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Well, looking at the history of reusable first stages, it seems you need a very sturdy solid motor casing, and a low staging speed, to survive the pressure loads if you just use "an empty rocket stage". Ask SpaceX, it seems they also overestimated their chances (F9's first stage is covered in cork to heat-shield it through reentry, but they have only recovered little pieces of it and telemetry that indicates it blows up when it hits the atmosphere).

The plasma idea needs a powersource, never mind the magnets (cryocooled if superconducting, mind you). An incredibly dense one that doesn't create more waste heat that it deflects. Good luck with that one.

Me, I lean on the idea that if you get it sturdy enough to survive aerodynamic loads (and control the heck out of it the whole way so you choose the precise angle of attack), then you can heat-sink your way through it with little added effort. Pressure-fed tank sturdy, for example. If necessary, an active cooling loop can work wonders (the shuttle used one with freon, that's where the heat went in the end), and an open one (I'm thinking perspirating heatshield) even more so.

So, my two cents can be summed by up by this idea that just popped in my head: spaceX got a real nice engine by forgetting their initial dreams of cheap(er) pressure-fed rockets... but their stages ended up failing structurally. With great empty weights, of course. That their are going to have to kill with structural reinforcements.

Rune. That's what happens when you forget the BDB creed. XD

Lots of people I know there... from what I hear, it's almost a spanish city, by the number of aeronautical engineers we export there. Which is sad in a way. But my french is non-existent, my english is good (or so I'm told), and even though I'll graduate in mostly aerodynamics and structures, rockets and rocket engines kind of won my heart a long time ago. Even of we dedicate <1% of class time to them... no one else dedicates more time, that I know of (in spain, of course).

Of course, if I can get some internship at ESA I really wouldn't mind! ^^

As to how much to go, 12 subjects. Which is not to say I will end 12 exams from now, my university has the habit of passing less than 50% of people in every exam. When I passed calculus, I was one of 40 among 200, and 40% of the new arrivals got expelled because they didn't pass anything the first year... Good times.

Rune. Idiotic system, I know. At least we know we can do ANYTHING afterwards.

Wing loading is aerodynamic force per unit of area, or at least that's what I've always been told. If a plane is flying level, it roughly equals the weight (roughly because of drag), but when executing maneuvers, it can go up or down, depending on whether the plane is accelerating upwards or downwards. Very important design factor, by the way.

Rune. Skiing... I miss that! Sigh, I guess when I'm a rich engineer and can't find the time, I will have the money.

Two (actually just one) quick cents on the subject:

As a useful comparison to trade against, take the classic full ablative heatshield on top of the stage. It may be much heavier for the same area, but the area is much, much smaller. An smallish additional inflatable skirt could put the rest of the rocket in it's aerodynamic shade and "keep it cool". IIRC, those are already tested at small scale, sub-orbital reentry speeds.

Actually, now that I write about it, make the full heatshield inflatable. No interface between solid and inflatable that way, so in accordance with the KISS principle. These may actually be up to three different options to include in the trade analysis (solid heatshield alone and some thermal protection on the rest of the rocket, solid heatshield + inflatable "skirt", full inflatable).

Rune. Awesome idea, I'm already waiting for that book. Go write it!

Err... what's the problem with having an elevator inside an atmosphere? Said atmosphere, through friction with the ground (and nothing else) would rotate with it and speed up (or down) until the relative angular velocity is zero. The elevator, by definition, would rotate at that same speed. Nothing moves with respect to nothing unless you pump energy into it.

Rune. I REALLY want to get back to my desktop computer soon. Posting by tablet is awful (which is why I'm not doing it, that and the holiday food).

Can it be from Fallen Angels? 'Cause if it is then I totally forgot about it, and I deserve a reprimand!!

Rune. I'm starting to get the feeling I should ask you about good sci-fi books I haven't read...

