New Mars Forums

Well, if ρ is about 100 times lower (and it is), then yes, you need a wind speed 100^(1/3) times bigger, or ~4.64, for the same power. The thing is, solar is about half as efficient on Mars as on Earth, and it is already dubious you would choose it over nuclear options. Plus, wind power is pretty much on the top of it's theoretical efficiency on earth already (>85% of theoretical Cp), so not much room for improvement. Some niche application is of course entirely possible.

Rune. Oh, and v^3 is much more clear (IMO) than v 3, if what you mean is velocity cubed. Made me look it up. ^^

Well, everything post-2008 got lost in... how are we calling it? The great crash of '11? Whatever you call it, you are in the same situation as all of us. In fact, I am thinking of making a blog just to archive my posts and not lose the gazillion of hours I invested in writing them up.

Rune. Also, welcome back!

First of all, Terraformer is talking about a solar-powered system.This thread is about nomad planets in interstellar space, right? So no-go anywhere outside the sphere of influence of a star for his plans (he talks about 500AU's from a Sol-type star).

Secondly, about your idea about using coils to harness energy from a huge rotating dynamo like Jupiter: That's all fine and dandy, but the energy has to come from somewhere. In this case, the de-orbiting force your magnet experiences. So maybe if you de-orbit some massive moon by using magnetic fields (it can surely be done, google electrodynamic tether), you would get it's gravitational potential energy back in the form of electric current in your coils. But then your moon is no more, on account of having collided with the planet. That seems like a waste of good processable mass (it's not trapped under a huge gravity field and several bars of pressure, for starters). Conservation of energy is always a bitch, I know.

So for true nomad terraforming, or colonization or whatever, you either use the planet's heat waste as powersource (cause by gravitational compression, mainly, and hence limited) or you have some external power source like fusion/fission reactors. Which, assuming you got to the nomad in the first place, crossing interstellar distances, is a fair bet.

Rune. I mean, Pluto is very much inside a star-system comparatively, these nomads are in the ass end of nowhere, quite literally.

I actually linked to it on the reusable launcher thread, thought it was a more suitable place. Nice story with lots of info!

The boosters don't stage slower so they have the range to get back, they stage slower because if they didn't, they'd keep on blowing on reentry long before they hit the water like they do now, unless you built them as stout as SRB's (actually, stouter, the SRB's stage slower). Plus, their lower speed lends itself to a 'pop up first stage' launch profile, so no flyback required at all, which always looked kind of silly to me in the first place. And no matter how you cover the engines, humidity at least would find a way in, it always does, so you still have to protect for corrosion the engine. Probably have to do that anyway to some extent, if you are going to use it form the cape a lot of times.

My main point of contention is actually the second stage performance. That has to reenter at high mach, so beefy structure and heavy ablative heatshield. And they want it to provide ~8kms/sec? It is starting to look like a SSTO, air-launched. Which actually, lends itself to the idea of treating the first stage boosters like airplanes. Short turnaround times, little to no work required to get it back in the air. Just do a visual inspection, get horizontal, stack it together and transport to the launch tower, fill 'em up and back to flight. A single booster (the heaviest part, so the most expensive) could service several second stages that way.

Rune. Damn, Falcon R is starting to look suspiciously like the DH-1.

Well, the CM+SM combo clocked in at a bit over 30 tonnes and our lunar Dragon is restricted to 16 by the launcher, so there's the source of discrepancy. Part of that comes from the SPS (the hypergolic main engine) being actually sized to lift a much bigger CM straight of the lunar surface (that was the original plan, direct return), part of that is the older, heavier tech, part of that was the fuel requirement (2,800m/s delta-v on it's own) to brake the whole stack, LM included, into LLO.

Of course, I think it's a bit optimistic to think you could get a 7 crew, 10mT lunar Dragon that can actually return to earth and keep them alive for two weeks under 16mT. But, with a lander a bit heavier than the old LM that brakes into orbit itself, and a much reduced crew, you could get 2-3 crew members to the lunar surface easy, I think.

I don't think anyone is sure on that one. Not enough data to do anything but rough guesses. Plus, orbital firings could be done with the smaller, not canted attitude Dracos, for a penalty on Oberth savings. And even their isp is, at this point, anyone's guess. But there is enough margin form an empty Dragon to a full one, that it seems possible with a reduced crew.

Rune. It's a mouthful of acronyms, speaking about Apollo. Look at that first paragraph!

