I got a question via email:
I have often heard of the difficulty of getting mass to orbit. Earth’s atmosphere and gravity are on the edge of being too much for chemical rockets. Unfortunately I have not found any discussions that analyze modified case scenarios such as “What if Earth had a thicker atmosphere?” or “What if the atmosphere was roughly equivalent but the gravity was 10% greater?” Would these be game stoppers for chemical rockets?” If we had evolved on Venus what method would be best for getting to orbit? Ultimately, are we in a sweet spot as far as our planet is concerned, too big to loose the atmosphere but not to big to be stuck?
It’s a misconception that it’s too hard to get off the planet with chemical rockets. Earth’s gravity is bad for single-stage, but as long as you’re willing to stage, it’s not that big a deal. What makes it expensive is the low activity rate, not the intrinsic capabilities of chemical propulsion. Ignoring the fact that it would have been very unlikely that we would have evolved on Venus, the best way might be a hot “air” balloon to the top of the atmosphere, and then take off from there. Designing a propulsion system that would work in that atmosphere would be no fun. Commenters may have other thoughts.
Chemical rockets are limited by the rocket equations, which in its essence reduces to the ratio of the orbital velocity to the thermal velocity of reaction products of chemical reactions.
That the orbital velocity is large relative to the “tails of the distribution” of the thermal velocities of gases in the upper atmosphere is part of how we have an atmosphere on earth. Other parts are the magnetosphere offering shielding from charged particles stripping off the atmosphere, perhaps the role of the biosphere is controlling the composition, and other factors.
But yes, to a rough approximation, that we have at atmosphere means it is “hard” to achieve orbit with chemical rockets. The Moon, where it is “easy” to achieve orbit and even with a single stage, lacks an atmosphere. And yes, having orbital velocity some multiple of the nozzle gas velocity makes achieving orbit “hard.” Multiple stage is “hard” in terms of the technical aspects of safely staging to all of the extra hardware. Achieving orbit with chemical rockets is “hard” in that even with staging, you need a high mass fraction and you need rocket engines with high specific output relative to their mass, the “controlled explosions” that people talk about. That is hard because one is always operating close to design margins for the thing just dissassembling itself in a conflagration (blowing up, to lay people).
That “easy for chemical rockets” worlds are airless, that worlds with atmospheres are “hard for chemical rockets” is not just a theoretical concern, it is a choice available for exploiting the resources of space.
One resource of space is uninterrupted solar power — the solar power space power station. One thing that makes this power a pipe dream is the cost of launching the power station, even if one manages to “travel down the cost curve” of mass-produced space launch, using ELV’s or RLV’s.
One proposed solution has been to make the power station from minerals and other resources mined from the moon. The lower “gravity well” of the moon makes it much cheaper (in principle) to use Moon material to make the power station than Earth material — maybe one could even used the proposed “mass driver” to fling Moon rocks into space and do the smelting “up there.”
But the Moon is airless, making doing anything on the Moon much more difficult than on Earth, including mining. So there is an engineering tradeoff. Yes, maybe after taking into account the tradeoffs, it is still cheaper to mine Earth materials and launch them from the steeper gravity well than to manufacture a solar power space power station from Moon materials, either robotically or with workers in space suits or pressurized habitats. Maybe it is cheaper because the cost of space launch is overstated based on experience with the small volumes. But the consideration of Moon vs Earth for exploiting the “space resource” of uninterrupted solar power is not an idle on.
I disagree. Airless bodies are much harder to build spacefaring cultures around, because the delta-v requirements are doubled.
Yes, if we had no air on Earth we could get to orbit with 7700mps deltav rather than 9000. But we would then require another 7700mps deltav to land, rather than the 100mps or so required to intersect the atmosphere. Airless round trip deltav is 15400mps. I submit that if you aren’t doing a round trip you aren’t doing anything that interesting.
I agree with Rand – orbit isn’t really that hard, we just haven’t been trying that hard due to low demand. The success rate of orbital vehicle design is not really that much different than air vehicle design – if you keep working at it, you get there.
