Considering the Saturn 5 rocket first stage was 95% fuel; If the Earth was more massive and gravity say 10% greater, would a moon mission still be possible using rocket technology ?
Would a (presumably) denser atmosphere make other technologies more feasible, e.g Skylon ?
Assuming the earth radius remains unchanged, 10% greater earth mass/gravity means 10% greater kinetic energy required for escape velocity. KE = 0.5mV[sup]2[/sup], so 10% greater KE means 4.8% greater escape velocity.
If I’m using the rocket equation correctly, then 4.8% greater escape velocity (for a rocket that’s 95% fuel) means the rocket needs to be 16% bigger. That assumes 16% more fuel mass, and 16% more mass in the rocket structure itself. Seems possible, but just barely.
There is no condition under which chemical rockets become completely impossible. For any set of parameters, you can always calculate some mass ratio which will work. Of course, at some fuzzy point it becomes impractical, but then, one could argue that the Earth is already well above that point.
For “impractical,” check out this XKCD “What If” scenario on getting to earth orbit using model rocket engines.
I suspect that the density of the atmosphere on an Earth-like planet need not be directly proportionate to the gravity of the planet. Venus and Titan both have denser atmospheres than Earth but have lower gravity. It is more likely that the atmospheric density of a planet is dependent on the amount of volatiles that were incorporated into the planet’s mass during formation, and also the amount of volatiles that were lost in the same period - this could depend very strongly on the late history of the formation of the world, and subsequent bombardment by comets.
Most Earth-like planets (if there are any) might have atmospheres that are roughly proportionate to the strength of the gravity- but we don’t know all the variables involved, or if Earth’s atmosphere is unusually thin or dense for a planet this big.
It is a little more complicated than that because higher gravity also means higher “gravity drag” losses (the impulse that is applied to get the launch vehicle up to altitude and until it reaches orbital speed), but your estimate is a good starting point. The specifics would require more details on the launch vehicle configuration—while the Saturn V was powerful, it was by no means mass optimized—but it would certainly require a better mass ratio.
There is a point at which the improvement of mass ratio for any given configuration by scaling results in negligible performance increase. However, you can always add more stages or side boosters to reduce the carried inert weight for upper stages and then scale the vehicle to readjust the mass ratio at any particular point. There is nothing impossible about a seven or eight stage except all of the interstage mass and propulsion systems that don’t provide any function at lower altitude have to be compensated by higher thrust and more propellant in the downstages.
The point at which chemical rockets become infeasible is if the exhaust pressure is less than ambient pressure or if orbital speed for the body is greater than can be achieved for the amount of total impulse available for even a unity mass ratio, and of course if gravity is such that the vehicle cannot even lift the weight of the propellant. Performance of chemical rockets powered by internal combustion are ultimately limited by the temperature of the combustion process and molecular mass of the products regardless of material limits or nozzle configuration.
The Skylon isn’t feasible regardless of the density of the atmosphere. Setting aside of how to make an engine that works from subsonic through scramjet and then pure rocket propulsion, the amount of propellant mass that has to be carried and the associated mass of the airframe which has to be aerodynamic in some form (and thus, cannot be optimized for mass ratio) which only provides a benefit at the thicker layers of atmosphere and then becomes inert mass that has to be carried to orbit and protected from heating on reentry mades it a non-starter despite the cartoon portrayals of it. Space launch vehicles are not airplanes, and it makes no more sense to try to make a rocket shaped like an airplane than it would a submarine which looks like a locomotive.
The only way a Skylon-type concept works is if you have a much more efficient propulsion system than chemical propellants can provide or if you can somehow contrive to make the lifting structures reconfigurable in flight and of almost negligible mass, thus optimizing pressure lift performance at transonic speed, supersonic shock riding lift at supersonic speed, and minimal exposed surface heating area upon reentry with no significant penalty for all of this complexity. There have been proposals for inflatable wing structures, but none that used real world materials or accounted for the design and operational complexities of such structures.
Stranger
There are certainly Earth-sized rocky worlds with the zone around K, G, and F type stars which would permit liquid surface water, and they are probably even relatively common. Whether they have any other conditions that specifically favor the development of life is still in question, but the only reason Earth might seem to be fairly unusual is our large, tidally locked moon.
Earth’s atmosphere is thought to be the composition it is because of the development of life, which naturally captured and sequestered atmospheric carbon greenhouse gases (primarily carbon dioxide and methane) while leaving the largely transparent diatomic nitrogen free and then adding diatomic oxygen as a waste product. Without that, Earth’s atmosphere would likely be as dense and oppressive as that of Venus. (It was once thought that Earth’s Moon was responsible for stripping away the heavier constituants but that has been largely discounted as a significant contributor.)
The thick atmosphere of Titan is a puzzler for planetary scientists. The moon should be too small to retain such a thick atmosphere even if internal geological processes were producing some reasonable amount of carbonaceous gases, especially since it lacks a magnetosphere of its own. The atmosphere is predominately diatomic nitrogen and appears to be the result of material absorbed either from impacts with transneutonian objects or from some unknown outgassing phase in Saturn’s lifecycle, and the methane component shouldn’t exist in any significant quantity without some mechanism which actively produces it. While there could be some natural mechanism we don’t understand, it is also plausible that methane is the waste product of some acetylene or other alkyne powered self-organizing process. It is far too early to make any predictions about whether such a process is indicative of non-terrestrial life but it is certainly a viable hypothesis.
