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This is in regards to the Tyranny of Rocket Fuel. Summed up: Heavier ships need more fuel which makes them heavier, etc. You end up with diminishing returns. This is compounded by higher gravity - Everything is heavier in the first place. There's an article that claims that, beyond 1.5g, conventional rockets become effectively unusable.

But what if we take that the other direction? Lower gravity lowers the weight, which lowers the fuel required, etc. So, you get more "Efficient" rockets.

Now, to my question. Say we have a civilization, at the beginning of rocketry. They're largely identical to humans, except for one key difference: Their planet is smaller and has gravity that is 1/6 that of Earth.

Would that civilization's rocketry/spaceflight advance at a quicker pace than we have seen in our own history?

Things to be ignored:

  • Differences in the planet other than what would be caused by the gravity and size. Assume the same resources, oceans, atmospheric pressure at sea level, etc.
  • Differences between this hypothetical species and humans. Just assume they're humans for terms of mass, food requirements, etc.
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    $\begingroup$ It may be difficult to handwave all the differences between this planet and Earth. For example, 1/6 of Earth gravity would dictate that atmosphere would spread much wider, which, in turn, would dictate that stable orbit must be much higher too. $\endgroup$ – Alexander Jul 17 '18 at 6:40
  • $\begingroup$ You ask two very different questions here: In the title you ask if it leads to earlier spaceflight, in the post you ask if the civilization's spaceflight advances faster. You still need relatively similar technology to ours for the first flight to space, but advancement is much faster (with space elevators being a more valid option). $\endgroup$ – Infrisios Jul 17 '18 at 9:21
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    $\begingroup$ @Alexander "1/6 of Earth gravity would dictate that atmosphere would spread much wider," That doesn't necessarily follow. Cases in point: Venus, Earth, Mars, Titan. Atmospheric density does not scale with surface gravitational acceleration to any significant degree. "which, in turn, would dictate that stable orbit must be much higher too." Orbital radius is purely a function of the masses involved and the orbital velocity. A noticable atmosphere farther out might however affect what orbits are practical (see the ISS needing periodic boosts to maintain its ~400 km orbital altitude). $\endgroup$ – a CVn Jul 17 '18 at 12:47
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    $\begingroup$ In addition to my answer, take a look at this from Space.SE. This is quite interesting in the scope of your question. $\endgroup$ – CKA Jul 17 '18 at 13:45
  • $\begingroup$ @Michael Kjörling - could you substantiate why do you think atmospheric distribution is not affected by gravity? This link states: "The scale height of the atmosphere is about 10.8 km, which is higher than Earth’s (6 km) because the surface gravity of Mars is only about 38% of Earth’s, an effect offset by both the lower temperature and 50% higher average molecular weight of the atmosphere of Mars." $\endgroup$ – Alexander Jul 17 '18 at 16:54
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I'd say yes because:

As soon as the human scientific level reaches a point where a certain technology becomes usable governments, companies and indiviuals will begin to tinker with it.

Let's compare the automotive industry with the space industry. In 1886 Carl Benz patented his 'Benz Patent-Motorwagen' - the first car. He built it with little assistance and his wife was able to drive the third version through germany in 1888. Let's jump forward 50 years to 1936. The automobile is a common sight in our society. (Rich) individuals use it, the government uses it as well as companies. There are many competetive companies that produce them. Many big companies and brands we see today (Mercedes, Audi, Opel, Toyota e.g.) started as small businesses that grew due to their success.

In 1957 the Sovjet Union launched the first satellite. This was no individual task. Thousands of people and a whole megacountry were behind the development of the R-7. The USA quickly caught up and for a long time space exploration was a matter of this two countries. If we jump 50 years forward (2007) again we see a picture that's quite different from the car's 'success story'. Some big companies and government owned space agencies still do all the contracts. Even very rich individuals can't get to space, even if theoretically they'd have the money simply because of the little capacity of manned spaceships we have. The whole individual and small business tinkering level that made the automotive industry so fast growing is completely missing because you need a very big amount of money, material and workforce to even enter, let alone succeed in this business.