I further submit that it is not gravity that is the problem, it is the inconvenient spheroid shape of the planet. If the planet was tube shaped, for example, launch vehicle design would be much easier. Just jump off the top, and maneuver to miss the other end!
I propose a new project for NASA – geoforming!
I propose a new project for NASA – geoforming!
Well, if we just sped up the rotation of the planet to about once every couple hours, it would be easy to get to orbit, at least at the equator.
Well, perhaps we’re just approaching this space elevator project from the wrong angle. I wonder what the maximum height of a pile of dirt is?
Well, if we just sped up the rotation of the planet to about once every couple hours, it would be easy to get to orbit, at least at the equator.
And yet evidence shows that the Earth is slowing, partially due to the actions of man.
This crisis has only one solution – we must require organizations to purchase rotational offsets.
Plus, of course, we need more funding for additional research.
Mike, I think a key parameter is the angle of repose. I see no particular problem with your solution except the rather astounding cost to build a 200 mile high pile of dirt. But then I don’t understand orbital mechanics. Don’t you also need to acclerate to something near escape velocity? This is the part where my understanding of space elevators disintegrates.
Yours,
Tom DeGisi
Thought for a cool payload for Falcon 1/9 – a stage recovery package – if your payload acheives it’s designed function, you get to keep the stage.
Yours,
Tom DeGisi
Yep. We keep throwing stuff at the Moon, which hits it and knocks it further away from the Earth, thus increasing the drag on the Earth’s rotation.
We need to establish a base on the Moon from which people can pick up the stuff we’ve thrown at the Earth, and throw it out into deeper space, so that the reaction will cause the Moon to drift closer. Getting it to less than 22,000 miles will cause the Earth’s rotation to start speeding up.
I favor a dense Klemperer rosette of orbiting neutronium spheres, chosen such that the effective orbital velocity out to the radius of the Moon is 73 microradians per second.
After importing enough comets to fill this volume with air and floating faerie cities made of spider silk and stainless steel, we can swim freely about in a habitable Smoke Bubble with a volume of 240×10¹⁵ km³. Assuming the Earth’s surface is not now unduly overpopulated, and allowing 1 km spacing between similarly inhabited layers, the habitable volume exceeds that of the Earth’s surface by 10⁹. In principle we need not slow population growth for another 1700 years, roughly, until we reach a population of 10¹⁸, but one suspects, like an overclocked Core 2 chip, we’d have serious cooling problems long before then.
Still, a population of tens of trillions, or roughly what you’d have in a Galactic Empire with 10,000 inhabited planets, seems quite plausible.
Since there was a mention of balloons, I’ve always enjoyed reading about the “airship to orbit” concept. Apparently there isn’t a “won’t work because of the physics” issue. Check out jpaerospace.com if you’re interested. I wonder what would happen if somebody put some real capital behind this idea.
Ah, like Al Gore, you’re a true capitalist! I propose we use the funds to feather our nests and use a token amount to finance a fleet of otherwise idle commercial vessels to circumnativate from east-to-west in perpetuity.
“I wonder what would happen if somebody put some real capital behind this idea.”
Jeff, someone did put some “real capital” behind JP’s ideas in the form of the Near Space Maneuvering Vehicle. While I’m not at liberty to go into details, the results were an unmitigated disaster. Google “Near Space Maneuvering Vehicle” and read the article that comes up titled “Air Force Backs Away From Near Space Maneuvering Vehicle.” I would post the link but I’m woefully inept at doing that in these forum posts.
I’m not saying the airship-to-orbit concept isn’t viable, but the high altitude regime has proven much more difficult than the dreamers predicted a few years ago.
It’s here:
Air Force Backs Away From Near Space Maneuvering Vehicle
Looks like they need rockets for propulsion.
