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The converse of this was kind of an open problem in the early days of rocketry. Given the theoretical rocket concept, was there a propellant combination with sufficient exhaust velocity to make an orbital rocket practical? The answer was not immediately obvious, and there's a Goddard paper where he talks about just how big the rocket has to grow as you lower the propellant velocity to get equivalent performance. Eventually you're burning entire mountains of gunpowder just to get a few dozen miles up.

It was a nice surprise (and a relief) to the early rocket pioneers to realize that we lived on a planet where gravity and chemistry would make orbital rockets possible. The rest was just engineering.

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For anyone interested in the history of rocket propellants, I highly recommend "Ignition!" by John D. Clark[0]. It has plenty of chemistry if you're into that, but even if you're not (like me) it's an enjoyable read.

[0] https://library.sciencemadness.org/library/books/ignition.pd...

Seconded! That is such a wonderful book.

Just one of dozens of amazing passages in this book (page 48):

> "... its density was a little better than that of the other acid, and it was magnificently hypergolic with many fuels. (I used to take advantage of this property when somebody came into my lab looking for a job. At an inconspicuous signal, one of my henchmen would drop the finger of an old rubber glove into a flask containing about 100 cc of mixed acid -and then stand back. The rubber would swell and squirm for a moment, and then a magnificent rocket-like jet of flame would rise from the flask, with appropriate hissing noises. I could usually tell from the candidate's demeanor whether he had the sort of nervous system desirable in a propellant chemist.)"

I think it's a bimodal distribution. On the one hand you have the unflappable who just calmly watch what happens. On the other you have the far too flappable who is already out of the lab and making good time out of the building and the state.

Exactly this. Either they have the right stuff, or they don't.

Both are the right stuff.

I meant what was shown in this movie [0]. In a nutshell, the ability to remain calm when the unexpected happens, to try to solve the problem, or at least to not make it worse.

[0] https://en.m.wikipedia.org/wiki/The_Right_Stuff_(film)

Although for some reactions, the latter is the appropriate reaction.

Ah so this is where the cartoon trope of jets of boiling fluid speeding up from a lab flask came from. I thought artists were exaggerating, but I guess not this time.

As a former chemist, I thought this book was a great example of "applied chemistry".

The theoretical aspects are challenging enough. But then you realize just how difficult the practical application of the theory can be. Sure, a mixture of fuming nitric acid and hydrazine will produce enough propulsion, but how do you dump tons of it into an engine without it just exploding?

The section on building high-precision detonation speed timing apparatuses (and occasional explosive deconstruction of same) made me realize how uncomfortably close "information we require" and "catastrophic consequences of collection that information" are in the field.

https://youtu.be/K0FLy2nI13E turbopump https://en.wikipedia.org/wiki/Turbopump

There are few things that get certain types of chemists and engineers excited like being able to find out - and not being in trouble with ‘the bosses’ if it explodes a few times along the way.

There is an excellent YouTube channel that explains the V2 rocket in detail. Almost down to the last screw. Highly recommend. https://youtube.com/@RocketPlanet?si=DdgyQ8HFnswrZgZr

His videos about the Steam Pot [0] and the mechanical system that scheduled each stage of the rocket [1] are extremely good:

[0] https://www.youtube.com/watch?v=5C9xipCTe8I

[1] https://www.youtube.com/watch?v=lAFMl5bkP5Q

There was also a quote about chlorine trifluoride in Ignition!

”It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.”

https://www.science.org/content/blog-post/sand-won-t-save-yo...

Wouldn’t it be fascinating if there were an advanced civilization on a planet with gravity that was much higher than earth that couldn’t build chemical rockets and was therefore forced to build nuclear rockets?

What if that actually made the exploration of their solar system easier, since once they left the gravity well of their planet getting to other planets with nuclear rockets was comparatively trivial?

These things are fun to imagine, but the real fun gets to be when you start talking about all the downstream effects. For example, if you can't build rockets you can't build GPS. Building a global communication system is much harder, which means things like shipping and flying are much more difficult. Not to mention that the gravity is much higher in the first place so flying is going to require way more fuel so how long does it take for them to get to that stage of civilization and how does their technological path differ? It gets even trickier once you start thinking about how the atmospheric composition will be different as gases follow similar escape velocities (e.g. Earth loses 3kg H/s but only 50g He/s) and it also determines what can even stay aloft. In general much of the technological paths are fairly straight forward, always iterating off of the current state (leaps and bounds are not common as they're more often a lack of domain expertise or not properly contextualized around the historical knowledge). But I think people forget how connected a lot of these things are. Then again, people often question why it is important that we build rockets, while asking those questions on their handheld computer connected to a global communication network. It's quite incredible how complex these interaction chains actually are and I think make you only admire the beautify of it all that much more.