You don’t mention Mars but by all accepted models of planetary formation Mars should have a much thicker atmosphere than it does today. The leading theory is that the lack of a magnetosphere allowed the charged particles from a much more energetic Sun earlier in its evolution to strip away gases leaving only a tenuous (but thick enough to be problematic) atmosphere of mostly carbon dioxide but a curious issue is the lack of any atmospheric nitrogen and nitrogeneous compounds which have been found only in trace amounts in soil to date. It is possible (and indeed, likely) that some mechanism has locked the nitrogen that had to be available on the primodial atmosphere of Mars to deeper within its strata and left the atmosphere thin enough to be blown away when its dynamo slowed down, but all of this is speculative without more information about the evolution of Mars that will only come with more exploration which examines exposed strata.
Stranger
Last I heard, the prevailing hypothesis for Earth’s thin atmosphere blamed it not on the Moon itself, but on the collision which formed the Moon. Is that what you were referring to?
Yes; the exact mechanism of this collision and the subsequent history of the volatile fraction is not known in detail. I think that if the Moon-forming collision were even slightly different then the atmosphere of Earth might have been quite different in quality. If the planetesimal that hit the Earth had been larger, or travelling at a greater relative speed, the volatile fraction of the planet would probably be dispersed more efficiently, and we would have a drier Earth with a thinner atmosphere; a smaller impact, and the reverse would apply.
Add to this the uncertainties over the volatiles brought to Earth by comets, and we can see that there are many ways in which our planet could be different.
There was an old hypothesis, popular in the ‘Sixties, that the Moon was responsible for stripping away most of Earth’s atmosphere leaving only the comparatively lightweight nitrogen and oxygen. This seemed to be largely based on the fact that Earth has a large moon and a light atmosphere, and Venus has no moon and a thick atmosphere. You can see this otion referenced in a number of science fiction works of the era and most notably that of Larry Niven. However, that hypothesis has been debunked by simulations which show that even when the Moon was much closer to the Earth it would have a minimal influence on the outgassing of heavy molecular species.
While the Lunar formation by the impact of a seperate body (dubbed Theia) with a protp-Earth is widely accepted, the idea that this caused the carbon species to boil off is questionable at best. The impact had to occur relatively in Earth’s evolution before the outer core and mantle were fully differentiated because of the distribution of elements found in Lunar samples and before the atmosphere was really formed. When and how the Earth’s atmosphere was formed is a pretty open question that we may never have a definitive answer to but one of the leading theories is that much of the water, oxygen, and nitrogen came from comets and meteors containing carbonaceous chondrites after the formation of the Earth-Moon system. If the surface of the Earth was still hot hydrocarbon gases should have formed and been suspended in the atmosphere while free oxygen would be rare, which would provide a rich primordial environment for developing the precursors to life.
All life on Earth tends to sequester carbon by nature becuase it is the primary structural backbone of cells, and we now know that much of the sedimentary rock that makes up the lithosphere is sequestered in organic remains of formerly living matter, which explains why Earth’s atmosphere is relatively free of carbon oxides and hydrocarbon gases while the dead Venus is subjected to a runaway greenhouse effect. What we don’t know is why Earth evolved carbon-sequestering life and Venus apparently did not.
Titan remains an enigma becuase not only is it to small to maintain an atmosphere by convention models but it is also the only object in the Solar system of its size to do so, which indicates current models of rocky planet and large moon formation and evolution are still significantly incomplete.
Stranger
A recent study suggests that the Earth was already oceany before Theia.
Scott Manley simulates an orbital rocket for five-times-Earth gravity (Youtube video)
Skip to 52:37 to see what his rocket looks like.
Bear in mind, too, that NASA had two basic plans for a lunar mission. Earth orbit rendezvous* EOR* and Lunar orbit rendezvous LOR (the plan eventually adopted). EOR involves assembling the lunar rocket in Earth orbit from launches of rockets about half the size of the Saturn V and might be more amenable to the proposed massive Earth situation.
Like most New Scientist articles that one doesn’t really accurately represent the source paper, and in particular neglects to address the basic premise that the research comes to stated conclusion by changing the statistical methodology in assessing ratios of oxygen isotopes in samples of lunar rocks. This is hardly conclusive even setting aside other mechanisms for variations in isotope ratios.
I could barely stand to listen to more than thirty seconds of that video and the inane commentary, but while that vehicle might fly in Kerbal it isn’t remotely feasible in reality; differential thrust and modal vibrations would tear that vehicle apart long before it even left the pad.
Stranger
Would an Orion style nuclear pulse system be able to overcome almost any gravity well?
Orion depended on a huge mass to absorb the shock wave as well as material for ablation.
The Starship & the Canoe by Kenneth Brower covers a lot of the challenges with that project.