Yours,
Tom
I worked for JP Aerospace about a year following this. The official story as I understand it is that the contract turned out to be quite demanding and the founder, Jim Powell decided to bow out of the next stage of the project. This created a schism in the group (some wanted to continue) that lead to the departure of a significant number of members.
In comparison, the Aviation Week story says:
The NSMV was transferred to “another company”, Global Solutions for Science and Learning. It’s odd that Aviation Week story didn’t report that.
Googling around, here’s words from J.P. himself:
[…]
I wasn’t there so I don’t know how it really went down. Even if I were, maybe I’d lie to you or have a heavily distorted view of it. But the last paragraph seems true to me. I saw the second and third vehicle (I visited briefly a couple of times in 2004 before the contract died, but wasn’t officially involved until late 2006) so they were built.
Turning of gravity, that’s smart! Why didn’t NASA think about that? Launching the Space Shuttle would become a lot cheaper. I know why, that would’ve made too much sense. Turning gravity off in real life isn’t possible I hear you object, wrong! Me and a couple of dudes use to turn of gravity quite often, do you remember those mushrooms GW mentioned, if you smoke enough of them gravity will be turned of for a few hours. It shouldn’t be illegal, if it can turn of gravity then it deserves to be studied by NASA, actually NASA should give me some money and then I’ll investigate “Gravity Negating Substances” for them. Just give me 10 million dollars and a few years to do some “science”.
I don’t think you can turn off gravity because it’s synonymous with the existence of the object generating it. But I believe if you acquire a sufficient amount of dark energy and paint it appropriately around, you can balance its negative pressure against whatever gravitational forces you’ve got, thus neutralizing them, like positive charges neutralize negative.
And I am so driven by altruistic green motives that I will do the necessary research without being paid by the public treasury. In fact, I will pay the government for the privilege of doing the work, so long as there are no fussy restrictions put on the sign of the amount of money I pay.
Carl, you assume here that dark energy can be “gathered” rather than just an inherent property of space. Our models tend to treat it as the latter.
Now, given your enthusiasm for such a project, I could be lead to contribute (up to sign) to such a worthy project.
Would Dark Chocolate work as a substitute?
Yes, maybe after taking into account the tradeoffs, it is still cheaper to mine Earth materials and launch them from the steeper gravity well than to manufacture a solar power space power station from Moon materials.
It all depends entirely on the scale. For building one (or perhaps even half a dozen) proof-of-concept prototypes, the statement is without a doubt correct. For building enough SPS to make a significant contribution to global new electrical requirements, the High Frontier option is doubtless the less expensive option.
Karl, I so do not believe in a cosmological constant. So I think the dark energy is indeed lying around, like fine black dust, to be gathered carefully up and exploited. Screw the greens who want to put a drilling ban on the stuff. They’re just weasels.
For one thing, you can actually build an FTL drive with it. GR does not prohibit things becoming distant from each other at infinite speeds — so long as the’re not moving, it’s just the space between them inflating. Which is what dark energy does for us! We just need to sprinkle it out the nacelles of our starship, and zap the parsecs expand behind us like slime from a slug or whatever.
Of course, enough of this messing with the local curvature of spacetime and we might just break it. Who knows how much bending it can take before it fatigues? Then we’ll be up chocolate creek wtihout a popsicle stick for certain sure.
Karl and Carl, in your opinion, is airship-to-orbit possible with today’s tech? How is drag overcome?
anon, I don’t know though I am optimistic. The mission profile is tough. Basically you need to float a flying wing vehicle up to near the top of where the buoyant part of the atmosphere ends. Then you accelerate it over several days to orbital speeds. Nobody has tried any part of this and that part of the atmosphere is poorly understood since only a few sounding rockets, some radar experiments from the ground, and perhaps some imaging from satellites higher up have ever looked at it.
I’m not allowed by NDAs to say much more, but there are a number of serious problems and engineering difficulties. I think the ones we know about can be overcome with near future technologies, but there’s a difference between “possible” and “competitive”. The whole system would need to be tested. And it would need to be capitalized.