extension doubling its 2.1 m diameter to 4.2 m would still allow for reasonable clearance versus the 6.6 m stage diameter. This would increase the nozzle ratio to around 100:1, significantly better than the ratio of the 450 s versions of the RL-10, and with a higher chamber pressure.

The nozzle extension and the necessary lengthening of the interstage would add mass -- 5 tons or so, at a guess -- but even with that penalty, achieving 450 second specific impulse on the third stage could increase translunar payload by around 2 tons.

It's not particularly surprising that this approach wasn't used for Saturn/Apollo; developing and deploying a variant engine would come at a cost that might be better spent elsewhere.

  • Was this possibility at least considered during the Apollo program, or during post-Apollo proposals for further Saturn development?
  • Is the difference in ISP between J-2 and RL10 attributable primarily to nozzle expansion ratio, or is propellant mix ratio or another factor contributing significantly? Would a 100:1 expansion ratio J-2 be able to achieve 450 s specific impulse at 5260 kPa chamber pressure?
Russell Borogove
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Remember when the J-2X engine was proposed for the Ares I and V rockets? That featured a nozzle area ratio of 55:1. This ratio was the same as that of the original J-2X proposal. However, unlike the new design that would add performance through gas generator and chamber enhancements, the old J-2X was a simplification study.

The modern J-2X and its 55:1 expansion ratio could achieve a vacuum Isp of 448s or 451s, although this would operate at a chamber pressure above 1,200psi instead of the original 763psi.

As you mentioned, the clustered S-II arrangement of five engines would not allow for any increase in nozzle dimensions. You are also correct about S-IVB interstage length, which limited the J-2 size. But we must remember, too, that there are RL-10 variants with extending nozzles.

The RL-10B-2:

enter image description here

The J-2X was to be essentially a standard J-2 with an extension kit attached to the original nozzle exit. In the stowed position the J-2X would have the same length as the J-2, and the effective area ratio would still be 27.5:1. Upon actuation of the nozzle, it would extend (in a required time of under two seconds) and produce a 55:1 area ratio nozzle. This increased nozzle diameter could easily pass through the S-II/S-IVB interstage. The studies required that the extension kit weigh less than 204kg (450lb). Using this expansion ratio, the J-2 engine's vacuum specific impulse at a 5.5:1 LOX:fuel ratio would be increased by 10 to 11 seconds. This would be sufficient for a 3,200 to 4,200 pound improvement in Saturn V payload (I believe this was to TLI), minus the weight of the system.

Three different methods of extending the nozzle in flight were proposed. One system involved a one-piece nozzle extension that would surround the engine when stowed and lower down with hydraulic arms when deployed (much like the RL-10B-2). This proposal was discontinued due to its similarity with the XLR-129 engine under study (the shuttle engine that lost out to the SSME). A similar proposal would have broken the extended nozzle into several 'rings' that would telescope down over each other. However, the added complexity of the system caused it to be eliminated.

More info here: http://www.secretprojects.co.uk/forum/index.php?topic=16981.5;wap2

That left the Airmat design. This would involve a nozzle constructed from a strong woven mesh that would inflate into the desired nozzle shape as soon as the engine was started. Exhaust from the gas generator would be routed through the mesh nozzle, and this would also cool the material. This gas could only vent inwards, allowing film cooling. not only was this design simple in operation, but it was also lightweight. Therefore, it would likely promise the greatest payload gain.

This graphic shows the basic principle of the 'Airmat' design, complete with the exhaust venting through the inner wall for film cooling: http://www.alternatewars.com/BBOW/Space_Engines/J-2X_Airmat_DWG.png

The three concepts that made the grade for final analysis: http://www.alternatewars.com/BBOW/Space_Engines/J-2X_NozzleConcepts.png

Hope this is helpful.

Al.

