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More specifically, is it possible for rocket engines comparable to those of the mid 1960s to exist in a world where in atmosphere jet engines are only in their early stages of development?

This world is broadly closest to the 1960s in terms of most aerospace technologies & early 1950s in terms of computing. However, gas turbines & therefore jet engines are significantly behind what you'd expect, being closer to the level of what you'd be seeing shortly after the end of WW2, significantly behind what we had in the real world by the point rockets where being developed. Knowledge around other fields of aerospace is still comparable to that of the 1960s aside from poorer knowledge of high speed aviation materials. Although test flights similar to those of the X-15 & similar have been conducted & materials needed for those do exist they are not as well developed.

Would this lack of knowledge of in atmosphere jet engines cause problems for the development of liquid fuel & solid fuel rocket engines or could they still be developed despite a lack of knowledge of jet engines without also causing further development of jet engines to a significant extent?

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  • $\begingroup$ If they know so much about aerospace, how do they know very little about its predecessor? $\endgroup$
    – Martamo
    Commented Jun 2, 2023 at 10:16
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    $\begingroup$ @Martamo wdym? aerospace technology existed before jets, prop planes existed & still do exist. Aerospace just means aviation & space technology together as it's largely considered one industry. $\endgroup$
    – OT-64 SKOT
    Commented Jun 2, 2023 at 10:27
  • $\begingroup$ I am confusing myself(side-effect of staying up all night). You are asking if rocket engines are possible if the gas turbines and jet engines are subpar while aviation and space technology are 1950s to 60s level? $\endgroup$
    – Martamo
    Commented Jun 2, 2023 at 10:34
  • $\begingroup$ @Martamo pretty much $\endgroup$
    – OT-64 SKOT
    Commented Jun 2, 2023 at 11:31
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    $\begingroup$ "being closer to the level of what you'd be seeing shortly after the end of WW2, significantly behind what we had in the real world by the point rockets where being developed." The A-4 (V-2) missiles were literally developed before that, and they formed the basis of both USA and USSR ballistic missile programmes. It's important to note though that by the end of WW2, jet engine planes were also flying happily, so I think the question's premise might be incorrect. $\endgroup$
    – biziclop
    Commented Jun 2, 2023 at 16:23

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There are at least four significant problems with this scenario:

  • Gas turbines were thought of by the late nineteenth century, and were an obvious possibility for aircraft engines by the late 1920s. Steam turbines had been proven as prime movers for ships and electrical generators by 1910 and a lot of knowledge was carried over into gas turbines; the problems in building the first gas turbines were mostly with the compressor and combustion systems, rather than the turbine itself.
  • All the basic ideas of gas turbine engines had been invented or discovered by the end of WWII. There's nothing in a modern engine that Frank Whittle or Hans von Ohain would not have understood in 1945, although the size, precision of manufacture and quality of materials would have impressed them.
  • The turbopumps that large liquid-fuel rocket engines use to feed huge amounts of fuel and oxidiser into their combustion chambers at high pressure are gas turbines. They are significantly different from jet engines, but if turbine development in general is retarded, they may not exist.
  • Jet engines are so advantageous for large and/or fast aircraft, as compared with piston engines, that a lack of development effort on them is difficult to justify in an alternate history. A lack of effort on rocket engines is far more plausible, if the German V-2 project had not been successful.

If you want liquid-fuel rocket engines as capable as the F-1 used in the Saturn V while jet engines are stuck in the late 1940s, that isn't very plausible.

A lack of both jet engines and liquid-fuel rockets is more plausible, but running a space programme entirely on solid-fuel rockets is difficult. You can't throttle them, or turn them on and off readily.

Addendum: Pressure-fed rocket engines exist, usually pressurised by bottled gas. They have an unfortunate intrinsic limitation. The pressure in the fuel and oxidiser tanks has to be higher than the pressure in the combustion chamber, or they stop working. Making the fuel and oxidiser tanks and pipes strong enough to contain very high pressure adds lots of dead weight, which is deadly for rocket performance. So pressure-fed engines have very limited performance. There are jobs they are fine for, once you're in space, but getting there with them is hard.

