Quickest time a Single-Stage-To-Orbit futuristic fighter-jet take could get into orbit?

Thank you for the answers everyone, they have all been a really interesting read. :slight_smile:

I didn’t realise it was such an engineering challenge!

No, you could build it largely from steel. It’s not as hard as you might think to build a 30 km tower. Mount Everest is 9 km high and it’s just a pile of rock. A truss structure made with steel could easily reach 30 km.

There are more advanced routes that could get you to hundreds of kilometers, such as pressurized balloon trusses made from boron-nitride fiber or the like, but they aren’t necessary here.

Maintaining a precision steered electromagnetic system tens of kilometers long is, while not trivial, at least something that’s been done before. See the LHC.

Space elevators are vastly harder, and in fact do require unobtanium. Specifically, they require carbon nanotubes manufactured both in a quantity and quality that’s never been achieved before. A space elevator tether is 35 thousand kilometers long.

The problem with this proposal (and similar accelerators) is economic, not engineering. I’ve never seen anyone really close the economic case when it comes to flight rate–for the hundreds of billions it would cost to build, you would never pay the interest on the loan with the number of flights achievable, even assuming the payloads are available. But the OP didn’t ask about economics, so I ignored it.

To be fair, developing a Kardashev III civilization is “just an engineering problem”, too.

Speaking just to Gs. …

If we assume a typical jet-like upright seating position for the crew, then the G forces involved in getting to orbital velocity are applied front to back (tits to spine) rather than the direction normally thought of as G forces in maneuvering flight which is head to seated butt. That orientation increases the max survivable level some, and the max long-term sustainable level even more.

I’m going to say WAG 12-14 G briefly (seconds) and 9+ for several minutes. Since tits to spine is the same direction that real NASA astronauts experience ascent and capsule reentry loads, certainly NASA has hard science-based numbers on this; I’m too lazy to hunt them down. I’d expect they have numbers as to bare survival, and numbers deemed tolerable for crew safety and crew functionality; NASA does expect their crew to be something other than inert meat in a pan on the way up and down.

For sure at those high loads it’d better be smooth; if there’s much low Hz vibration impressed on the base loading they’re going to jelly-ify the crew’s brains, if not the rest of them.

Describing Mount Everest as “a big pile of rock” is sufficiently obtuse enough to give geologists fits. Everest is in fact a highly complex structure of three distinct formations (the Qomolangma Formation, the North Col Formation, and the Rongbuk Formation) totalling over 16 trillion tons of material occupying 60 billion m[SUP]3[/SUP]. By contrast, the world record for the largest single pour of concrete, an office complex in downtown Los Angeles, was 16,208 m[SUP]3[/SUP]. I’ll leave it to the reader to calculate the relative difference in scale.

Claiming that a “trust structure made with steel could easily reach 30 km” (the tallest extant steel-framed structure is 828 meters tall) is frankly completely risible. Just constructing the footings for a tower of that height alone would be an engineering challenge exceeding the most massive construction projects ever built, and while I haven’t attempted to even conceptualize a design of this 200 km long, 30 km tall support structure for an accelerator, I’m morally certain that trying to build it using standard structural steel elements (square tube, C-channel, I-beams) would result in negative margins long before you could reach even 10 km, notwithstanding the problem of supporting it from seismic movement. In fact, if it were possible to build such a structure out of any material, you’d probably want to build it in such a way that it moves freely on the ground, or floating in the water with neutral buoyancy to protect it from inevitable movement of the ground under such loads and natural seismic activity. This is notwithstanding the incredible difficulty of constructing such as structure in essentially no atmosphere, necessitating workers to operate in pressure suits. It would literally be easier to build and maintain a transatlantic underwater tube than this hypothetical megastructure.

The Large Hadron Colloider is 27 km in circumference, is built underground in a concrete-lined tube, and accelerates hadrons (mostly individual protons) to energies up to 6.5 TeV. This is absolutely nothing like the kind of rail gun accelerating payloads of many tons over a 200 km length to near orbital speed. In fact, just the waste heat this system would have to dissipate would be challenging by itself. The loads it would have to resist would be greater than any static structure has every been built to withstand, and just a rough order of magnitude guess at the power requirements is causing the California power grid to flicker in fear.

