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Re: What/Where did you fly today? - Part 3

Postby Thorsten » Wed Jul 20, 2016 5:22 am

When arriving at a planet, it would jump (or warp?) to a position at about the same altitude, cancel out the gravity and slowly descend into the atmosphere - no heat shield needed.


It's certainly going to need a heat shield for that (operating the technology in that way would be... dangerous).

Thing is - the planet moves around its sun. The sun moves in the galaxy. If your hyperdrive keeps real-space state of motion, the departure and arrival velocity will be very different.

The planet also rotates. There's no such thing as positioning a physical object at 100 km altitude at rest - keeping stationary in space relative to a surface point isn't even an inertial system (you can't initialize it with a state vector and it will stay) - you constantly need to accelerate, anti-gravity or not. The required acceleration is also a function of altitude and latitude (there's Coriolis force to reckon with).

Also, the atmosphere will (partially) co-rotate with the planet, which will blow you off-track.

JSBSim will do all that, and whatever technology you have aboard will have to compensate for the real-world physics. The acceleration values you imagine may simply be inadequate for the flight pattern you envision.

Antigravity sounds more difficult, but I think I can define it as a vertical thruster with thrust depending on gravity field strength and the angle between the vertical ship axis and the gravity field lines.


It would depend on whether is is some force countering gravity or whether it is inertia suppression - the two are rather different.

This is what most science-fiction ships seem to do, and this is what an Elite or Oolite ship would do if it could land.


Well, Elite used to have plenty of orbital stations as well, and I'm guessing they're called that way because they orbit :-)

Getting into orbit will probably the last thing I'll try -


It can't hurt to think about the actual flight profile early on, because it's going to be frustrating if you work out the technology, tune it for atmosphere flight - and then discover it can't do what you actually planned with it.
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Re: What/Where did you fly today? - Part 3

Postby Fritz » Thu Jul 21, 2016 12:19 am

To the Moon with the DC-3...

Image

(actually it's just TerraSync being too slow again, but I like it!)

Thorsten wrote in Wed Jul 20, 2016 5:22 am:It's certainly going to need a heat shield for that [...] Thing is - the planet moves around its sun. The sun moves in the galaxy. If your hyperdrive keeps real-space state of motion, the departure and arrival velocity will be very different. The planet also rotates. [...] Also, the atmosphere will (partially) co-rotate with the planet, which will blow you off-track.

Of course you are right, but I think, you make things a little too complicated for what I'm planning to do. Just imagine what would happen to a balloon slowly rising to orbital altitude. It would have a speed relative to the surface dictated by wind speeds. If it rises so far that pressure and wind becomes negligible, it will keep it's last speed (I'm ignoring coriolis effects, but we won't go much higher than 500 km anyhow). So our balloon (or anti-gravity ship) drifts in a known altitude above a known position on the surface, and it has a known speed relative to the surface. It's not orbiting, its simply flying very high, and it is still using a coordinate system relative to the surface with latitude, longitude, and altitude. Arrival from a hyperspace jump would be the the same, but only if our speed is low and not affected by relative speeds of planets, stars, and galaxies.

We don't know how hyperspace drives will work one day (if ever). So we have a certain freedom of defining how they work without having to fear that reality will prove us wrong during our lifetimes. :) Science fiction authors have created several methods: superluminal speed (ignoring relativity), warping (made prominent by star trek), jumping/teleporting (like it is done in Elite), natural wormholes, and "star gates" or other kinds of artificial wormholes.

I can't easily say why, but I always preferred the idea of jumping. This method is especially suitable for use in games because you don't have to model a continuous journey through the galaxy, but you can simply load the target planet (or target solar system) while doing the jump. It could even need notable disk loading time without disrupting the game flow, and you don't need visual special effects like wormholes or warp speed effects. Last but not least, you don't need particle deflectors or other fancy stuff to prevent the impact of very-high-speed particles. Jumping keeps our ship as simple as possible and it allows for almost every imaginable outer design. And, as I said, I want to keep the ship as low-tech as possible.