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  • $\begingroup$ Exactly, while pressure-fed rocket engines are certainly a thing, and are used in many applications even today, they have a very hard upper limit in terms of overall thrust. $\endgroup$
    – biziclop
    Commented Jun 2, 2023 at 21:03
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    $\begingroup$ @biziclop: Thanks, added. $\endgroup$ Commented Jun 5, 2023 at 13:19
  • $\begingroup$ While heavier and less efficient than gas turbines, a piston engine could provide much more pressure than a gravity-fed rocket engine. It would be a middle-of-the-road approach. $\endgroup$
    – Rekesoft
    Commented Jun 6, 2023 at 10:27
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Yes, certainly. In our world rockets of one sort or another were in use literally centuries before the development of the jet engine, for fireworks and in warfare (e.g. the reference to "the rockets' red glare" in the US national anthem). Goddard experimented with modern liquid fuelled rockets in the 1920s. This was around the same time that the use of jet engines for airplanes was being investigated, but the developments were quite independent and one could easily imagine an alternate history in which jet engines languished while rockets advanced more quickly.

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    $\begingroup$ I think the key thing this answer is referring to, but never actually says is SOLID rockets, versus liquid rockets which are the ones more closely associated with jet engines. $\endgroup$
    – DKNguyen
    Commented Jun 2, 2023 at 20:50
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    $\begingroup$ @DKNguyen The V2 rocket used liquid fuel. Still predated jets (although only by a little). $\endgroup$ Commented Jun 3, 2023 at 0:54
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    $\begingroup$ In one sense, yes, Goddard's 1920s rockets were modern. But in another very important way they weren't: his engines were pressure-fed, while modern rockets use turbopump-fed engines, which provides much higher performance. $\endgroup$
    – Mark
    Commented Jun 3, 2023 at 3:13
  • $\begingroup$ @gomennathan The Messerschmitt 262 became operational in April 1944, the V-2 bombing of London started in September the same year. So technically jets predated it, but we're talking months. $\endgroup$
    – biziclop
    Commented Jun 3, 2023 at 10:58
  • $\begingroup$ @biziclop The V2 wasn't the first liquid fuel rocket. You're comparing a mature piece of equipment to a prototype. $\endgroup$ Commented Jun 3, 2023 at 20:48
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Yes, if something in your world hindered the development of flight. One obvious candidate is lower pressure - say, sea level is at Earth's 10km. That means early propellers are out, no brothers Wright, jet engines have to wait until the technology matures, there is no early military adoption and no breakneck development of flight during the WWI and WWII equivalents.

Hydrogen baloons still work, and are used for meteorology, but their lifting capacity sucks. No dirigibles.

But... the rockets work the same way as here. By the 1960s equivalent, first practical jet engines are being produces, with some military investment, because the possibility to carry a human operator is clearly there. But these are very early attempts.

While rocketry is well advanced, being heavily invested into and used in the wars, with V1 & V2 equivalents proved and mature. Note that on Earth, German rocket programme was way ahead of its time, with major personal contribution of von Braun. In this world, there is no von Braun, rocketry does not get this immense boost from a developed ultramilitary dictatorship, instead it continues "the usual" path of mistakes and setbacks. But it is still quite comparable to ours, just because some of the effort that went into bombers & fighters on Earth goes into rocketry. Just without von Braun and the Nazis.

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  • $\begingroup$ I like this. Change the planet physics just a little, and planes have a huge disadvantage. Biology might have some issues at that pressure, but it could work. (Or find some intermediate value.) Meanwhile, rockets get a little easier with less air drag and perhaps less gravity. (Which would explain the reduced pressure.) $\endgroup$
    – Brianorca
    Commented Jun 6, 2023 at 23:14
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Not likely.

The technology and metallurgy needed for both is very similar. So to have an alternate history where the one would develop and the other would not need some explanation.

The ISP of the propellant is the most important factor. It tells you how much velocity the rocket can build up while burning it propellants. Note that the due to the "tyranny of the rocket equation" small increases in this number lead to large increases in performance. The rocket equation is an exponential function.

Second, the stage of metallurgy must be relatively low and the incentive to build fast planes must not be there. Otherwise the same scientist who are developing rockets would have developed the jet engine. This requires a rocket engine that works with simple materials and works on a low pressure because pressures of 50 bar and above can only realistically be achieved with a turbo pump. Without a pump the big storage tanks need to handle this pressure and they would become to heavy.

Solid rockets are not the thing you're looking for. They are to heavy and don't reach the high velocities needed for orbit. Unless you use many stages which result in a very big rocket with a very small payload. And the rocket is difficult to throttle. With the best chemicals they have an ISP of around 285s. They can be used as a first stage, but not as a second or third.

