Imagining what a starship would be like

[mdiinican]

I pretty much agree with someone_else

In a hard scifi environment, I expect spacecraft to remain rather spindly affairs, with all the pressurized sections being roughly cylindrical or spherical for strength. Staging will probably remain not uncommon. The price of material delivered to the far reaches of interplanetary space will quite likely outweigh the price of an engine and some empty fuel tanks, just like it has for sending things into earth orbit. Every Kg of spacecraft you bring to your destination that you aren't actually using is a kg less of cargo you could bring instead.

I don't think it's unlikely that larger spacecraft might have some onboard manufacturing ability. There are a number of rapid prototyping technologies that can produce metal parts in a variety of alloys, as well as many more that can print stuff like plastic. IIRC, the ISS is due to eventually receive such a machine that works by depositing metal in a fashion similar to the operation of a MIG welder. These devices can get quite small, and may be practical for replacing any number of smallish metal bits on a spacecraft at a net overall reduction of mass when compared to carrying spares of every small to medium sized metal or plastic part that is likely to break at some point. I do not think that the capability to manufacture the majority of electronics on site will be a feature of spacecraft in the foreseeable future. The production of integrated circuits, or future integrated photonics is a rather more involved process.

I'm no expert on nanotechnology, but I figure it will share some traits in common with other things that share the same general size and perform roughly similar tasks, specifically bacteria and fungi. They do seem to be able to adapt to a fairly wide range of environment, though they need a power source, probably in the form of an energy-rich chemical solution. They do seem quite adept at producing interesting chemicals like proteins or enzymes or oils or alcohols given the raw materials plus what they need to live, but I'm not sure how good they would be at building specific macroscale things. Specific breeds could probably be made to produce a certain pre-set shape through processes similiar to developmental biology, but I have doubts about how programmable that would be, let alone how accurate or fast, or how flexible the design parameters could be. Another solution could be to use a process similar to lithography and build things up in layers, but I don't know how much of an advantage nanotech would be at that over more conventional alternatives.

Spacecraft are not liable to have very effective metal armor, not only for mass reasons, but also because of the effects of cosmic radiation. High energy protons and alpha particles from the sun have a nasty habit of causing heavier elements to give off secondary radiation when hit. Steel or depleted uranium components to composite armor like in a tank are probably not worth it when you consider the increased X-ray flux the crew will be getting at all times. Aluminum seems to be about as heavy as you want to go. Lighter elements tend to make better shielding against particle radiation. Plastics and water make good radiation shielding because they are made up of a bunch of lighter elements. Impact shielding is probably best left to methods like whipple shields. The crewed segments of a spacecraft might be constructed as cylinders surrounded by tough fiber-reinforced polymer insulating material that also works as radiation shielding, and that would be surrounded by spaced armor to act as a whipple shield. The insulating layer would also act as the inner layer of the whipple shield and would absorb the vaporized cloud of material produced by the initial impact.

About the detection. You can certainly detect many spaceship sized objects at interplanetary ranges but in all likelihood won't find them. Telescopes look at very narrow sectors in the sky. Tracking a known body and finding something out there in the darkness is totally different. There is a ton of stuff out there right near Earth etc that we simply miss. The overwhelming majority of NEOs, rocks larger than aircraft carriers, are believed to be still undiscovered.

Anyone building off-planet colonies in a realistic setting is probably doing it at least in part because they are concerned about extinction level events happening to their planet, and will therefore probably have more government run dedicated asteroid finding and tracking telescopes than our (zero). Sure, we have a bunch of amateur astronomers finding new stuff all the time, but they have to look through our atmosphere and asteroids and such can be quite dim, especially the carbonaceous or very small ones. Plus there's the matter of having more telescopes up there for keeping track of the elevated levels of dangerous space junk that comes with increased space travel, as well as telescopes for tracking all the actual traffic, for various reasons like space traffic control.