Organic Marble
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Alastair Haslam
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    This is a very interesting answer. Some links to sources would make it a very interesting and authoritative answer! protip - telling people to search for images when you could embed them in the answer is bad form. – Organic Marble Jan 05 '17 at 03:08
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    Wow, that Airmat concept is interesting. I guess the extensible nozzle strategies, even if complex, would be far lighter than the longer interstage that would be needed for a large fixed nozzle. According to Alternate Wars, Airmat would go to 48:1 expansion ratio, (or 70:1 on the J-2S), with Isp in the 430s or 440s -- not quite the 100:1 and 450s I'm looking for. Do you know if they thought a 30% larger nozzle would work with the S-II engine configuration, or would they have had to redesign the stage? – Russell Borogove Jan 05 '17 at 03:29
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    (Organic Marble) - Sorry about that one, I'm still getting to grips with the forum. It's been very helpful to me, so I'll try to make sure I can return the favour to you guys in the future. – Alastair Haslam Jan 05 '17 at 03:45
  • (Russell Borogove) - I was shocked that they proposed a rocket nozzle composed of what may be loosely referred to as a wire mesh! And I suppose the S-II redesign for a larger nozzle would mostly concern the diameter of the engines rather than the length of the interstage. I understand that the Saturn V-3B proposed a shorter interstage due to its smaller toroidal engines, so I guess changing that component could have been more acceptable to a degree. – Alastair Haslam Jan 05 '17 at 03:50
  • As you can see here http://spacerockethistory.com/wp-content/uploads/2016/02/4-S-II_Inboard_1963.png the base of the stage clusters the engines fairly close together, leaving quite a bit of room between the outboard edges of the engine bells and the interstage. Seems like the extensions would collide when expanded, unless the engines were mounted further out. – Russell Borogove Jan 05 '17 at 04:01
  • Perhaps it was deemed okay to avoid implementing the J-2X extensions on the S-II. The greater nozzle area ratio seems to give a vacuum Isp of about 435 to 436 seconds, right around J-2S territory, but with a greater engine mass and size due to the added nozzle system. It could be that the J-2X was to be an S-IVB upgrade only, intended as a low-cost interim to simulate J-2S performance before finally changing both upper stages to the J-2S. – Alastair Haslam Jan 05 '17 at 04:27
  • @AlastairHaslam helpful reminder - put the @ symbol before the person's name instead of the parentheses you've used above. Then the person is sent an alert - a little red flag at the top of the page - so they know you've added a comment addressed to them. Also, can you edit your answer and add the links to this answer, here, and not just "do better in the future"? Answers here are considered 'living documents' and are often maintained and even improved over time. Thanks! Very nice answer by the way!! – uhoh Jan 05 '17 at 05:30
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    @uhoh all completed. I hope that clears it up. I'll get used to the proper etiquette soon enough. Thank you for the help! – Alastair Haslam Jan 05 '17 at 11:19
  • @AlastairHaslam no problem, we all were new once. Welcome to the site! – Organic Marble Jan 05 '17 at 13:50
  • @AlastairHaslam Thanks for the reference and images! I've edited your post to show the images inline. You just use the little picture icon in the frame of the editing box to do this. Great answer! – Organic Marble Jan 05 '17 at 13:54
  • @OrganicMarble that's excellent, thank you for your edit and the welcome! – Alastair Haslam Jan 05 '17 at 14:00
  • I'm still somewhat wondering if a 100:1, 760psi / 5260kPa J-2 could reach 450s, but this does answer my first question. Thanks! – Russell Borogove Jan 05 '17 at 16:18
  • @RussellBorogove I'm afraid the calculations would be beyond me, but I did find something that might answer this. http://www.astronautix.com/r/rl-10a-4-2.html gives details on the RL-10A-4-2, an RL-10 variant featuring an 84:1 expansion ratio and a 566psi chamber pressure - capable of 451 seconds vacuum Isp. Now, the RL-10 is not the J-2, but it does show it's possible to do so with less. – Alastair Haslam Jan 06 '17 at 01:57
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    @RussellBorogove PWR's newer J-2X program had an engine design featuring a 92:1 nozzle expansion ratio. Engine length increased from 3.38m to 4.70m, while diameter increased from 2.03m to 3.05m - too large for the S-II and S-IVB. Vacuum specific impulse was to be 448 seconds, but even with the massive area ratio it required a chamber pressure of 1,337psia. Therefore it seems unlikely that the 763psia J-2 could surpass 450 seconds, even with a 100:1 nozzle. http://alternatewars.com/BBOW/Space_Engines/Reference_Spacecraft_Engines.htm – Alastair Haslam Jan 11 '17 at 09:16