There is a low tech liquid fuel that doesn't imply having a turbo pump. Hydrogen peroxide in combination with kerosene. Because it is not cryogenic the tanks can be pressurized to around 10 bar with normal compressed nitrogen. A low pressure rocket motor for the upper stage can be build with this. Made of copper and cooled with the fuel it burns. Because it uses liquids valves can be used. It doesn't have the performance of cryogenic fuels, but it comes close with an ISP of 319s in vacuum. It might be enough.

http://www.astronautix.com/h/h2o2kerosene.html

Still there is a large overlap in technologies. So a Manhattan project like zeal must be present to only develop a rocket and not a jet engine in the same time period.

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    $\begingroup$ You're exaggerating the disadvantages of solid fuel. The M-V (en.wikipedia.org/wiki/M-V) had a payload to LEO of a couple of tons. $\endgroup$
    – John Doty
    Commented Jun 3, 2023 at 12:59
  • $\begingroup$ @JohnDoty Maybe. An other example would be the Minotaur IV which has a better payload to launch mass ratio of 50. Both need 4 stages. And both are influenced by the need to launch ballistic missiles. The military needs solid rocket technology as per the wiki article on the M-V. A more conventional LOx/RP1 will easily reach this ratio with 2 stages. (And maybe a small orbital insertion stage) It is hard to find data about actual hydrogen peroxide rockets because after WWII cryogenic rockets became possible. It is very difficult to compare pre WWII liquid and solid rocket technology. $\endgroup$ Commented Jun 5, 2023 at 14:40
  • $\begingroup$ M-V only needed three stages to LEO. M-V had nothing to do with missiles: it was a development out of the Japanese Ministry of Education. $\endgroup$
    – John Doty
    Commented Jun 5, 2023 at 17:48
  • $\begingroup$ Yes, solid rocket motors are less efficient and need more stages than liquid fueled ones, but since we have many successful examples, we know it could be done. $\endgroup$
    – Eugene
    Commented Jun 5, 2023 at 18:18
  • $\begingroup$ Point is: the military always encourages the operational use of solid rockets for other purposes. So their use is not purely on their merits as an orbital platform. This is a general pattern. So it was no surprise to find a chapter on the wiki page for the M-V rocket where the military has put political pressure to keep the rocket operational while the space organizations deemed it unnecessary. Just want to note that this bias exist. $\endgroup$ Commented Jun 7, 2023 at 11:34
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What happened if rocketry had gone the way of the Big Dumb Booster? In the sixties, there was a feeling that the difficulties of 'Rocket Science' could be overcome by making a larger, simpler lift vehicle. The 'Sea Dragon' was a particularly massive proposal that would have been filled and launched while floating at sea. This approach was abandoned. The article goes into why.

Rocketry has been shaped by people such as Goddard, Von Braun, Korolev, and others. If they had been replaced by some charismatic and inventive designer of Big Dumb Boosters, then they may have got into space when people were trying to get into space at any price, and then have developed the design, and trimmed down the weight. Then making a sophisticated lightweight rocket might have been seen as the high risk option.

I can't say I am convinced. But maybe.

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  • $\begingroup$ You can sort-of see that now. While the Starship Super Heavy booster is certainly not the kind of "dumb" that was envisioned for the BDB, it still shares some similarities in that it is actually not very efficient. Critics have described it as "brute-forcing its way to space". But, of course, the Raptor engine is one of the most complex engines ever developed, so that definitely does not fit the BDB mold. The first Starship prototypes were literally welded together by a water tower company, outside, on a dirt parking lot in the middle of nowhere, TX. $\endgroup$ Commented Jun 4, 2023 at 10:53
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As the other answers have established, large liquid-fuel rockets, capable of space exploration rely on turbo-pumps. Pressure-fed rocket engines are useful in upper stage motors or other mid-to-low thrust applications but they can't produce the sheer brute force that the first stage of a large launch vehicle requires.

And if we have turbo-pumps capable of pumping the fuel for a large rocket, we must have jet engines too, as they face very similar technical challenges.

Frame challenge: the early success of pulsejet engines meant that there was little financial incentive to replace them with turbojets.

Pulsejet engines are much, much simpler beasts than turbojets so theoretically they could've been widely used in flight before turbines were developed to a level suitable for either aircraft propulsion or rocket fuel pumping. Specifically, pulse-jets can be built with no moving parts at all.