I don't especially like the idea of jumping to a pe-selected absolute position by simply chosing the destination on a map, like it is done in Elite/Oolite. It's ok for a game, but it seems more plausible and flexible (usable in uncharted territory!) to point the nose of the ship in the direction you want to go and jump over a pre-selected distance. Longer journeys could be made by several successive jumps, getting shorter on approach to the target planet. You would certainly need a distance selecting control with a logarithmic scale! This method is similar to how the short range jumps in the original game of Elite worked, that have been replaced by the continous "torus drive" in Oolite which simply increases speed.

Even neighbouring stars can move relative to each other with speeds of more than 100 km/s, and the orbital speed of Earth around the Sun is almost 30 km/s. You would need very strong heat shields to brake from that! Of course we could simply use our fancy thrusters to slow down outside the atmosphere, but they would have to be very strong, and we would have to do some very critical calculations to not mess up the approach.

But we are doing science-fiction: We can simply define the jump drive to compensate differences in kinetic energy caused by relative speeds of planets, moon and stars - these energies would anyhow be orders of magnitude lower than the energy (probably) needed for the jump itself. So our ship jumps to a position with a speed of 0 relative to the centre of the gravitationally dominating object, i.e. the star, planet, or moon the ship will fall to if we don't prevent it. We could make it a little more realistic and calculate interpolated speeds for positions in between two or more objects (e.g. L1), but at these locations there is nothing to see, and we wouldn't notice slightly "wrong" speeds. After all, we don't even have a working speed indicator outside an atmosphere! We would just make a short jump nearer to the object we intend to land on.

For landing we would have to accelerate to the rotational speed of the planet (not to much higher orbital speeds). For Earth, this speed is about 465 m/s or 1674 km/h (40000km/86400s) on the surface, and only a little more at 200 km altitude. FG can't do it (yet?) but if it was possible to jump from somewhere to Earth, the simulator would have to give our ship the rotational speed in a westerly direction, of course depending from latitude and altitude. It isn't a very high speed: Every modern fighter can do this, and for our space ship it would be no problem at all: While descending slowly into the denser parts of the atmosphere, the air resistance would accelerate us very gently (or brake us from the flight simulator point-of-view). I assume, FG would do this without additional code because it should treat the space ship like an airship flying at a very high altitude.

Orbiting is something completely different of course, but a ship equipped with the technology I imagine wouldn't need to do it, and it wouldn't need the computers and precise maneuvering needed for orbiting. It wouldn't even need to be able to accelerate to supersonic speeds, and the main thrusters would mostly be needed only for atmospheric flight. A ship like this could and should able to be flown "by feeling"; the only critical thing (apart from interstellar navigation :roll: ) would be to prevent too high speeds during the approach. Testing my ship will prove me right or wrong, but test flying will be one of the interesting parts!

[Antigravity] would depend on whether is is some force countering gravity or whether it is inertia suppression - the two are rather different.

I'm thinking about countering gravity, like some magnetic or electrostatic effect, not about simply reducing mass to make the ship "invisible" for gravity. It is science-fiction of course, but I want it to be plausible. The ship creates its own gravity field "somehow" (I wish I would know how to do it! :)). In outer space it could perhaps even be used as artificial gravity, so you can walk around in the ship, but that's not important for FG. Near a planet, the artificial field interacts with the surrounding field. If the gravity devices are creating 9.81 m/s², the ship will float stationary above the surface. If they create 5 m/s², it would fall as if Earth's gravity would only be 4,81 m/s², and if they create 10 m/s², the ship would rise with 0.19 m/s² (if we ignore air resistance). And if the ship rolls or pitches to 90°, the antigravity effect would be zero, the ship would fall like a stone. At least, that's my rough plan at the moment...
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Re: What/Where did you fly today? - Part 3

Postby chris_blues » Thu Jul 21, 2016 1:19 am

Fritz wrote in Thu Jul 21, 2016 12:19 am:To the Moon with the DC-3...