The notion that the issues with this idea (which doesn’t even begin to approach the degree of rigor to be regarded as a “proposal”) are “economic, not engineering” boggles the mind. I take from this comment that you do not have any background in construction or structural/civil engineering, because if you did you’d understand the fundamental problems with the essential premise. But don’t take my word for it; contact the civil engineering department at any university and ask a professor or graduate student to estimate the feasibility of this concept. Just be prepared for uncontrolled laughter in response.

Stranger

We’ll get Musk on this right away. He’ll bring top men, top men I say.

True, from a certain way of looking at it, but there’s certainly an economic step-change. A trillion-dollar project is possible by large nation-states. But even a Kardashev Type I civ (let alone Type III) is more like a quadrillion-dollar problem; infeasible even if all the engineering was straightforward. Making it economic (via self-replicating manufactories, etc.) is not yet “just an engineering problem”.

http://boards.straightdope.com/sdmb/showthread.php?t=254792&highlight=mountain

Stranger, how do you feel about ablative laser propulsion? You need telescope mirrors that can take laser impingement and give you spot size of about a meter in radius at 100 kilometers. I’ve actually read that the mirrors are possible with present tech. It’s really really really big - billions of dollars worth of laser modules for even a small system - but every module is independent so to my rough assessment, having only done electronics and software engineering, it sounds feasible.

Sure, but all of the stuff you mentioned makes Everest worse than an engineered structure. So 9 km is in some sense a lower bound for “just a pile of rock”, and certainly a lower bound when taking better materials into account.

Comparing to the Burj Khalifa is unfair. It is a slender skyscraper designed for human habitation. As such, it spends much of its mass and internal volume on things like elevators and offices, and furthermore has to take human comfort into account. A steel truss tower wouldn’t resemble it at all. The Burj Khalifa is furthermore nowhere even approaching current engineering limits, as we might expect from an ordinary building designed for use by “regular people”.

Structural steels can reach 2000 MPa yield strength, though more common alloys are perhaps 700 MPa. Let’s say that 500 MPa is reasonable. By my math, this gives a mass ratio of 110–that is to say, with the required taper factor (lower parts of the tower require more cross-section than higher parts), you need 110 kg of structure to support 1 kg of “payload” at the top. The tower mass is sensitive to the compressive strength; at 400 MPa, the mass ratio is 360, while at 600 MPa it’s only 50. In any case, none of these figures are so extreme that they make the concept untenable, and certainly none of them imply that unobtanium is required.

I’m fairly sure a graduate student in civil engineering would find this an interesting problem. I certainly won’t pretend to have worked out the details; I will just say that this is very much unlike a space elevator in that even just at a napkin-math level, the materials required for the cable are past the limit of what’s known. So however hard you think this problem is, the elevator is worse. Not to mention hypothetical engines with both a 5000 second Isp and high thrust (aside from Orion designs).

BTW, I do in fact agree that the construction is probably the hardest problem. We have no experience at all in construction in both a vacuum and under gravity. Construction in space is slow but at least somewhat tractable. But with gravity the challenges are unknown. It seems to me that construction would have to be largely robotic, which may or may not qualify as “just engineering”. Though perhaps one could built it “top down”, where the entire tower is raised on jacks, starting with the topmost section and ending with the bottom. The Japanese have managed to deconstruct office buildings this way.

You are correct that the Khalifa Tower is not comparable to this hypothetical structure in that, as a habitable structure that doesn’t bear any external loads other than wind and seismic, whereas this 30 km tall, 200 km long structure is going to be reacting huge operating loads, modal responses, enormous wind shear, and its own structural mass. Given the scale of the conceptual structure, it wouldn’t surprise me if it would take more steel than everything produced for construction to date, and building footings alone would probably take decades worth of cement production. While not the kind of demonstrably physically unworkable problem as a spaceplane SSTO the size of an F-22 (which can be dismissed just by looking at the rocket equation and calculating the necessary mass of propellant), this is equally impossible from any practical standpoint, regardless of economics.