There are only so many viable transfer orbits and launch windows for a given realistic acceleration and maximum change in velocity, so even in the vastness of interplanetary space there would probably be a surprisingly high chance of getting close to another ship. I don't think it would take that much extra software to get all those sorts of sensors to recognize things that are out of place. Any one telescope doesn't have much chance of finding any given thing, but it's just a matter of having enough to cover the whole sky. There's also the fact that there really isn't a whole lot of anything in the solar system above and below the ecliptic plane, so you can mostly ignore those parts of the sky. Sure, you could be sneaky and fly up or below the ecliptic, and try to get the drop on something from there, but that's going to use an awful lot of fuel, and if anyone sees you on your way out, that's going to raise some serious eyebrows.

[Decimator]

In a hard scifi environment, I expect spacecraft to remain rather spindly affairs, with all the pressurized sections being roughly cylindrical or spherical for strength. Staging will probably remain not uncommon.

Spindly is putting it mildly. This particular craft is indeed staged, as well.

[sirocco]

In a hard scifi environment, I expect spacecraft to remain rather spindly affairs, with all the pressurized sections being roughly cylindrical or spherical for strength. Staging will probably remain not uncommon.

Spindly is putting it mildly. This particular craft is indeed staged, as well.

From what I understood, this ship is supposed to rotate at the some point to have the engines in the opposite direction of flight (so that it can slow down on arrival), right?

But wouldn't that rotation phase be the most dangerous part of the flight? First, you don't have whipple shields on the sides to protect you. And you also have to take into consideration the additional transversal stress on the structure.

So just how does it work?

[Decimator]

It doesn't rotate. That's why it has two engines.

[sirocco]

Oh yeah you're right. It's James Cameron's version which was rotating...

Though I wonder if this is better (i.e cheaper on long term) to have 2 engines instead of making it rotate.

[jollyreaper]

The Cameron model has gimbaled hab modules (see the Venture Star.) When under thrust, the arms rotate flush with the hull so "down" is towards the tail. When on coasting phase or in orbit at the destination, the arms are rotated perpendicular to the hull and spun.

So as far as deck plans go for a realistic ship, it'll be a break from the 50's space rocket era ("down" pointing at the tail) and the post-rocket era ("down" pointing at the keel of the ship, layout like modern navy ships). The habitable portion of the ship will be very small in comparison.

The question of shuttles remains interesting. Tail-sitters or aerodynamic shuttles with landing gear? The Battletech dropships are some of the best modern examples of tail-sitters and "down" is towards the engines. The flight profile for dropships is completely ballistic, either up to orbit or to a destination on the other side of the planet. There's no flying around like an airplane. You can't beat an airplane for fuel efficiency if you need to just loiter around but you need some kind of runway, even if it's crudely hacked out of the wilderness. A tail-sitter can be a little less picky. The compromise of something like the Valkyries from Avatar that have aerodynamic flight but also VTOL capabilities seems a little complicated. I think it more likely that a dropship might carry atmosphere drones if necessary or smaller manned aircraft for larking about but wouldn't be able to fly like that itself. Something like a Star Trek shuttlecraft where you can use it like a helicopter or like a business jet or fly to the next planet seems a little on the fantastic side.

[Decimator]

Though I wonder if this is better (i.e cheaper on long term) to have 2 engines instead of making it rotate.

Since you can cut down so much on shielding, it probably is. Note that the front engine is not decelerated, either. It's ground up and used to smash any space junk in the craft's path while it's decelerating, since the droplet shield doesn't work anymore when decelerating.

[someone_else]

In a hard scifi environment, I expect spacecraft to remain rather spindly affairs, with all the pressurized sections being roughly cylindrical or spherical for strength.

Also for most living modules to be GASP-OMG-SHOCK...... inflatables! Like Bigelow Aerospace's stuff. They have already two full-sized things in orbit, and work fine. Add whipple shields and you're ready to go.

Spindly is putting it mildly. This particular craft is indeed staged, as well.

That is an interstellar craft. It has even tighter mass budget than interplanetary crafts. The Valkyrie is more or less the best you can get with a self-powered engine, and its engine is made of unobtanium and assumptions.
It is also designed by the same guy that was called by Cameron to design the Venture Star, that thankfully has decided for a more sensible approach.
Titanic lasers at home speed the Avatar's VS up at 1.5 gees for 6 months or so (with a truly titanic laser battery, since each Gw of light produces only 6.7 newtons of thust on the sail), then it brakes with the antimatter torches. To get back home it refills with Pandora-made antimatter (EEEK!!! wait a sec, where do they make the antimatter in that shithole camp?) and does the reverse to get back again.