Ah, alien obduction! I love Science Fiction! :)
Don't hesitate to let me know if I'm incorrect or just annoying! As long as you do it gently! :)
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Re: What/Where did you fly today? - Part 3

Postby Thorsten » Thu Jul 21, 2016 5:06 am

It would have a speed relative to the surface dictated by wind speeds. If it rises so far that pressure and wind becomes negligible, it will keep it's last speed (I'm ignoring coriolis effects, but we won't go much higher than 500 km anyhow). So our balloon (or anti-gravity ship) drifts in a known altitude above a known position on the surface, and it has a known speed relative to the surface. It's not orbiting, its simply flying very high, and it is still using a coordinate system relative to the surface with latitude, longitude, and altitude.


In fact it would not (which is what you're going to see in JSBSim).

An anti-gravity ship outside the atmosphere doesn't feel any force, so it does what an object which doesn't feel any force usually does - it moves with constant speed along its last velocity vector. It holds inertial attitude and velocity, not earth-relative.

Whereas the trajectory to keep stationary above a point on earth is an approximate circle in earth-centered coordinates and an elongated and curved sine in sun-centered coordinates. There's an acceleration needed to keep an object on this trajectory, and that's different from the acceleration you need to get barely supersonic, it's the acceleration required to match curvature.

Also, your attitude in earth-relative coordinates (LVLH) rotates with respect to inertial at about 0.07 deg/s. Even to keep attitude relative to the horizon is going to cost you some non-trivial coordinate transformations.

If you arrive directly above target with velocities matched, by the time you 'gently descend' from 500 km into the atmosphere, your landing site will be thousands of kilometers away.
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Re: What/Where did you fly today? - Part 3

Postby Fritz » Thu Jul 21, 2016 9:07 pm

Thorsten wrote in Thu Jul 21, 2016 5:06 am:
... It's not orbiting, its simply flying very high, and it is still using a coordinate system relative to the surface with latitude, longitude, and altitude.

In fact it would not (which is what you're going to see in JSBSim).

Ok... than I might run into problems. :roll: So FG changes the coordinate system at a certain altitude? Probably I will have a look at the Space Shuttle or some sci-fi spaceships if the time has come. To be honest, I haven't even tried the UFO. At the moment, I'm writing forum postings still working on my model in SketchUp, so there is a very long way to go! Perhaps the whole solar system will be available in FG until I can make my first take-off. :wink:

An anti-gravity ship outside the atmosphere doesn't feel any force, so it does what an object which doesn't feel any force usually does - it moves with constant speed along its last velocity vector. It holds inertial attitude and velocity, not earth-relative. Whereas the trajectory to keep stationary above a point on earth is an approximate circle in earth-centered coordinates and an elongated and curved sine in sun-centered coordinates. There's an acceleration needed to keep an object on this trajectory, and that's different from the acceleration you need to get barely supersonic, it's the acceleration required to match curvature. Also, your attitude in earth-relative coordinates (LVLH) rotates with respect to inertial at about 0.07 deg/s. Even to keep attitude relative to the horizon is going to cost you some non-trivial coordinate transformations.

That's right if FG doesn't simply use flight simulator coordinates at orbital altitudes. And I don't even want to think about orbit calculations including whatever my gravity devices will do. :roll: In theory, normal orbiting would be possible if the artificial gravity field lines are kept 90° to the Earth gravity field lines, so that no antigravity effect is felt. But the pilot could "adjust" gravity by rolling and pitching, and at a given orbit, the ship could be slower than a real object. And if the (auto) pilot doesn't keep the ship aligned at the same angle to the surface all the time by constantly using the thrusters, the result would be a very strange orbit... Kepler would turn in his grave!

If you arrive directly above target with velocities matched, by the time you 'gently descend' from 500 km into the atmosphere, your landing site will be thousands of kilometers away.