If you imagine you could make a rocket that could travel to space and back about as big as a truck, you'd need a lot less material and people. A small team could build one. You'd need weaker rocket motors with less fuel which would mean you could do it with less expensive materials and bigger quality tolerances in the parts you use.

So I think yes, easier access to space would mean that we would advance much faster in the technology we use.

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    $\begingroup$ I'm not sure your reasoning quite holds. Let's take a closer-to-Earth example: homebuilt airplanes, rather than homebuilt spacecraft. Yes, there are actually people out there who build their own airplanes from scratch. The number of such people is vanishingly small. Probably more people buy and assemble airplane kits. Even the number of such people is really tiny. Not only is an unpressurized airplane, let alone operating it, many orders of magnitude less complex than any kind of spacecraft; the risks involved are also far less dire. $\endgroup$ – a CVn Jul 17 '18 at 12:54
  • $\begingroup$ Surely few people build there plane from scratch but there are a lot of small-, middle- and large-sized planes as well as different sized companies. Otto Lilienthal, the Wright brothers as well as their fellow pioneers started small because it was possible that way. Also in fact an airplane is much more complex than you think. The reason why spacecraft are so expensive is that you've very little room for errors because a.)High forces are involved (acting against Earths gravity all the way up) b.)Sending replacements is very expensive or impossible because all launches have to beat gravity $\endgroup$ – CKA Jul 17 '18 at 13:20
  • $\begingroup$ "Also in fact an airplane is much more complex than you think." That depends on the airplane. A modern jetliner or fighter jet indeed is extremely complex, but that's at the upper end of the complexity range. At the other end you've got aircraft that don't even have electrical systems, let alone lots of fancy instrumentation. It's perfectly possible to fly a simple aircraft with little or no more instrumentation than in an ordinary car, in good weather and as long as everything goes well. The complexity is for when you depart (no pun intended) from those conditions. $\endgroup$ – a CVn Jul 17 '18 at 13:27
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    $\begingroup$ @MichaelKjörling I don't think so: we already have groups of hobbyist that are able to send missiles pretty high, make it way easier (like 1/6th of g and 1/10th of escape velocity) and you will have a lot more people doing it and putting thing in orbit. Moreover, give the simpler material and engine technology you need, you will probably have spaceflight earlier and a rampant space race colonization. $\endgroup$ – Gianluca Jul 17 '18 at 13:29
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    $\begingroup$ @jamesqf - every one of those individuals reached space in a vehicle that was originally designed and funded by a state sponsor. They didn't get there on their own. $\endgroup$ – Jules Jul 17 '18 at 18:30
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the simple answer is yes. It will be easier for rockets to achieve either orbital or escape velocity from the lower-gravity Earthlike planet.

By comparison with planet Earth with an escape velocity of 11.2 km/s the hypothetical planet will have an escape velocity of approximately 2 km/s. Chemical rockets have exhaust velocities of 4 km/s and this means rockets launched from Earth into space will have large mass ratios. Mass ratio is the ratio of the mass of fuel to the payload. That's a simplified version of the concept, but it should suffice.

On a planet where the escape velocity is actually lower than the exhaust velocity of a chemical rocket the mass ratios of space vehicles will be much lower again. This is due, mainly, to the compounding factor of burning fuel to lift more fuel that needs to be burned to achieve even higher velocities. Lower gravity will ensure not only much less fuel needs to be burned, but the additional amount to reach those even higher velocities, especially velocity above exhaust velocity itself.

The net result is smaller, also less dangerous and difficult, rockets can be launched into space with bigger payloads. Therefore, the technological and economic barriers for working space vehicles propelled by chemical rocket engines will be considerably less than on planet Earth.

In conclusion, yes spaceflight can be achieved earlier. provided other push factors are in play (but they're not technical factors involving rocketry).