Also the lasers keep any serious alien threat at bay (and redirect asteroids, and push around solar sailers with ease). With that power they can melt about anything that dare step in the outer system.

The question of shuttles remains interesting. Tail-sitters or aerodynamic shuttles with landing gear?

Avatar lander nonsense aside....
The choice is dependent on fuel consumption. Which then depends on what engines you have for them, but I think that landers will tend to look a lot like 50's rocketships.

In general, aerobraking is a desperate way to save fuel, mostly due to the fact that we have nothing in orbit ready to refuel the craft and the fuel used to brake the craft would weight far more than a reentry shield in its payload mass budget. (yet noone factors the cost savings of using orbital fuel depots launched by commercial launchers to refuel the thing in orbit.... but anyway)

Since here we are dealing with a multi-ton spacecraft that wants to land a tiny vehicle from orbit, fuel consumption is maybe less of a concern.

-Got chemical rockets? Landing with such vehicles is annoying, but you can manage to pull it off without significant aerobraking. You will need to mine fuel from the planet's surface if you want to get up again though. (or aero-brake disposable fuel tanks before and then land reasonably close to them)

-Got nuclear rockets? the same as above. But more fuel-efficient so they will be somewhat smaller. But when they run out of fissile fuel (and they tend to run out of it somewhat fast) you must reprocess the fuel rods, not something you can do in the wilderness like mining water ice.

-Got advanced nuclear rockets like a nuclear lightbulb (closed-loop nuclear gas-core engine)? Then you can land on your thrusters AND carry enough fuel to get in orbit again, Libery ship. Although not an easy feat to design, make and run, these are a pretty nice choice since they have a far better fissile burnup. They can also go on Thorium, that is found at least on the Moon.

-Fusion? Too far in the future to be sure, but Fusion is so appealing due to its high ISP, not due to its thrust and is also a tricky, mindboggingly complex, heavy and expensive engine. Imho it's not worth it for landers.

-Antimatter? No. Just no. Worse than fusion in any respect (for a lander, of course).

-Adding air-breathing engines and wings to fly it around? Regardless of how you get on the ground, in the "flying around" configuration you are carrying lots of useless stuff (rocket engines and rocket fuel) and even if you hope to find the rocket fuel on the surface, you still have lots of useless volume (the fuel tanks) that will give your "aircraft" the size and performance of an Antonov for truck-sized cargo capacity. So much for small dirt runaways .
Unless you plan to launch aerobraking drop tanks and manage to link them up solidly in the wilderness. You see how stupid things are getting here.

-Adding only wings? May be a good idea, but not wings designed to fly the thing around like the Space Shuttle (so it will be a tail-sitter anyway). Those wings are there to aid aerobraking (it starts aerobraking much before the space shuttle, in high atmosphere, and thus the thermal loading when enters the thicker parts of the atmosphere would be more limited, allowing you to place less thermal insulation) and help it a little in ascent, so they are designed for hypersonic use. Wings that look too small to fly it subsonic.
You need designers with balls of steel and lots of development cash to make something like this, but should be worth it in the long run.

-------------------------------

In general, when you are dealing with rocketry, the more specific work a vehicle does, the better perfromance will be, so I'd choose the "tailsitters that go from orbit to ground and possibly back again with planetary vehicles in their cargo bays". My engine choice would be chemical rockets or at worst NERVA-like nuclear ones, cheaper to design, test, build and mantain than anything else, although I'm fascinated by the nuclear lightbulb and I'd love to have one.
In any way, I'd use aerobraking for reentry but with a shape that isn't a PITA like winged designs also meant for flying subsonic.
I'd also try to make my lander plumper instead of a high tower, since I want it a more stable lander for landing without landing pads.

You want some pics of what I'm thinking? We all want cool pics. Rombus and Pegasus. (scroll down for the pics)
They are reasonably close to what I'm talking about (the lander must do the reverse that these vehicles are designed to do). I can accept drop tanks anytime.


A related thought, when the mothership gets back home, the landers instantly become wasted mass (since Earth-launched stuff and space tugs can do the same work much more cheaply), and would be a good idea to leave them there. The Avatar VTOL shuttle-like landers are all left on Pandora, for example (and most got cannibalized).
You can always sell them to the locals and make a good profit.