Yes, I thought about this already. It's one of the problems a pilot will have to solve by experience, if he doesn't want to lose time by flying thousands of kilometres inside the atmosphere, probably at subsonic speeds. But if the pilot knows how long a gentle (or should I say survivable?) descent takes from a given altitude, he can aim his last jump to a good starting position, perhaps somewhere above continental Europe if he wants to land in London (at higher latitudes, the rotation speed is lower anyhow). I don't think that such a descent would take more than an hour, probably even significantly less, depending on how daring the pilot is and how powerful I make the thrusters. And we can already start chasing our destination while doing the decent, i.e. during this hour.
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Re: What/Where did you fly today? - Part 3

Postby chris_blues » Thu Jul 21, 2016 9:09 pm

I think it's best to stick with the shuttle. These SciFi spacecraft are not realistic in any way. For instance, if you put the throttle to zero, it just stops...
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Re: What/Where did you fly today? - Part 3

Postby Fritz » Thu Jul 21, 2016 9:17 pm

I suspected this. And that's exactly what I don't want to do!
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Re: What/Where did you fly today? - Part 3

Postby chris_blues » Thu Jul 21, 2016 9:21 pm

Good to hear that! It's good for testing or exploring the scenery, but for actual orbital flight it's non-sense...
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Re: What/Where did you fly today? - Part 3

Postby Fritz » Thu Jul 21, 2016 9:29 pm

Yes. But perhaps I can modify these ships with "my" technology and use them to test how FG behaves at higher altitudes. Hmmm... :roll:
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Re: What/Where did you fly today? - Part 3

Postby Thorsten » Fri Jul 22, 2016 5:29 am

So FG changes the coordinate system at a certain altitude?


Um... no. Nature doesn't either. What changes is that the atmosphere is co-rotating with the planet, so you feel a force pushing you along the direction of rotation as soon as you enter it. But even airplanes feel centrifugal and coriolis forces in JSBSim at all times - they just usually happen to be small relative to typical aero foces such as high altitude winds.

If you look into the JSBSim part of the property tree you'll always find eci speeds and positions - that stands for Earth Centered Inertial (I think...) - the sim always knows both systems and does calculations of a particular force in the easier one (aerodynamics in co-rotating system, centrifugal and coriolis in inertial).

To my knowledge YaSim doesn't know any of this (but then again, it's probably not flexible enough to design anything fictional).

Yes, I thought about this already. It's one of the problems a pilot will have to solve by experience, if he doesn't want to lose time by flying thousands of kilometres inside the atmosphere, probably at subsonic speeds.


It's just not how anyone would operate that kind of technology. It's a bit of driving your jet airplane via a motorway to the nearest cliff to launch it by going over the edge - unnecessarily wasteful and dangerous.

(I know lots of the real-world science, and I do write SciFi, so I end up thinking about how a fictional bit of technology would be operated a lot).

First, an anti-gravity device probably is expensive hightech, and maintenance is going to cost and is needed by the hour of operation. A heat-shield in comparison is cheap technology.

Second, complicated devices can fail. If you're on an anti-gravity assisted descent and the device goes, it's total loss of ship and crew. Whereas a ballistic entry with a glide phase offers plenty of redundancy and abort options. In addition, a heat shield is an essentially dumb device and if weight is not a concern, well, you'd just make a thick one.

Third, we had avionics to aim a Shuttle towards a landing site in the 1980 - a procedure by which an interstellar ship just relies on the experience to guesstimate the planet's rotation correctly is not really plausible.

Given a cheap and safe option and an expensive and dangerous option to operate a piece of technology, it's just not clear to me what kind of future society would possibly habitually go for the second one.
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Re: What/Where did you fly today? - Part 3

Postby Fritz » Sat Jul 23, 2016 12:31 pm

Thanks to the moderators for extracting this topic!

Thorsten wrote in Fri Jul 22, 2016 5:29 am:
So FG changes the coordinate system at a certain altitude?

Um... no.

That's a relief to hear! So it's not like a game that switches into "space mode".

What changes is that the atmosphere is co-rotating with the planet, so you feel a force pushing you along the direction of rotation as soon as you enter it. But even airplanes feel centrifugal and coriolis forces in JSBSim at all times - they just usually happen to be small relative to typical aero foces such as high altitude winds.

Yes, you'll probably not notice this in a DC-3...