So in this scenario when pump-fed rockets come along, pulse-jet flight is already well-established and quite advanced. Thus there is much less to gain from a switch. You don't want to replace thousands of perfectly good aircraft just to put a slightly better turbojet engine on them.

Now there's a certain level of hand-waving needed here, as there are very good reasons you don't see pulsejet airliners flying above our Earth. Noise and vibration to name just two, not to mention that even the best designs are way less efficient than modern turbofan engines.

To compete with turbofans you'd need pulse detonation engines, and as one may guess from the name, they come with their own set of problems.

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Pressure-fed rocket engines exist; while less efficient than turbopump-driven engines, France's Diamant orbital satellite launcher used pressure-fed engines in the first stage. While there's an upper limit to the size of them, you could gang more and more of them together to get the necessary thrust for a large launcher. You wouldn't be able to get to the moon on them, in practice, but low Earth orbit would be reachable.

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Rockets are older than you think!

The first rockets were made in ancient China. The ca. 1900 invented hero of Wan Hu even is said to have designed a rocket chair to get to the stars... though technically he might have never existed. What most certainly did exist was the technology of gunpowder rockets and fireworks, invented during the Song dynasty around 1000 AD - about 950 years before the invention of the jet engine.

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Sure. We choose what we develop.

We could have focused our world creativity on developing harp-based music, but instead we focused on guitar-based music. It's an arbitrary choice.

At the end of WWII, rockets were looking better.

The Me262 was a desperate and very costly experiment whose engines were good for 1-2 flights. Fuel economy was terrible and had little prospect of improving. This is a problem, because - ask Ukraine - a slightly faster fighter that uses 10 times the fuel puts a heavy load on the logistics chain - and amateurs talk strategy, professionals talk logistics.

The V-2 was a competent production craft that was absolutely delivering the goods with good efficiency and obvious advancement prospects.

The speed of sound is not a barrier for rockets but in 1945 was a huge barrier for jets. Aerodynamic effects right at the speed of sound (Mach 0.99-1.01) are brutal, and this may be seen as impracticable to overcome.

A rocket will always be fast enough to catch a jet, but a jet will never be fast enough to catch a rocket. The military may conclude that no matter what they do with a jet, the enemy's rocket will always be able to catch it.

A central planner looking at both of them, and having to choose one for national commitment, wouldn't be unreasonable to choose rockets and call the jet a dead-end. That might mean that on into the 60s, they made Apollo-like investments in rockets, certainly reaching the F1 engine by 1965.

They may even make a treaty with would-be adversaries not to waste their mutual national industry on the pursuit of jets. Which would stop development cold, obviously.

Carve up our city for longer runways? No thanks.

Postwar, it was fashionable for cities to destroy tens of thousands of homes, gouging up whole towns in pursuit of longer, longer and longer runways at airports. All this to accommodate the new jetliners.

What if they didn't? What if towns with harbors or lakes were perfectly happy with flying boats? Which worked great, and were enormous as they had no structural limits like a land plane.

Well now the longest runway you can count on is 4000' or so. The high speeds of jetliners become impractical for such short runways, and water landings as well. That kills the commercial market, which means the military would need to pay all the freight for jet development.

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Yes

(Other answers have covered the same topics, but not together, so I'm summarizing and adding world parameters necessary to make it likely)

There are 2 viable ways to do it, Solid Rocket motors and pressure fed Big Dumb Boosters:

There are a ton of real world, solid rocket only launch vehicles: https://en.wikipedia.org/wiki/Comparison_of_solid-fuelled_orbital_launch_systems so we know for a fact it's possible. If your history has a faster adoption of rockets, e.g. something like Ammonium perchlorate could have been discovered and made in the 19th century, leading to rocket artillery gaining a lead over guns and Ballistic missiles being a thing before airplanes, or a world with lower gravity, but vicious storms, also leading to early, solid fueled ballistic missiles, but no planes. Then it's a small step to start adding stages to you ballistic missiles, until someone says: "Hey, I bet this could reach orbit!"

Using the BDB approach, orbit is also doable. The design of the Sea Dragon had no turbo-pumps, tank pressurization + the literal weight of the fuel in the tanks(roughly speaking, for every 10m of liquid depth, the pressure at the bottom increases by 1 atm, and the fuel tanks were ginormous. As fuel is drained, acceleration and thus gravity on the fuel increases, which balances out the lower hydrostatic pressure). However, unlike with SRMs, a viable path is waaay harder, since you can't start with small rockets and work your way up, you have to start with a skyscraper sized rocket.

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