Which actually leads me to thinking that a space-adapted race really doesn't want to bother with planets and their big ass gravity well.

Resource extraction from asteroids or moons is a lot easier than carting it out of a big gravity well. Building stations in zero G rather than having to worry about orbital degradation and the like. Atmospheres get in the way of solar energy collection.

Earth is probably only useful as a vacation destination. Unless of course all those UFO reports are actual physics-defying antigrav drives with little green men.

The nice thing about gravity wells is they naturally concentrate things along density gradients.

The bad thing about gravity wells is they naturally concentrate things along density gradients.

Interestingly, development of rockets has only made a bunch of the things you mention cheaper (to the multiple orders of magnitude), not impossible.

Eg. determining location through radio signal triangulation can tell you a location pretty well, but would require placing a lot of signal stations throughout the world. Eg. remember the time-synchronisation mechanisms for watches through AM signals (including in hand watches).

Similarly, we did build a global communications network by placing expensive undersea cables across the world, but systems like StarLink are much cheaper (once you get to economies of scale for launching satellites).

So, like many things, rockets have accellerated discovery and progress, but are ultimately not the be-all solution: they work in tandem with the rest of science and engineering (including cultural development).

GPS done via land radio systems would be so flakey and expensive it would still likely not be implemented. Easy to jam too. And subject to control by terrestrial authorities.

Putting a dozen satellites in orbit - and out of reach of local authorities - is so much cheaper and more reliable, it’s not just a matter of cost - it’s an entirely different product.

Same with starlink. A big part of its advantage is someone can’t just walk over and cut a cable. And no one needs planning approval to put a cable in.

Line of sight to low orbit is about the only way to accomplish that - maybe some kind of high altitude ballon/plane could (loon?) but they’re so comparatively easy to shoot down that it makes it a very different kind of situation.

It seems like you are looking at it only from one side.

Undersea cables are probably more expensive than satellites today, but we'll still continue to put them in. And nope, someone can't just walk in and cut a cable sitting at 5000m under the surface.

Detecting a StarLink terminal is relatively easy from the ground, and someone can just walk in and demolish it once they locate it.

Basically, all tech has pros and cons.

Of course. But to Russias recent chagrin, blowing up/severing a fixed and very expensive cable (or underwater pipeline) is a generally far easier proposition than tracking down mobile and intermittent sources on the ground.

Still possible. But orders of magnitude harder. Nothing says that starlink ground station needs to stay in one place, after all.

Undersea cables get cut all the time, from shipping to nation states.

Trains don’t make cars obsolete, anymore than cars make trains obsolete. Taking out train tracks is much easier and more effective than taking out all possible roads though.

> Undersea cables are probably more expensive than satellites today, but we'll still continue to put them in. And nope, someone can't just walk in and cut a cable sitting at 5000m under the surface.

The cable has to come out on to the surface somewhere, though.

But barring that, dropping IEDs from a fishing boat, with a time fuse and some weight, isn't hard; the trickier part would be knowing where to drop them so they land near the cable. But there are tricks for that too.

Dragging an anchor seems to work pretty well.

D'oh. Of course!

All too easy to get bit by the curse of overcomplicating things.

You might be surprised to learn that Enhanced LORAN recently became operational around the UK's coast, specifically because satellite-based PNT is so susceptible to interference and jamming.

Not at all. It's only being installed in specific, high value areas within a specific jurisdiction. And mainly as a backup. Notably by a party which doesn't control GPS (albeit a close ally).

LORAN has also been used near airports in developed areas for a long time.

That isn't the 'base case' though.

The US military initially developed and launched GPS because of the reasons I stated, and it is still widely used as a base case for exactly those reasons.

Yeah and to add to this I think people are forgetting why Bell was given a sanctioned monopoly. Because there were just cables everywhere. When people say natural monopolies exist in markets with network effects, that can mean literal networks of cables that will block out the sun. Sure, this stuff will improve too, but I think people are also forgetting about the increased surface area and the increased gravity which makes each of these cables required to be thicker or require more support.

https://history.stackexchange.com/questions/58872/did-they-r...