I kind of half-remembered the old Saturn's numbers, and guessed I had remembered them optimistically. Still, 20mT out of 53mT (37.73%), I'd say my guess was pretty close to the mark, right? (Though the Saturn has a more efficient upper stage and this changes things somewhat, in Saturn V's favor). I also half-remembered an Apollo LM was less than that (14mT, turns out), so a similar lander would be easier to build.

And yeah, an upper stage of course would increase thrown weight. As well as development costs, and complexity, and eventually the cost of each launch. You just have to run the budget numbers (the very detailed, real budget numbers, with salaries and taxes and such included) to pick between them. Rough guess again, in the beginning you want to avoid large development costs. Until you have the depots in place and can refuel from of-earth sources, for example. That would be a very good time for refuelable, advanced upper (more like, in-space) stages to come available. Of course, for that, testing has to come first, as the first missions are launched to establish the refueling infrastructure.

Rune. Plus, I was going for cheapest, fastest and dirtiest. I think I won at that. ;P

Errr... you know, if energy is too expensive to make a metal-making infrastructure stick, we might as well pack up and go home. But (there's always a but):

You can import that energy! A few kilograms of highly enriched Uranium would keep a megawatt-sized (or gigawatt, if built locally, it's the optimum size on Earth) power plant running for a VERY long time. If you need carbon and your options are to build two bases far away from each other so one can supply CO2 ice to the ore-producing one, then you are way better off just building more powerplants, importing high-density nuclear fuel, and accepting a high(er) energy price for you steel and aluminum, and pay it by not having to import two whole bases and a transportation system to go from one to the other. Talking about importing steel or any construction material from earth when they can be made using energy is nuts, IMO. Energy, using nuclear systems, can be packaged so densely that it makes importing any other thing, even rare platinum-group metal to use as catalysts, a waste of payload mass by comparison.

Plus, all of a reactor main components can be built locally if you have a healthy heavy metal-making industry, so you have potential for expansion and truly big, 1GW commercial powerplants with decades of life. Imagine the kind of industry those powerplants could sustain. And the reactors can keep it running almost no matter the final energy price of the metal. The only problem would be how to get Mars unhooked from the expensive nuclear fuel from Earth. One solution is for someone to make fusion work economically, but I won't hold my breath for that one. Even if I have some short of hope for polywell still. Plenty of Deuterium on all ice, I'm sure.

IMO, the energy price is a nice consideration to have in mind, because the less energy that has to be imported, the tighter your energy budget is, the better in general. But if you have to pay the mass bill somewhere, if you are going to add significant complexity or a big number of processes/specialized machinery to save a few kilowatts, then think twice, since energy is one of the few thing that could be imported from Earth "economically".

Rune. Of course, it would be nice if the energy budget of local solar power turned out positive, manufacturing and maintenance energy costs included. I won't hold my breath on that either.

I think they mean to eventually refuel these H2/LOX stages at depots at either LLO, EML1, or LEO, supplied from the moon. Or all of the above. But since what we are discussing here is how to build the infrastructure to actually mine and process those fuels, I coincide that that is looking at the problem the wrong way. Since the fuel is going to be lifted form Earth until it is produced elsewhere, it only makes sense to use storable propellants, at first. Dense hypergolics while you are at it to improve simplicity and reliability are nice. The isp advantage is weighted against the duplicity in propulsion systems, the heavier cryo tanking, and general boiloff issues to call a real, solid, "worth-the-extra-complexity" delta-v advantage at this stage, methinks. Note that no commercial spacecraft (or otherwise, I think) intended to operate in space for prolonged periods of time and numerous burns uses H2/LOX propulsion.

Hell, skip the whole upper stage completely and launch stuff directly to TLI with the Falcon Heavy. It is basically a three-stage vehicle anyway, so the throw weight to TLI should be around half than the one to LEO, give or take a few tons. And if Falcon 9 is safe enough to put a crewed Dragon on top, Falcon Heavy should be, too. Or a lander. Or a return propellant tank to put the Dragon on its way back home, if you can't pack it in the crewed launch. A crewed Dragon would have the capability to dock with all this stuff at LLO (it'll have active docking systems to go to the ISS), so lunar orbit rendezvous is certainly feasible. Orbits don't decay as much at the moon, so the components can wait there for the crewed launch, and a preliminary launch of commsats can give you the kind of sensor environment to be very sure of your dockings. What is that, three Falcon Heavies per mission at most, and the only development required a ~20mT lander? Practically loose change for governments. Very hard for private investors, if all they get out of it is a few boots on the moon (certainly less than seven pairs, in a 20mT lander) and a few rocks back.