If you look into the JSBSim part of the property tree you'll always find eci speeds and positions - that stands for Earth Centered Inertial (I think...) - the sim always knows both systems and does calculations of a particular force in the easier one (aerodynamics in co-rotating system, centrifugal and coriolis in inertial).

That sounds interesting! If understand it correctly, this would mean that my space ship would behave correctly, without additional code needed. It would rise and rise, and at some point, aerodynamic forces would become completely negligible. The Python's shape doesn't seem to be very stabilizing anyhow, so I will have to use thrusters to control the ship and to keep it stable even at lower altitudes (and I probably will have to implement a simple kind of gyro-based auto pilot that keeps the ship stable around the three axes - this would also work in space). A pilot rising to space wouldn't notice anything changing apart from the air resistance gradually disappearing. And if he doesn't want to do something particular at this altitudes, like entering an orbit and approaching an orbital station, he wouldn't have to think much about it.

First, an anti-gravity device probably is expensive hightech, and maintenance is going to cost and is needed by the hour of operation. A heat-shield in comparison is cheap technology.

I dare to disagree! We simply don't know how expensive a gravity device will be, and, more important, we would need the gravity devices anyhow. They would even have to be stronger because of the weight of the heat shield. So no cost savings at all!

Second, complicated devices can fail. If you're on an anti-gravity assisted descent and the device goes, it's total loss of ship and crew. Whereas a ballistic entry with a glide phase offers plenty of redundancy and abort options. In addition, a heat shield is an essentially dumb device and if weight is not a concern, well, you'd just make a thick one.

A heat shield can fail itself - I remember one killing 7 astronauts. You could make a safer one by using it only once, but that's not a good idea for reusable ships because it would increase ground times and costs immensely. But yes, an advanced society could probably make a safe and reusable shield.

But even then, a heat shield would be useless on Moons or planets without (significant) atmosphere, and it would pose severe design restrictions on the outer shape of our ship. And even if the shield doesn't fail, the control system could, turning the ship into a position for burning up or breaking apart. Speed kills, but if we use anti gravity, we don't need speed!

Of course a gravity device can fail. But so could some other part - especially the hyperspace drive that could leave us drifting helplessly in interstellar space, unable to call for help by radio (my ship will have two hyperspace drives for this reason!).

Turbine engines can fail too, but even in the "primitive" world of today it is so improbable that twin-engine planes can and may cross oceans with passengers. My ship will have more than one gravity device (at least four, perhaps six or eight, my planning hasn't got so far yet). And additionally, there will be vertical thrusters, needed e.g. for a soft landing, so there will be some redundancy, perhaps enough to make a survivable splash into a lake or even a soft landing (this will depend on how much cargo is carried).

We don't know how a gravity device would work. If you believe the internet, it could be based on super-conducting coils, rotating parts, ketone thrust, and/or whatever happens inside magic carpets. I'm currently thinking about some heavy spinning parts, because I want to have something like rpm in my displays, and I want it to react very slowly, i.e. much slowlier than the thrusters. A device like this would share one potential hazard of a turbine - lubrication failure - but it wouldn't have to deal with extreme heat, leaking fuel, hail, or Canada geese. In my ship, pilot error will definitely be a much greater risk!

Third, we had avionics to aim a Shuttle towards a landing site in the 1980 - a procedure by which an interstellar ship just relies on the experience to guesstimate the planet's rotation correctly is not really plausible. Given a cheap and safe option and an expensive and dangerous option to operate a piece of technology, it's just not clear to me what kind of future society would possibly habitually go for the second one.

I don't imagine a star-trek-like "clean and perfect" universe where every forest tribe has access to high technology, but a world more like the Elite universe, perhaps even backwards in some way, and less organized compared to today. Space ships would be used more like intercontinental sailing ships in the 16th century, and not like airliners today. A ship in an universe like this may have to land on "primitive" planets without any infrastructure or perhaps even on uninhabited planets without an atmosphere. So it must be able to operate completely independently, and it must be able to be used in unknown terrain ("where no man has gone before"). This even excludes any dependency on radio navigation, and about the only hi-tech device we could use as a navigation aid on unknown planets is a radar altimeter - simply because there might be nobody on the surface to give us a QNH. Of course, this doesn't prevent us from using more advanced techniques when approaching a space harbour on a well-known hi-tech planet.