[Decimator]

That is an interstellar craft. It has even tighter mass budget than interplanetary crafts. The Valkyrie is more or less the best you can get with a self-powered engine, and its engine is made of unobtanium and assumptions.

The momentum rules are exactly the same for interplanetary craft. Every gram of structural mass you save translates to many hundreds of grams of saved fuel and remass. You might be able to dispense with certain aspects of the Valkyrie for an interplanetary craft, but the core concept of a tensile spacecraft remains.

[someone_else]

I just wanted to point out that if every gram of an interplanetary craft is worth X bucks, then every gram of an interstellar craft is billion times X or more, so the savings may not be that great in an interplanetary craft.

[sirocco]

why a billion times more?

From the sun, Pluto is at 50 AU max while Proxima Centauri is somewhere around 270 000 AU, which makes it 5400 times farther than anywhere we could go in our own solar system.

Of course, in the solar system you could have several refill stations placed strategically so that it would be quite cheap to travel around. Out in the interstellar void it would become more difficult (except in some specific case like wandering planets ejected from their original system).

[someone_else]

Of course, in the solar system you could have several refill stations placed strategically so that it would be quite cheap to travel around. Out in the interstellar void it would become more difficult

Exactly because of this, since rocket equation is logarithmic and even the best engines ever imagined aren't sufficiently fuel-effiecient to carry enough fuel to reach the speeds needed to do the voyage fast enough. (i.e. the crew doesn't die of old age)

Try this at home: let's imagine to do a coast-to-coast cruise through the US by car, without refuelling your vehicle. How much of your car must be used to store fuel for the trip? How stupid becomes using a SUV to do this voyage instead of just using it to go to the mall?


A little more wanking based on this link.

A fusion torch's theoretical maximum speed with a 100% efficient engine and optimal mass-ratio (between 4 and 5, so that the fuel is 4 to 5 times the dry weight of the ship) is a little less than 12% c. If you want to slow down at the arrival, then your max speed is now around 6% c. Which means you'll need around a century to go to Alpha centauri, with instant (unrealistic) acceleration from 0 to 6% and to 0 again.

Antimatter-using engines have slightly better or even inferior performance to this due to low efficiency (too much stuff decays to neutrinos too damn fast), the paper linked in one of my posts above goes more in depth. I personally think Valkyrie's performance cannot be above around 42% lightspeed (it's the theoretical maximum for engines of its kind), and the description is dodgy on that.

(warning, theoretical maximums tend to be waaaaaay off any reasonably achievable performance)

And that's it. No engine can go a lot faster than the above since it cannot bring enough fuel to do so without impacting negatively the performance. It's a little like approaching lightspeed, the close you get to the engine's theoretical limit, the more fuel you must burn to get the same increase in speed. You reach a point where is no more worth it.

So, they thought of a handy trick. What if we give speed to the craft with something that doesn't add mass to the vehicle (like adding more fuel does after a certain point) and have the ship use its engines and fuel only for braking?
This way you double the max speed. But must install a very costly infrastructure to "push" it (the most common being titanic laser stations).
Now, being light annoyingly "light" (lame pun ), every GW of light bouncing on a sail will produce 6.7 newtons of thrust.
So, if you want to shine enough light to accelerate 100 metric tons (a Space Shuttle fully loaded) by one milligee per second (0.00981 m/s) you need a laser cannon generating a beam of 981/6.7=146 GW.
Notice how that is the beam's power, not the laser cannon actual consumption.
Also you need gigantic optic arrays to focus the beam at the distance needed.

After a little while, the spacecraft gets too far from the lasers and and no optic array can reach it.
This means that either you have truly hellish lasers that manage to accelerate the craft at multiple gees (like Avatar's craft, that is accelerated at 1.5 gees for 6-8 months), or find another trick.
So other people suggested weird stuff like manufacturing hundreds of smaller robotic solar sails and using the titanic lasers to accelerate them up to relativistic speeds (30% or more c) and aim them at the backside of the spacecraft, that will have a magnetic nozzle where these sailbots smash and vapourize, transferring the kinetic energy to the bigger craft.