The way I see it, methane gas offers too many pluses to think of anything else. First off, you are already producing it for rocket fuel if you have any sense. The ingredients are simple and abundant as hell: atmosphere, energy, and water, and you can sabatier/electrolyze your way into liquefied methane and oxygen. Everywhere you have a settlement, you are going to have the three of them, or you are not going to have a settlement.

And you can run fuel cells out of the stuff, burn it in combustion engines, use as rocket fuel, even get gas burners for cooking. Pretty much a powersource for everything, scalable from tens of watts (fuel cell) up to megawatts (rocket engine or turbine engine/generator). And storage is pretty simple, pressurized tanks of methane and oxygen, preferably stored on opposing sides of the colony. Handling and transportation also don't seem an issue, we are pretty good about handling those on earth, you can move bottles of the stuff around by hand with a more than decent power density. And you are going to have to produce pipes for other things anyway. You can also forget about trying to store hydrogen, which is a bitch. And also on the plus side, none of the ingredients burn by themselves in martian atmosphere if there is an accident... I only see pluses, apart from the fact that you have to redesign all combustion engines to supply both oxidizer and fuel form tanks. That seems pretty easy.

Of course you are going to get a pretty awful efficiency out of the process, with much less than 50% of the input energy being able to be retrieved, compared to just using it as you produce it... but I don't think you can do any better here on Earth anyway.

Rune. No need to get complicated, methinks. So KISS.

I think I have it on favorites, in case I had to dig up the stuff you put there about ramjets. I recall your fuel experiments with the beetle were also worth looking... Will keep it there!

I actually don't have a reference for chutes or the absence of them... they are not in the video and Dragon is going to retain them as backup, that's all I know for sure. If you have heard something about that somewhere I can look at it, point the way!

I'm sure crossfeeding their way into a three stage booster also had something to do with that, but yeah, it's amazing what they have done with the engines. And good to know hey will have to show those numbers are real soon.. as in next year the hardware must be delivered, right? Big stuff happening... kind of makes you wonder if the haste with which the Falcon Heavy is going to be fielded has something to do with the lack of flights this year. They must be really pushing their factory output, in terms of finished cores, methinks.

A couple of things. First, it's more accurate to call the booster a resurrected Falcon V. It's going to have nowhere the "uumph" of its bigger brother, no matter what the advantages of air launch are. It's quite smaller, about 55% less than the F9, going by the number of engines, or a bit more if the air launch lets you go with lower T/W.

And the little delta wing in there, I am guessing, is there to perform the pitch-up maneuver any air-launched rocket requires. Taurus has a similar wing, and even though it could in principle be used to fly the booster back, I doubt it's even used for that purpose. No, I expect the booster to be expendable, at least until SpaceX proves reusability on the big boys, F9 and F9H, and then to be modified using their technology. Just an informed opinion, BTW. I have no inside info, just my personal "prophecies". And I would have never seen a vertical landing first stage coming, so I fully expect to be proven wrong again.

Rune. Ok, it's official. I'm re-hooked to this forum.

Have you seen the last video SpaceX released? In case you haven't, here you go, some eye candy. I understand completely your points, I even agree to most of them, the only problem I can see being now you are still not reusing two stages. And, as you said, ocean recovery doesn't go too well with reusing a stage quickly or for a great number of flights (Elon Musk has suggested thousands, as an ultimate goal, which I agree, sounds crazy). But they are not planning to recover on the ocean! Instead, the stage lands on rocket power on a pad, in the cape right next to the processing building if you believe the video. Never mind the horizontal distance traveled before staging.

So yeah, supposedly the video is not 100% accurate, in the words of Musk himself. And somehow making it back a few hundred miles on rocket power doesn't sounds like something easy (I hear they are looking for another launch site, which could be related). And they are talking about making it survive atmospheric reentry by slowing it down after staging. And the second stage is supposed to have a full heatshield, and a retractable engine bell (that was the thing that left me the most O_O). Oh, and Dragon has somehow acquired an active docking system besides the fancy launch escape that lands it on the same launchpad on the cape. At least one chute is kept in the Dragon for backup. As I said, eye candy.