Rune. And that would be the absolutely minimum hardware to develop to return to the moon. Decent MCPs suit would be cool to have, though, for a few million extra bucks and the extra publicity of superhero-like astronauts.

Well, we now know a nominal abort firing is about 5 seconds long. We also know, at last, that the abort G's will be close to solid rocket escape systems. About 6 G's, give or take. So the landing sequence is not going to last less than that, since it will be done at a fraction of the thrust. It could last up to 6 times longer, if the chutes stopped the capsule in mid-air and neglecting air resistance. So less than 30s of landing sequence, more than... say 10. A half-minute controlled descent would take away the landing ellipse? That's the question. Of course, that's assuming this new system has its own fuel provision, and all of it is used in both maneuvers (it would make sense to empty the tanks on an abort, that's for sure). They don't really have to keep the orbital propellant and the abort/landing propellant separated for any reason that I can think of, but that is not going to stop me form extrapolating performance as if they did.

Rune. Let's speculate away! It's free, and sometimes you get it right and feel good about yourself.

What do you know? Just a few days later, there is a press release dedicated to "SuperDracos". I comment it here.

Rune. Not a peep on T/W, though. But they are deep-throttleable.

Hi guys!

I'm just going to put together a bunch of your points and answer them in one go, because I think they all refer to the refueling of the ship, and that eventually leads directly to the overall architecture of the ship's mission:

and

and

Well, I just budgeted 10% of the fuel's weight to the tanks and left it at that. Should be enough for those recommendations, right? As I said, all the work is approximate, which is the best I can do without actually designing a ship's components. Cryogenic handling over years is also a big source of weight, and even then you can't expect the system to be perfect so you will lose some percentage. And making them reusable may, or may not, be the best idea.
If we are refueling from Earth (and until a refueling stop is built somewhere else, we will), then it could make sense to just leave the tankage section of the fuel tankers docked to the ship and discard the now useless service modules. That way you don't lift any tankage empty. You could even say the tanking is "free", because the refueling ships are going to need them anyway. I'm picturing some tug like progress, Cygnus or ATV, but devoted to fuel. Unless, of course, the unfueled ship is catching dumb tanks launched to orbit, which is risky, because they need a minimum attitude control to be fished, and if you don't circularize, you have very little time to catch them before they reenter. And even then it makes sense that those dumb tanks are also the flight tanks. If our ship is based at the Phobos refueling station and only stops at LEO to pick up cargo and crew and pass maintenance, then that's another thing entirely. But somebody has to build that Phobos refueling station first, and transport each and every piece from earth. By the way, note I don't need no fancy fuel depots. The ship is the fuel depot itself, as it should be.

I think you mean aerobrake, in spirit at the very least. Aerocapture is doing it in one go, and is quite a hairy proposition (not impossible, though). Aerobraking over several passes once you have captured propulsively still saves you most of the capture fuel (capture into high mars orbit is a couple hundred m/s, and I expect on Earth it would be similarly low), and has been done by unshielded satellites. If I didn't use it at the end of the mission is to build in a couple of km/s of delta-v margin, and so I didn't extend the mission for the couple of months required to perform aerobraking maneuvers into a low orbit. Least risk, least complexity, 90% of the benefits: KISS. Also, don't pick your materials until you have a clear idea of the requirements of the component. That one was hammered on me by my teachers.

My point exactly. Which is why I offer methane as an acceptable, storable variant. Ammonia could work too, but has less performance. Plus, everywhere you find water ice, you also find methane, including the lunar poles if we go by LCROSS data. Processing it is waaaay easier. Only fractional distillation required.

Errr... you mean for a chemical H2/LOX rocket, right? Because if not, carrying ice that is only 11% propellant by weight is just nuts. And won't give you a mass ratio close enough to go to mars, ever. You don't need that much oxygen! And the hydrogen will still escape as it is being produced, if you can't produce it quickly enough. More Oxygen you need to carry for nothing. Hell, just run plain water in a NTR and you can do better (isp ~500). But at isp ~500 you either need to refuel on mars or stage the mission and screw reusability.

Have I already pointed you here? If not, there you can find a Transhab designed for mars missions (so 18 month habitability, and can be extended to 24). Bottom line, as you can see on page 61, the system clocks in at 35mT for a crew of six, and is already designed for 1g at the end of an extensible boom (clever design, BTW) and a power-rich environment. And a solar radiation shelter, EVA systems... the works, of course. The "control deck" can very well be a laptop with wi-fi and LAN access to the rest of the ship, BTW.