This way you can keep using your lasers to accelerate the vessel for the whole first half of the voyage (so you can have way smaller titanic lasers).

If you are travelling to an inhabited (and badass) system, then they may use their own titanic lasers (and/or relaitivistic robotic sails) to slow you down, and in this case, the speed of travel is determined by how powerful the accelerating and decelerating lascannons are plus your engine's performance.

But it's the only way to make faster STL voyages without cheating (using FTL drives or engines with handwaved performance).

So, to sum it up, this is why I say interstellar spacecraft cost a zillion more:
In the Space Shuttle example above, you have to add to the Space Shuttle's cost the cost of the sail and of the 146 GW laser shining on it (plus ancillary stuff like its power source, and its radiators). And that ain't cheap. Although can then be used to do lots of other things.

As I said, you must be a truly badass race to do this.

[Guardsman Bass]

Also you need gigantic optic arrays to focus the beam at the distance needed.

After a little while, the spacecraft gets too far from the lasers and and no optic array can reach it.
This means that either you have truly hellish lasers that manage to accelerate the craft at multiple gees (like Avatar's craft, that is accelerated at 1.5 gees for 6-8 months), or find another trick.

All of this is probably the case with the Avatarverse human civilization, since they have the ability to create a lot of anti-matter, and the fluff mentions them having a fairly extensive presence throughout the home solar system. The arrays would presumably be in the inner solar system, along with a bunch of solar-powered particle colliders.

So other people suggested weird stuff like manufacturing hundreds of smaller robotic solar sails and using the titanic lasers to accelerate them up to relativistic speeds (30% or more c) and aim them at the backside of the spacecraft, that will have a magnetic nozzle where these sailbots smash and vapourize, transferring the kinetic energy to the bigger craft.

How big of a sailbot are we talking about? I question whether your starship would survive having large relativistic projectiles effectively impacting with it.

[sirocco]

@someone_else: Well yeah! And even thinking about it, an (advanced) interstellar craft would have both systems, sails and regular engine. While the former is for travel between systems, the latter would rather be for maneuvering when necessary.
OR it could just be some kind of carrier for smaller crafts way more adapted to interplanetary travel.

When I thought about the cost, I just thought about actually building and maintaining the fleets of spacecrafts, not really the whole infrastructure that would make the travel possible.

And I think that before going interstellar, we'd need to be able to easily travel in our own solar system. Say, for example what tech would be better (cost, durability, theoretical acceleration) to travel the 50 AU separating Pluto from the Sun?

[someone_else]

Note that lasers can do better than that if you can bounce the beam multiple times.

It's called Photonic Laser Thruster now, and a guy is working on that here.
But imho the biggest problem of this setup is keeping the laser aimed correctly at interplanetary distances, since you are aiming it at a meter-wide mirror and not to a 10-km-diameter sail.
I usually disregard all sentences containing the word "Mars", they are usually lies that disregard any kind of limitations the hardware has.

Anyway, if the above is going to be useful, then Avatar-like ludicrous accelerations (1.5 gees) with less than lasers are possible.

How big of a sailbot are we talking about? I question whether your starship would survive having large relativistic projectiles effectively impacting with it.

The size depends on what it must push. It is basically a small sail with a chip-sized control system. They guy talked about them massing 1 gram or so.
The actual spacecraft's "engine" is a magnetic nozzle or a simple pusher plate (the push is too tiny to require any kind of shock absorbing system ala Orion), and there is a tiny gas thruster whose only role is using tiny amounts of gas to hit and vapourize the relativistic sails before impact. The things turn into plasma and the magnetic nozzle does its work.

There is also a counterintuitive idea I'm afraid I don't understand.
The guy's original wanking 1 and 2

Say, for example what tech would be better (cost, durability, theoretical acceleration) to travel the 50 AU separating Pluto from the Sun?

I'd like to question why in the heck you would want to go on Pluto so regularly , it is quite a feat.

I'd say magnetic sails or Electric sails (another one. Both related, and both work by creating a field (either magnetic or electric) that allows them to interact with the solar wind. Which means protons mostly, and since those have a mass to speak of, the sails have far better thrust than solar sails, with around the same mass. Theoretically.