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No, simply because there really isn't that much room to do things quicker than they actually were done. It takes a certain level of mechanical sophistication - accurate machine tools, materials, &c - to build a working rocket engine. That was reached in WWII, with the German V2 (1944).

Going from that V2 to manned space flight took only 17 years (1961), and from there it was only another 8 years to Apollo 11. So how do you get much faster than that?

After that, the reasons for the pace of space flight become as much political and economic as technical. Absent something like the US-USSR "Space Race", governments aren't willing to devote that much effort to it, and private entities don't see much potential for a quick return on investment. There just isn't all that much up there, really :-(

Now for the practical (communications & weather satellites) and scientific space missions, the cost of launching the spacecraft is only a small part of the cost. A launch might cost (roughly) 50-250 million. (All amounts US dollars.) Building a communications or weather satellite might cost from 250 million https://www.globalcomsatphone.com/hughesnet/satellite/costs.html to 11 billion for the 4 GOES-R weather satellites: https://arstechnica.com/science/2016/11/americas-new-super-expensive-weather-satellite-launches-saturday/ For a scientific mission, a billion dollars is relatively cheap, e.g. Cassini 3.26 billion, New Horizons 700 million, the Webb Telescope 8.8 billion and climbing....

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On a planet where gravity at surface level is 1.6m/s^2, launching yourself into space isn't the problem. Assuming that the planet's radius is about 1/10th and it's mass is about 0.16% of Earth's (which makes the planet slightly denser than Earth but makes the calculations easier), you can reach orbit at around 1km/s and escape from orbit at about 1.5km/s. You can realistically reach 1km/s with a black powder rocket, a technology that was invented circa the 12th century on Earth. With motivation (and it's likely that people would realise pretty quickly that what goes up often takes a very long time to come back down again, and therefore start attempting to send larger and larger objects up faster and faster, just to see what happens to them), it could well have been possible to launch a primitive orbital vehicle by, say, the 15th century.

The problem that a 15th century space explorer would have is that there's not very much atmosphere up there. Anything alive that they sent up would die. Building a pressure vessel that can withstand vacuum is actually a harder problem. Leonardo Da Vinci sketched a design for one at right about the right time, but like many of his ideas the materials were not available to actually make it. The earliest known actual pressure vessels were built in the 18th century, thanks to advances in metallurgy and manufacturing techniques.

So, without changing history of the developments of the various technologies you'd need, the earliest space exploration could realistically begin on a low-gravity Earth would be the 18th century. Significantly earlier than it actually did occur, but not as soon as you might expect at first glance.

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  • $\begingroup$ You seems to assume that one technology branch can advance way faster than any other, but I think this is wrong: as soon you being to send larger and faster object up, you need to work on the materials since you need them to stand the launch (and possibly the reentry) so you need to advance also this branch. And so on with any other branch. $\endgroup$ – Gianluca Jul 17 '18 at 13:06
  • $\begingroup$ @Gianluca: Why do you think other branches would advance significantly faster? Realistically, your black powder rocketeers send up a few rockets, discover that there's nothing much up there, and don't bother sending more until they develop tech that can use communication & weather satellites. $\endgroup$ – jamesqf Jul 18 '18 at 16:25
  • $\begingroup$ @jamesqf because you are building the rocket itself. Just to be able to send up a rocket you need a sum of technologies from other branches like navigation, sensors, materials and so on, otherwise how do you decide that there is nothing up there ? And once you have the technology, the next step is use it in some other way and make it better (if nothing else, to earn more money) $\endgroup$ – Gianluca Jul 19 '18 at 6:47
  • $\begingroup$ @Gianluca: But the answer is saying that you could build a rocket to reach space with 15th century tech. Now assuming you do that, and find what you find with 15th century tech (basically nothing :-)), how do you make money off it? For a parallel, consider Antarctica. It was possible to reach the continent using 18th century tech, but there weren't more than occasional visits until the 20th century. It wasn't until the mid-20th century that long-term bases became practical, and only because of tech developed for other reasons. And AFAIK they stil don't make money :-) $\endgroup$ – jamesqf Jul 19 '18 at 16:52
  • $\begingroup$ @jamesqf my point is that the 15th century tech would not be "our" 15th century tech if they are trying to build a rocket. As for the money, doing a parallel with our 15th century, well, I just have a material to build a better armor for your army. And since we are talking, I also have a way to find the enemy army well beyond their sight range and hit them well before they can hit you. Are you interested ? Joking aside, just to go to space and return back with a "nothing" as response, you need a tech more advanced than the one of our 15th century in more than the single field $\endgroup$ – Gianluca Jul 19 '18 at 21:50
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Yes, but not dramatically faster.