So at least the atmospheric part of the flight would be very archaic, and the pilot would have to rely on simple styles of navigation, like looking out of the windows, using a map, and "IFR" (I follow roads and rivers ;)). I don't know if I'll succeed to make my ship like this, but I'll try, and I'll certainly find out where I imagined things to be too simple. Flying like this will be more fun - there is a reason why I prefer the DC-3 or Super Constellation to modern airliners, where most of the challenge is programming the auto pilot.
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Re: Spaceships and space flight

Postby Thorsten » Sat Jul 23, 2016 5:59 pm

It would rise and rise, and at some point, aerodynamic forces would become completely negligible. The Python's shape doesn't seem to be very stabilizing anyhow, so I will have to use thrusters to control the ship and to keep it stable even at lower altitudes (and I probably will have to implement a simple kind of gyro-based auto pilot that keeps the ship stable around the three axes - this would also work in space).


Fire up the Shuttle, use AP for liftoff and then take CSS and *gently* fly it - you can test how it feels. The PFD even has markers for inertial and earth-relative speed displayed, so you can watch how they initially differ and then merge as the Shuttle starts moving faster than the rotation.

(The Shuttle is also yaw-unstable at supersonic speeds, so if it wouldn't be constantly driven to zero beta by the DAP, it'd spin out of control quickly - same problem).

The Python looks streamlined for atmosphere entry - there's no other reason to build a wedge-shape.

We don't know how a gravity device would work.


You told me it's not inertia suppressing. That's nearly all I need to know - the engineering is whatever you invent, but I know what it does for the equations of motion.

There's a number of other constraints you can derive from the fact that someone bothered to build the device in the first place but doesn't use it as main drive. Since you told me you don't want to alter basic space travel physics, I actually know a hell lot at this point (probably much more than your internet sources...) since I have a folder of concept studies for fictional spacecraft equipment calculated :-)

Turbine engines can fail too, but even in the "primitive" world of today it is so improbable that twin-engine planes can and may cross oceans with passengers.


As you write yourself: I don't imagine a star-trek-like "clean and perfect" universe where every forest tribe has access to high technology, but a world more like the Elite universe, perhaps even backwards in some way, and less organized compared to today.

You're Captain of a space ship in a universe where spare parts are a few lightyears away. You have just arrived in a basically unexplored world (or is it? You don't know, could be outlaws hiding out here, could be smugglers,...). And the engineer you hired in the last system isn't exactly familiar with that piece of equipment (in fact, you're not even sure his diploma is genuine).

(I suppose that's the Elite setup in a nutshell...)

You play it fancy - or you do it as simple and as risk-free as possible?

Space ships would be used more like intercontinental sailing ships in the 16th century, and not like airliners today.


Right - so you really think they didn't play it safe or planned for multiple redundancy?

This even excludes any dependency on radio navigation, and about the only hi-tech device we could use as a navigation aid on unknown planets is a radar altimeter


You'd have an inertial navigation system of course and star trackers for attitude - how else would you even find the planet (or stabilize your rotation rate for that matter)? So you could designate a waypoint from space and simply let the INS compute errors to it.

Being the future, it's probably a lot better than the Shuttle's IMUs in terms of error accumulation.

A heat shield can fail itself - I remember one killing 7 astronauts.


That's because mass is a huge concern on chemical rockets - so the heat shield is as light as the engineers could make it with very little margin for error (in fact, the toughest material is only used at the leading wing edges, and it had to be lightweight, so it is brittle, which turned out to be the weak spot).

If you'd slash half the payload capacity from the Shuttle and give that weight over to a heat shield, it's not going to fail...