While both require some research and development, they don't seem to be so hugely, horribly, devastatingly complex as even the dumbest, crappiest fusion torch that cannot go beyond Mars (even on paper). That is still 40 to 50 years from now anyway.

Also, less-than-titanic-but-still-pretty-damn-big lasers that push solar sails or even that Photonic Laser Thruster could be pretty cool to have. In downtimes, both could be useful in redirecting asteroids we want to mine either in cool orbits or to smash on the moon. (the lasers keep shooting at the asteroid, and a piece of its surface is vapourized and becomes a jet that in months to years of shooting pushes the thing where we want, with the PLT you just bolt a mirror on the asteroid to receive the beam)

And while all this happens Earth has something like 3-4 of these orbiting and doing their work, possibly some fuel depots launched by commercial rockets, a moon elevator and moon-exploiting would finally have moved its first steps.

Welcome to the age of Space Sail .

[sirocco]

Say, for example what tech would be better (cost, durability, theoretical acceleration) to travel the 50 AU separating Pluto from the Sun?

I'd like to question why in the heck you would want to go on Pluto so regularly , it is quite a feat.

Exactly because it is quite a feat. To be able to get to interstellar, I think we need first to get a grasp of how difficult it is to travel around.

For example, I know how far Moon is from Earth but sometimes I just can't comprehend why it is so difficult to get there. It seems like all those scifi shows just shut down my sense of scale...

Getting to Pluto would be hard to achieve the first time. Being able to make regular flight there would probably require to improve greatly the original design. But when we get there, it would then be time to think about crossing the interstellar medium. And only then would it make sense to me.

Though your sail solutions seem to be to travel away from the sun. But how do you make the return trip? say...from Pluto to Earth?

[someone_else]

How big of a sailbot are we talking about? I question whether your starship would survive having large relativistic projectiles effectively impacting with it.

This paper and this presentation discuss similar ideas with a little more numbers around.

While both require some research and development, they don't seem to be so hugely, horribly, devastatingly complex as even the dumbest, crappiest fusion torch that cannot go beyond Mars (even on paper). That is still 40 to 50 years from now anyway.

A clarification. Fusion drives are pretty much the best engine theoretically available (that doesn't require antimatter-manufacturing facilities, that are another beast entirely). Inertially-confined fusion drives as the one described in the paper I linked in my first post of this thread could theoretically do a round trip to pluto in two years (one to go and one to get back) with a spacecraft composed by only 50% fuel tanks.

You also have Orion that nuking-its-own-ass reaches a pretty nice theoretical performance (but won't be developed any further for obvious cost and political reasons).

Still, I think it is more than likely their looong and costly development will let other (cheaper) technologies lots of breathing space to develop, trive and become prevalent, for 150 years at least. (that is, 50 years for the first ground-based fusion reactor, and 50 years for the first decent fusion torch in space. Another 50 years for it evolve enough to be able to reach Pluto. And I'm being very optimist )

For example, I know how far Moon is from Earth but sometimes I just can't comprehend why it is so difficult to get there. It seems like all those scifi shows just shut down my sense of scale...

It is a cost issue. Partly linked to completely stupid mission architecture faithfully mimicking Apollo. Apollo wasn't meant to be affordable nor particualrly useful, it was a "national pride" operation that aimed at putting US flag on the moon.

Making a refueling facility on the moon is an actual priority, not what NASA thinks is "science" (putting men in costly suits on the moon). That is the most important thing the moon can do for us. With that up and working, Earth-moon space becomes a giant playground for even the crappiest of the chemical rockets, and so you can use cheaper launch vehicles since space tugs will go take anything from even very low LEO (so to make cheaper launch systems) and bring it where it must belong with lunar fuel.

That would set the ball rolling for space exploitation (read "profit"), and real science. Possibly to colonization, but I'm not so sure of that.

Though your sail solutions seem to be to travel away from the sun. But how do you make the return trip? say...from Pluto to Earth?

I could have said "like normal craft equipped with wind sails go against the wind" but it would have been not exactly right.
Up there everything works in an alien way. All stuff orbits something, and what matters isn't "height" (the distance from the surface fo the thing you're orbiting), but "orbital speed" (what allows you to keep a particular "height" indefinetly).