Stranger

The F-15 eagle was adapted to haul a really big honkin missile up to its ceiling limit, so figure subtract the time it took for the eagle to get that high from the altitude that you want to fly space superiority missions.

But to me, the real limitation is life support. How much canned air are you bringing with the canned sunshine.

I think we’re just quibbling about what it means to be “economic”. Taking decades of cement production means that it could, in principle, be built. It’s not a billion years of production or something way out there. If there were some existential threat that somehow required one of these, it could be built.

Personally, I think your estimates are way too high, especially as the upper 2/3 is mostly there to keep the air out and doesn’t need to support the peak acceleration loads, but even if I was a factor of 100 low it wouldn’t change the overall picture: possible, in principle, but the economics don’t make sense.

You seem to have completely missed my point. Aside from the extreme economic challenge of just pouring enough material for the footings or forming enough steel to form a structure of those overall dimensions, a structure 30 km tall and strong enough to resist environmental and operating loads is physically impossible to construct from steel trusses. Even aside from the complexities of actually building such a structure, the necessary area of the footings to distribute the static weight of a 30 km tall structure would be hundreds of kilometers wide, and even relatively small amounts of modal response or seismic displacement would cause the structure to tear itself apart. It just isn’t even remotely feasible even given infinite resources.

Stranger

Hundreds of kilometers wide? No, that’s ridiculous, and would massively decrease the efficiency of the structure. 10-15 km wide is more reasonable.

The whole point to a truss structure is that it can be an extremely low density. This is desirable since it improves resistance to the bending moment without increasing mass. Although the material cross section increases exponentially with depth (rather like the rocket equation), this does not imply the overall width has to increase exponentially. The excess can be put into extra cross-section of the load-bearing members.

Maybe if I have some free time I’ll sketch out something with a bit more detail, but you seem to have imagined something that I didn’t propose.

Here’s a paper by Geoff Landis and Vincent Denis about a launch system reaching to 25km.


According to the paper it should be ‘easy’ to build a 25km launch tower; this may be true, assuming you have an unlimited budget.
And what are the results? Well, the launch tower still doesn’t get you to space- you still need to launch a fully-fueled SSTO rocket at the top, but the good news is that you can deliver 122% more payload to orbit. Nearly twice as much. A lot of expense for almost nothing.

Note that Landis and Denis’ scheme does not include an evacuated launch tube- it is just a dumb tower with a launchpad at the top. To include a launch ramp you’d need to extend the tower 200km sideways and strengthen it enough to support the stresses of acceleration. Still not impossible, but a lot more expense.

nothing much to add and I agree in general but it is a curious fact that the first flight of the Hurricane was Nov 1935 and the first flight of the Spitfire a mere 4 months later.

So yes, older but not by much.

I guess this is relevant. From Slashdot:
“New Mexico-based ARCA Space Corporation has announced that it is developing the world’s first Single Stage to Orbit (SSTO) launch vehicle that can deliver both a small payload and itself into low Earth orbit, at a cost of about US$1 million per launch.”

I can’t tell if they are serious, but they have pretty pictures on their website. So I guess it isn’t an April Fool’s joke.

SSTOs are firmly within the realm of possible, but not at all economical. IIRC, there are several former and current first rocket stages that could probably get themselves into orbit, if they weren’t carrying a second stage or payload. That’d just be one hell of an expensive stunt (maybe one Elon Musk would be tempted to do, with a used Falcon 9 stage that’s due to be retired…)

ARCA is proposing a capability that seems plausible, at first glance: 50 kg launched on an expendable SSTO. It looks like they’re planning to achieve this by using lightweight composites for the rocket body/tanks, and an aerospike engine, on a relatively small expendable rocket. The other big difference, from past decades, is that there is now some demand for launching such small payloads.

But even then, is there much reason to make a SSTO? All those technologies and changes in the launch market can also be applied to multiple-stage vehicles, of which there are several competing designs in development for this payload class.

Regardless, all the plausible SSTO designs leave you with an spent rocket in orbit, and perhaps a little bitty payload. Not a SPACE PLANE INTERCEPTOR that can do the ZOOM PEW PEW you see in cartoons. Just a big empty tube and a used engine, with no fuel.