Of course a gravity device can fail. But so could some other part


So knowing that you can be run over by a car, there's no point in washing your hands because you have one risk in your life anyway and germs do not matter then? Not sure whether this is how you organize your life, but most people do not :-)

Turbine engines can fail too, but even in the "primitive" world of today it is so improbable that twin-engine planes can and may cross oceans with passengers


Well, you're talking twin engines. They actually can reach an airport with one engine for starters - first safety net. Second, they don't just fall from the sky in case of a twin engine failure, an airliner has gliding rations above 1:20 - from 12 km cruise altitude, they can go more than 200 km - often enough to reach a runway. Second safety net. Third, in case they're over water they can actually ditch and have floating devices aboard to keep passengers alive till rescue arrives - third (tenuous) safety net.

I doubt they'd be certified if they had only one engine and would inevitably crash if that failed.
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Re: Spaceships and space flight

Postby Hooray » Sat Jul 23, 2016 6:20 pm

Turbine engines can fail too, but even in the "primitive" world of today it is so improbable that twin-engine planes can and may cross oceans with passengers


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Re: Spaceships and space flight

Postby Fritz » Sat Jul 23, 2016 9:44 pm

Thorsten wrote in Sat Jul 23, 2016 5:59 pm:The Python looks streamlined for atmosphere entry - there's no other reason to build a wedge-shape.

I don't think whoever designed that shape thought much about this! :) Actually, the Python isn't made for atmospheric flight in Elite, but the story is, that people wanted to have cool looking ships and didn't want to fly around in bricks (or Borg cubes). But the shape will probably do better than some airplanes from the pre-ww2-era and should allow speeds of several 100 km/h even at low altitudes. It is almost perfectly symmetric, so there shouldn't be any unwanted pitch moments. Perhaps the shape is even stabilizing in some way, because it is "fatter" towards the back. I still wonder how I could define reasonable aerodynamic properties of this thing in JSBSim, but I'll get back to this when I've finished the basic work. And I'll try if FG gliders are able to slip...

There's a number of other constraints you can derive from the fact that someone bothered to build the device in the first place but doesn't use it as main drive.

It is the main drive for atmospheric flying, like the wings of the space shuttle! I could (and probably will initially build) the ship with much stronger vertical thrusters instead of the gravity devices. But then we wouldn't have artificial gravity during our interstellar flight, and thrusters could fail themselves, and if one fails, we would lose lift force to correct pitching and/or yawing moments. Actually, I imagine the thruster to be more prone to failing (and overloading) because the gravity devices would be built to be able to run continuously for days or even weeks during long flights. So the chances of failing during a 3/4 hour descent are quite low anyhow.

Since you told me you don't want to alter basic space travel physics, I actually know a hell lot at this point (probably much more than your internet sources...) since I have a folder of concept studies for fictional spacecraft equipment calculated

And I thought I was the only one to be crazy enough to seriously think about these things! :) But I don't rely on internet sources. Actually most ideas do come from Elite (the hyperspace jumping), and from some pseudo-scientific magazines a friend of my parents used to have, when the internet didn't yet exist (like countering gravity by rotating heavy disks or free-energy devices using fancy arrangements of magnets and spinning parts). I even started to simulate this kind of flying in the late 1980s on my C-64 (without graphics of course, just some cockpit instruments), but the project was never finished and sadly seems to be lost, although most of my original C-64 diskettes have survived until today.

You're Captain of a space ship in a universe where spare parts are a few light years away. You have just arrived in a basically unexplored world (or is it? You don't know, could be outlaws hiding out here, could be smugglers,...). And the engineer you hired in the last system isn't exactly familiar with that piece of equipment (in fact, you're not even sure his diploma is genuine). (I suppose that's the Elite setup in a nutshell...)

That's not bad description!

You play it fancy - or you do it as simple and as risk-free as possible? [...] Right - so you really think they didn't play it safe or planned for multiple redundancy?

I will have redundancy, and a heat shield wouldn't really help if the gravity devices fail, simply because the ship would need the latter for flying level and for landing. The ship wouldn't burn up, but it would crash anyhow, falling to the ground with the terminal speed dictated by it's aerodynamic shape. Big parachutes would probably a better thing to have than a heat shield!