Any space engine (and sail) will have to thrust more or less parallel to the surface of the object you're orbiting to increase the orbital speed, and thus to "gain height" permanently by going in a higher orbit (that requires a faster orbital speed).

Anything thrusting perpendicularly from the surface of the object it is orbiting will make its orbit more elliptic, and that is generally stupid (unless you are doing it on purpose to do some trick like slingshots and oberth maneuvers), since will usually lead you to deep space or crash on the thing you're orbiting (more rarely).

For the reverse, to "get down on Earth" (or get back from Pluto) so to speak, you need to kill the orbital speed, not to thrust towards the surface (that will make your orbit more elliptic again).
So, since you know the direction of your orbit, the engine will orientate itself to thrust in the opposite direction, but still more or less parallel to the surface of the object you're orbiting.
Aerobraking does the same, but uses air friction to decrease the bulk of the orbital speed.

Here a better explanation with animations of what the sail does to "get up" and "come down".

[jollyreaper]

Regarding the lander talk, it makes you wonder just how unrealistic the Firefly design is. I think it's likely one of those ideas that seems good at the time but in practice would be a nightmare, much like the V-22. So on the bright side you get your VTOL performance using the same engines you go to orbit with. The engines are air-breathing so I'm guessing there's some kind of efficiency there. I don't think they ever said what the engines ran on.

The only thing we could say for sure is that making rotating engines would have to be more complicated than a pure tail-sitter. So the question is "What's worth the sacrifice to make this a belly-sitter?" Does it make the cargo-handling easier? Any other advantages?

The thing that's killed tail-sitter aircraft in the past is that there just weren't any good control systems to make them intuitive.

This guy was meant to fly off the decks of cargo ships or smaller warships.



It was difficult to land even on a good day and with a proper airfield. Operating off the pitching deck of a ship would be awful. But they're looking to revisit the idea with UAV's.

[someone_else]

Regarding the lander talk, it makes you wonder just how unrealistic the Firefly design is.

I'd say Avatar Landers are more realistic, although still sitting atop a big pile of handwavium. At least those have a realistic-ish shape to do a decent reentry with (although I don't know how optimized they are to reenter in Pandora's atmosphere only. ). From the lore, they seem to use jet engines to VTOL around and accelerate flying up to hypersonic, then ignite scramjets that bring them close to orbital speed, and use only a little help from their "fusion engines" to actually reach orbit. Their numbers are largely made up (although don't look ludicrous), but it's the concept that matters.

In real life, Scramjets would be the best airbreathing engines to help you reach orbit (since they can bring you to mach 10 and possibly up to 20 while still airbreathing), but since they don't work at less than mach 5, someone must design a hybrid engine or carry both and switch between one an the other as needed.
Also, vehicles fully optimized for hypersonic flight, fly like a brick when going subsonic (they cannot have the wings needed for subsonic flight).
Probably an approach like Avatar's Landers is the best for them. You turn the engine intakes upward and (relatively) slowly VTOL down, or accelerate the brick while still angled downwards enough to sustain its subsonic flight up to mach 4-5 where its shape starts to have enough lift to fly properly hypersonic.
That should save boatloads of fuel. But it isn't an area where I know a lot.
Goes without saying that if you aren't landing on Earth-like worlds with both a more or less specific pressure and a decent oxygen content, your scramjet landers must be either custom-designed (good luck making the flight tests) or are useless.

The closest thing to that (beyond NASA's and Boeing's scramjet things) Skylon's engines (currently in development) should be air-breathing up to mach 5.5, and then, since you're getting hypersonic and jet engines don't work hypersonic, it starts to work as a normal rocket. Since orbital speed is around Mach 22 and rocket equation is logarithmic, it isn't that bad.

The only thing we could say for sure is that making rotating engines would have to be more complicated than a pure tail-sitter.

Those also require an upgrade to its frame, since the rocket must now be able to resist other kinds of stresses than usual.
That would add structural mass to an already mass-sensitive design as-is.
May not be a killer, but you must have very good reasons to do it.

So the question is "What's worth the sacrifice to make this a belly-sitter?" Does it make the cargo-handling easier? Any other advantages?