> Interestingly, development of rockets has only made a bunch of the things you mention cheaper (to the multiple orders of magnitude), not impossible.

With finite time, lifespan and resources, "cheap" is often equivalent to "possible". If you look at the connections between inventions and developments that GP mentioned, it's usually the case that the necessary prerequisites don't involve just knowledge, but something getting cheap enough to be available / worth building.

If something is worthwhile, it gets developed and gradually becomes cheaper.

If you were handed an internal combustion engine car without petrol, it would be extremely expensive to get that fuel for the first time.

That even happened with satellites!

Interesting thought. I think ground-based GPS wouldn't be too difficult though - we already have most of earth covered by GSM/3G/LTE, and with updated towers you could have something as precise, if not more, as GPS. Of course the coverage wouldn't be 100%, and navigating in ocean would be more difficult.

Planes would be replaced by trains and aquaplanes for sure. Our modern fastest trains (TGV, Maglev) are only half as slow as the fastest commercial planes. Also, you might have rocketry on such a planet, just not for orbit, and for things that right now we use jets for.

The biggest issue with be probably no detailed aerial maps, and in later stages - no space mining, so such civilisation would be limited to resources on their own planet.

Also, I'm imagining that such a civilisation would send out more signals into space to encourage someone to come and visit them, and hopefully dropship resources from orbit :D

Imagine two civilisations living like that in symbiosis - one on the orbit, able to drop things to the one that is lower, but being able to extract only information / art / mental labour / energy from below.

Higher gravity -> denser atmosphere -> airships start making more sense? In those conditions, flying could've been actually cheaper and safer, even though slower than what we have on our planet.

If you could get to space with a fusion engine, then why would taking satellites into space on said rocket be any different then it is for us (to build GPS)?

Wouldn't LTA blimps work BETTER in higher gravity for flying?

A limit often ignored for high-gravity balloons is, the pressure gradient inside the balloon reaches a point where it tears it apart. When the atmospheric gradient is compressed to some point, the distance between the bottom of the balloon and the top can create enormous forces on the fabric.

So depending on the gravity we're talking about, blimps are out!

All projectiles become much shorter range weapons. Maybe once they've got gunpowder they can finally fight at range, though each shot would require a lot more gunpowder relative to the same shot on earth. Maybe it sort of washes out if you figure the inhabitants are all stronger and more sturdy as a result of the gravity.

Such an effect may even highly discourage ranged combat in the first place. I'm sure you'd still have ballista but bows? Probably crossbows. But there's definitely a butterfly effect for sure.

It feels like we've finally found a reason for the old sci-fi trope of future battles using melee weapons

I'm wondering if siege weapons work at all

Dune has a take on this that the movie doesn't do a great job at explaining. But the shield is made to prevent high velocity projectiles and energy weapons. This way the shield can be hidden and fool enemies because the user is still able to interact with daily objects. Like you can eat while wearing the shield and since you'd have body guards it's much harder to get in and stab someone wearing a shield. The Apple version of Foundation has a similar shield but it is more sensitive. Dune's shield has obviously affected many subsequent works.

That civilization could have invested in rail transit and tunneling instead. Positioning isn’t so hard on fixed roads, although fixing the, in the first place under oceans could be a problem. They might figure out triangulation using their planet’s magnetic field or something. It’s also completely possible that life isn’t viable at all on non-Earth like planets.

Remember that the gravity is higher. Your mountains are more dense and your oceans have more pressure. You're not living on a world with 14psi. You're not living in a world with the same ground, air, or ocean composition. All these change. So all your drills have to be thicker and harder. All your cables need to be stronger.

> once they left the gravity well of their planet getting to other planets with nuclear rockets was comparatively trivial?

We are actually on that planet. Spacecraft have what is called delta-v, which is basically a measure of what orbit changes they can perform given the amount of fuel they have onboard. For example getting from the ground to LEO has one measure, and getting from LEO to moon orbit has another.

It varies somewhat by the specific rocket to get into space (due to drag and effects of higher gravity), but once you are there it's basically the same for all spaceships.

It takes around 9.6km/s (no relation to gravity, just a coincidence) of delta-v to get into LEO, however once you are there it's fairly cheap to get around the solar system. To get from Earth LEO to a captured orbit around Mars needs a delta-v of around 5km/s - yes, less than to get into Earth orbit. To get out further to Neptune would need around 12km/s of delta-v.