Rune. Is glad to talk nukes!

I am intrigued enough by the prospects to explore that vertical-launched ramjet variant a bit more. I especially like it, because a ramjet has an easier time flying back to the launch site under its own power, however it lands (parafoil gliding on skids?), than a rocket does. And I don't like ocean recovery, or having to transport stages across continents to reuse them.

First, I would like to ask how hard it would be to bend further the standard gravity turn of vertical launch trajectories to keep the ramjets operational for their full, up-to-M6, envelope inside atmosphere, then stage and pull up in rocket power. More aerodynamic heating, for sure, but if we are talking reusables, like we are, then they will have to endure worst on their way down. If we can get the staging done at those speeds and altitudes, then it should be no problem to solve the rest of the trip to orbit with dense fuel engines and reasonable enough mass ratios (7-8 with kerolox and a vacuum nozzle, for the ~7km/s remaining to orbit) to consider a single stage that can be reused. You know, since we make good expendable ones with MR 30, and just tens of flights out of the same airframe would be already a big revolution. Such a "second stage" would also be a single-stage moon and mars lander, by the way. Just saying.

Then, I would challenge the percentage of thrust assigned to the ramjets. In fact, I would challenge that doing a parallel burn with the rockets is a good idea in the first place. First off, we all agree the second stage(s) takes the lion's share of the delta-v to orbit, and makes most of its work outside the atmosphere. Therefore, if we don't want to get into the trouble of an altitude-compensating nozzle, the engine should only be used outside the atmosphere (or at least, the dense portion of it). Not only that, but at the speeds the ramjet is going to take over, the body of the rocket could give a significant amount of aerodynamic lift to counter the gravity losses of the slower accelerating ramjet at a very reasonable attack angle. That would pretty much solve the low thrust and frontal thrust issues inherent with ramjets. Also multiply the drag losses, but you know, nothing is free and right now they are in the order of hundreds of m/s. We are going to have a structure though enough to take it, if we plan on reusing the whole thing, after all.

And, the first stage is the one we are SURE of recovering. It's the biggest, most complex, most expensive part of any launch vehicle (ok, maybe the shuttle was more complex than it's launch vehicle... but only if you count the SSME's as part of the shuttle, not the launch vehicle). We should be trusting it to do as much as it can, so we can get the most benefit out of reusing it, hopefully, with airplane-like procedures and lifecycles. That way, we can build only a few, even if they are big, and still get a huge benefit in terms of final cost to orbit.

And takeoff thrust, thanks to your "solid engine inside the combustion chamber" method, GW, is certainly not the problem. Maybe keeping the G's low enough for humans is, instead? Handling the loaded booster prior to liftoff can be a problem (solids are HEAVY), but again, modularity and parallel staging is clearly the route around that as you said.

I ran into a problem here, however. The second stage also has to be reused, and reused fast and easy, so it has to land properly, not splash down in the ass-end of nowhere. And it's going to be strapped for mass ratio, so not a lot of room to add fancy landing systems like wings, undercarriage and that short of stuff. Those kinds of things belong in the ramjet booster(s), which is the airplane-like part of this vehicle. Increased structural weight hurts less in the lower stage, and all of that. A rocket is NOT an airplane.

Seeing as how it is big as hell (MR 7-8, remember? Even if it is a dense fuel...), maybe you could land it gliding as a lifting body with only the undercarriage and control surfaces required. Maybe a parafoil for good measure and reasonable approach speed. Throw is a cargo bay, probably, so you don't have the aerodynamics of hell to contend with, although carrying stuff on shrouds has worked well until now. But the really Heinlenian way to go about landing would be to turn back on the main propulsion system and land majestically on a plume of rocket awesomeness and hydraulic pistons. Problem is, I insisted on having the engines designed for vacuum. Thrust is not an issue here, since the stage is mostly empty, but the expansion ratio is going to mess up the engine seriously. Like backpressure choking the engine and blowing out the combustion chamber, right? Thing is, again you can design around it (me likes aerospikes, promise! They go well with VTVL) but you are going to take a mass/performance hit somewhere, for sure. And I can't help the nagging feeling that it's the thing that makes this architecture not the best one. Of course, I'm expecting the lot of you to polish it until it is!

Rune. So please go ahead. And have fun while you are at it!