World with 1/6 would be similar to the Moon, where velocity to reach an orbit is 1.68 km/s and escape velocity is 2.4 km/s (for Earth, those numbers are 7.9 and 11.2 km/s respectively). If this is not a Moon-sized world, but a hypothetical low-density planet with the exact size as the Earth, those numbers will be 3.27 and 4.6 km/s (but average density comes up as fluffy 0.938 g/cm3).

As the result, launching satellites and even interplanetary missions would be be considerably easier. A simple one-stage rocket can do the job. For a Moon-world, its inhabitants can even use Jules Verne-style space cannons to send out probes. All in all, space can be reached with XIX century technology and even earlier if they are determined to do so.

However, the early probes can be only unmanned and not quite sophisticated ones. A number of advances in electricity, radio and material science will be needed to launch a man into space (and bring him or her back alive). Manned mission would have to wait until mid-XX level. However, some daredevils may try it earlier, and suborbital uncontrolled flights are still possible at XIX century tech level.

After space flight details are worked out (circa 1970 tech level in Earth), there would be a big potential for quick development unseen here on Earth. Sending probes to space would be cheap, it would not require an expensive state-sponsored program (though rocket science would not get much easier).

In addition, the concept of Spaceplane would be much more viable in low gravity. Their Space Shuttles may be able to launch horizontally, use wings to climb to stratosphere and then go higher into the orbit. Lower orbital speeds would make reentry easier as well.

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  • $\begingroup$ "For a Moon-world, its inhabitants can even use Jules Verne-style space cannons to send out probes" ... not significantly earlier than rockets to space became viable technology for us, actually. The most powerful cannon developed during WWII [*] had a muzzle velocity of around 1.5 km/s. Scaling that up to something that could fire a usefully sized orbital vehicle would have taken substantial effort, so I doubt it could have been done by any date much earlier than the 60s. $\endgroup$ – Jules Jul 17 '18 at 18:40
  • $\begingroup$ [*] -- it's perhaps ironic that this cannon was developed as response to the bombing of Peenemünde, the event that effectively stopped all experimentation with large scale rocket engines until after the end of the war, and that those rockets were being developed by Werner von Braun, who would eventually lead the development of the rockets that did finally put man into space. $\endgroup$ – Jules Jul 17 '18 at 18:44
  • $\begingroup$ @Jules 1918 "Paris Gun" had a muzzle velocity of 1,640 m/s, and I mean the cannon will be used to launch simple probes, never large satellites or manned missions. $\endgroup$ – Alexander Jul 17 '18 at 18:47
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I'd say yes, but less than you might expect. Rockets in one form or another have been around since at least the $13^{th}$ century in China but it took a long time for anyone to even think about using them beyond line of sight. The main thrust for developing rocketry in the modern era was political; first for their shock value in over the horizon warfare and later for the propaganda and ideological value of the space race. So while lower gravity will make pushing into space easier once the decision to go that road has been made that doesn't of a necessity mean that the decisions behind the space race would be made, earlier or at all. Further to that the other technologies that you need to go to space aren't any easier to come by because the gravity is lower so their development will still be a decisive factor in your progress.

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