Oddly enough nuclear rockets aren't particularly powerful and tend to be extremely heavy. Their strength is that they're highly efficient, so they can just keep going with relatively little fuel. Chemical rockets, by contrast, tend to be extremely high power but also extremely inefficient. Here's a few comparisons:

----

NERVA [1] / Nuclear / 1969 / 246kN thrust / 18,000 kg mass, 841s ISP (seconds of specific impulse - higher is better/more efficient, a little is a lot) / The only completed possibly launch viable nuclear rocket engine, as far as I know.

F-1 [2] / Chemical / 1959 / 7,770kN thrust, 8,400 kg mass, 263s ISP / Powered the Apollo rockets

Merlin [3] / Chemical / 2007 / 981kN thrust, 470 kg mass, 282s ISP / Powers the SpaceX Falcon 9 in a group of 9

Raptor [4] / Chemical / ?? / 2,640kN thrust, 1,600 kg mass / 327s ISP / Powers the SpaceX Starship in a group of 33

----

So what really matters in a rocket, for getting off Earth, is its thrust to weight ratio. NERVA isn't inefficient because it's dated (which was part of the reason I included the F-1), but simply because nuclear itself has an inherently poor thrust to weight ratio. However it just keeps going and going and going, which makes it absolutely awesome for travel once you're already in space.

It's even "fast" in space, because of how travel in space works. You don't just keep thrusting in space; instead you make a limited burn and then coast to where you're going, making a final reversal burn towards the end. So even if it takes hundreds of times as as long to reach a higher cruising velocity, it'll end up getting to the destination long before a chemical rocket, for any sufficiently distant destination.

----

[1] - https://en.wikipedia.org/wiki/NERVA

[2] - https://en.wikipedia.org/wiki/Rocketdyne_F-1

[3] - https://en.wikipedia.org/wiki/SpaceX_Merlin

[4] - https://en.wikipedia.org/wiki/SpaceX_Starship

> It's even "fast" in space, because of how travel in space works. You don't just keep thrusting in space; instead you make a limited burn and then coast to where you're going, making a final reversal burn towards the end.

The dream of course is that you keep thrusting, accelerating until the halfway point, then flip around and burn to decelerate. In that scheme, your thrust doubles as artificial gravity too!

> we lived on a planet where gravity and chemistry would make orbital rockets possible

It's kind of insane luck. Bit heavier planet and we wouldn't be able to have a single satellite before building nuclear engines.

I'm not sure how nuclear engines would help?

A nuclear reactor is a bit like an ion drive: great for long distance space travel, but not great for getting off a planet.

Unless you mean the kind of nuclear engine that consists of detonating atomic bombs behind you? See https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

Thrust to weight of a nuclear engine is fairly poor, so they are best suited for upper stage or in-space work. A heavy-planet rocket might use chemical propulsion in a lower stage just like we do and a high-energy nuclear upper stage (or two) where the really high Isp would be quite useful.

Maybe something like NERVA

For non-manned launches and those that can be hardened to withstand extraordinary g-force, something akin to setup that resulted in the missing (900 kg) borehole cap of Operation Plumbbob may do the trick.

Acceleration to 66 km/s is probably a little bit overkill, even.

https://en.m.wikipedia.org/wiki/Operation_Plumbbob

Orion drive should work fine.

> A nuclear reactor is a bit like an ion drive: great for long distance space travel, but not great for getting off a planet.

What are you basing this on? NERVA was for getting off the planet. It had a thrust of ~250 kN. In comparison, a SpaceX Merlin engine has a thrust of ~900 kN, while ion drives have <1 N of thrust.

High fixed (non propellant) drive weight compared to chemical rockets makes it pretty inefficient due to the gravity well - thrust/weight ratio vs time matters a lot when you’re quickly climbing out of the well. And it is very difficult to do that quickly with nuclear without exceeding our materials science abilities and causing a nuclear accident.

  Additionally, atmospheric density and friction matter a lot in these situations, and getting out of high density atmosphere and ‘up’ as quickly as possible pays large dividends.

Once you’re in a very low friction environment and ideally already moving near orbital or extra orbital velocities, taking your time is all good, and maximum end-to-end efficiency and power density matters more - you can have as much time as you want.

If you really cared about getting off the rock and not so much about containing the radiation you could build some VERY thrusty nuclear rockets.

Project Orion being perhaps the logical conclusion of that line of thought.

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