Cargo handling? That depends on how it is supposed to reach orbit. If it goes up like a conventional rocket, then imho it makes more sense to load it while it is standing upright. You need to balance the cargo to ensure a safe flight and standing upright makes easier to measure the vessel's center of balance as you move cargo mass around the cargo bay (unless you are dealing with pre-balanced standardized cargo pods).
If you go up like skylon (more or less horizontally) then cargo-loading isn't a problem.

The only advantage I can think of for belly-sitting is stability when parked. The average rocket is very "fluffy" without fuel onboard. But you could solve that by making the craft plumper and by planting cables in the ground to stabilize it like with a tent.

The thing that's killed tail-sitter aircraft in the past is that there just weren't any good control systems to make them intuitive.

Huh. I guess better computers changed a few things from those days.These guys are practicing VTOL flight with rockets, and so far they had good success. (and won a million bucks for the feat)

This guy was meant to fly off the decks of cargo ships or smaller warships.

I want one .
It reminds me a more recent (failed) idea. The Rotary rocket, also called Roton, and some limited specs.

Its main feature were the helicopter blades. Those were supposed to help it on ascent (and ended up just compensating their own weight), and do something useful on an aerobraking descent (work like an helicopter for a soft controlled landing without using buttloads of fuel). It did some flying around with the rotor (and the helicopter pilot said it had a pretty limited field of view, btw), but investments ended and the company went bankrupt before testing any more stuff, 11 years ago.

That's a concept I have a soft spot for, but of course landing with enough fuel to get up again is probably out of question. Blades won't hold the added weight regardless of how big you make them.


There is also another point I only hinted at with landers. Fuel to get up again.
For a Surface-Orbit-Surface vehicle, things are easy, it starts with fuel, consumes most fuel to get in orbit and then, when is light and empty, it gets down again (by aerobraking).

For a lander, a Orbit-Surface-Orbit vehicle, you don't need necessarily fuel to get down, but you most certainly cannot bring enough fuel down with you to get in orbit again.
We are talking of multiple hundreds of tons of fuel or more, and aerobraking a so much massive vehicle is quite a feat.
Unless you have overpowered engines (gas-core or liquid-core nuclear or better), that let you go down with a powered descent and carry enough fuel to get in orbit again. Or badass airbreathing turbojets and scramjets.

This raises the question "how can I get up again"?

You need to drop expendable fuel tanks (before committing to reenter with the vehicle), as bigger reentry capsules (although I have some doubts that you'll be able to make big enough capsules to carry the whole fuel load, although using hypersonic parachutes, that should weight less than a heat shield it may be doable).

If you plan to use your landers very often or the mothership cannot carry enough fuel for them for some reason, then setting a fuel-manufacturing forward base is the only way. Albeit far slower one.

[jollyreaper]

My ideal tail-sitter design is squat like a Battletech dropship, not a 50's atomic rocket. Should be stable enough.

As for getting off the ground, I'd liked the idea of them being fusion rockets. (with the caveat that this is what I thought when I was a kid, not in light of realistic design constraints in the here and now.) My thinking was that the vehicle would have hybrid engines, air-breathing rockets. On descent it's using onboard fuel. It deorbits, uses aerobraking as much as possible, deploys flaps in the lower atmosphere to dirty up and slow as much as possible. The air intakes would open up and the fusion drive would superheat the air so it would finish braking. The advantage is there's no combustion, the fusion fuel stock is onboard or maybe you use antimatter instead.

Once on the ground the ship would start compressing the local atmosphere to stick in the reaction mass tanks. Upon launch it starts as an air-breather blasting hot air out the back. Once it clears the thicker atmosphere it transitions to pure rocket mode and burns off the stored reaction mass.

Given all of the complications involved, I might do less violence to physics just declaring it uses an antigrav drive. But that's what I thought was pretty strict SF when I was younger.

[Sarevok]

Regarding scramjets for reaching orbit I am not sure if they would work at all or be economic if they do. Objects at velocities where scramjets operate experience tremendous heating and drag. That kind of destroys any savings scramjets incur by using the atmosphere as source of oxygen. The atmosphere is an enemy of spaceflight - most rocket tracjectories are designed such at they leave the atmosphere asap and then do the main burn to achieve escape velocities.