But if you take anything from the video, or the conference that followed it (and I can't seem to find on youtube), it's the vertical landing under rocket power from terminal velocity with no chutes involved. That's really revolutionary, if realizable, and given it only has to land 4,082% of the fully fueled, stacked vehicle propulsively... Of course it won't be just that, they are talking about beefing up the stage also, but you get the idea it's an argument agaisnt increasing the dry vehicle mass stage. In other words, a trade is involved. Go figure. Seems their engineers are betting that trade goes in favor of more fuel left in the tanks, and lighter tanks. Me, I wish them most of luck in their endeavour and hope their calculations prove accurate.

As to how are they going to manage to modify the stages so much maintaining the payload, I am divided over if it's sheer magical thinking, or the performance increase of the Merlins is really going to permit VERY stretched tanks. In case you also haven't heard about that, the new and improved version of Merlin, the 1D, expected to go into service some undefined time in the future, is claimed to have 168% the thrust of the 1C that has already flown. That is an enlarged first stage with potentially up to 60% more fuel on it. Or a good deal of increased structural integrity. Or a heavier second stage that is almost a shuttle. Or a combination of the above that actually works as advertised, and then it's all business and glory to them.

Rune. The press conference after the video was an informational gold mine. And very... persuading? Musk almost made a SpaceX fanboy out me.

Here's something truly on topic... and it's actually news, I read it like 5 minutes ago and haven't heard a peep about it before:

Microsoft Co-Founder Paul Allen Unveils Giant Plane for Private Space Launches

My take? Gosh, they are actually going forward with the spaceship one approach. I hope the air launch end ups giving them more than it costs... and if it does, I see it performing more because of range availability than anything else. Something which, by the way, I don't doubt is a big improvement, cost-wise. I just doubt if it is a worthwhile improvement.

Rune. And Paul Allen... there's more geeks with money every day, right? That's a good thing.

Well, I'm afraid I can't help you, since I got it out of thin air and a bit of thinking. My reasoning went something like this: you always have the same propellant flow (by volume) through the same engine and nozzle, so mass flow in the exhaust is directly proportional to molecular mass. Therefore, T/W is directly proportional if the speed of the exhaust remains constant. For chemical rockets, that's not really very far off reality, but you could always adjust for the difference in exhaust speed... isp1/isp2 should be the correct factor to adjust with. But of course no source is going to use such a crude method... I guess I could cite anything about conservation of momentum? It's also where you derive the rocket equation from... XD

"Brute force" trajectories would take about as much delta-v as is the difference between the orbital speeds of mars and earth, so about 29.8km/s (for earth) - 24km/s (for mars) = 5.8km/s, without taking into account entering and leaving either planet's orbit (whichever orbit you choose to park in, the delta-v requirements vary).
As to how much it takes to actually go into orbit and leave and such, I will refer you to this handy picture, with the most common delta-v's in cislunar space and mars. I got all the numbers from wikipedia, I'm that lazy.
For comparison, the "standard" minimum energy Hohmann is about 1.5km/s, again without the departure and capture taken into account. You should have enough data with that to plug in the dV in the calculator, and get the mass ratio of pretty much any trajectory between the earth, moon and mars. Just add the dV of all the steps your imaginary ship takes, and then plug the number in the calculator like jumpboy says.

Rune. Informed common sense almost never fails. Emphasis on "almost"... big one.

Ok, you got me there, I should have said you have to look at the density of the exhaust products to figure out relative T/W between different engine fuel choices. Well, actually at their stoichiometrically averaged molecular weight, not the density, that's also incorrect. Whatever else I said, disregard as wrong.

Rune. But wait, haven't I already said that? Like in the post you actually quote.