Gravity devices must be extremely reliable, even in a not so reliable world. Engines can fail, wings can break off, tires of cars can explode at 200 km/h, pilots can enter wrong waypoints in sophisticated auto pilots, and don't let me get started about helicopters (to be honest, I wouldn't dare to fly in one because I don't trust them). But these things usually don't happen. It's calculated risks that most people will take, And a pilot in the Elite universe, who will very likely die by being shot down by pirates, will surely take it! :)

You'd have an inertial navigation system of course and star trackers for attitude - how else would you even find the planet (or stabilize your rotation rate for that matter)? So you could designate a waypoint from space and simply let the INS compute errors to it.

Navigation in space is one of the interesting parts, and sadly it is the part that won't be possible to simulate in FG, because we would need a 3D representation of the galaxy with several 100 billion stars. As I mentioned, my hyperspace drives will need the ship's nose to be pointed in the direction we want to go. Going to a nearby star would be relatively easy, if we can see it with our bare eyes or with a small telescope (my first cockpit sketches include a telescope "aiming" screen. We can simply aim at the star and select a (known) distance in light years for our jump drive. Of course there will be errors, caused by bad aiming and because we can't enter the distance accurately, because the stars move relatively to each other, and the planets move too. So we will be near our target system after the first jump, but with a big probability somewhere well outside the inner system. We'll probably have to make one or two shorter jumps to be near enough to be able to see the inner planets. Earth is a very bright planet, it could be easily seen with bare eyes from everywhere in the inner solar system, and we can assume that this will be the case with other earth-like planets too. So we could make few jumps, each shorter than the previous one, to reach our target planet.

I don't know yet how long it will take to "charge" the hyperspace system - in Oolite it's 15 seconds - but I'll probably make it depend from the jump distance, but not with a linear correlation of course. Anyway we would be so fast (and inexact!) that we wouldn't have to think too much about how fast stars and planets are moving. We wouldn't do space flight as in reality, we wouldn't need to calculate orbits and movements of planets. If we can see our target, than we'll be able to reach it, even if our approach usually won't be optimal. But it doesn't have to be, because we don't burn fuel doing all this - we have free energy!

Of course a gravity device can fail. But so could some other part

So knowing that you can be run over by a car, there's no point in washing your hands because you have one risk in your life anyway and germs do not matter then? Not sure whether this is how you organize your life, but most people do not :-)

If you argue like this, you should insist on getting a parachute when you enter an airliner, and wear a helmet when you drive a car! ;)

Last but not least, the Python simply does not have a heat shield, and I intend to use the model with as few changes as possible. Its underside could be a little heat resistant, perhaps by using titanium instead of aluminium, to reduce the risk of pilot misjudgements ending in burnup, but that's all I can do!
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Re: Spaceships and space flight

Postby Thorsten » Sun Jul 24, 2016 5:35 am

Actually, the Python isn't made for atmospheric flight in Elite, but the story is, that people wanted to have cool looking ships and didn't want to fly around in bricks (or Borg cubes)


Last time I checked (and that's an eternity ago on the C-64 I guess) Elite didn't have any real-world physics. Planet's didn't orbit, they were just at a fixed location in space. Thruster operation didn't add momentum vector, it set the momentum vector (if you cut throttle, you'd be stationary). Stations didn't orbit either, they were just there. And you couldn't land on anything anyway.

So sure - you start out with cool-looking ships.

But once you decide to have real-world physics, you might as well ask what the shape means. It's a decent lifting body outline for hypersonic aerodynamics. It has a nice blunt body to detatch a shockwave, it has no vertical stabilizer fin, meaning it'll be yaw unstable, but then for a normal hypersonic attitude it'd be outside the airstream anyway.

I still wonder how I could define reasonable aerodynamic properties of this thing in JSBSim


Put the shape into a computational wind tunnel.

It is the main drive for atmospheric flying, like the wings of the space shuttle! I could (and probably will initially build) the ship with much stronger vertical thrusters instead of the gravity devices.


The issue with VTOL by thrusters is that for a ship that heavy, you end up frying the landing site badly. There's enough energy dissipated to melt stone.

I will have redundancy, and a heat shield wouldn't really help if the gravity devices fail, simply because the ship would need the latter for flying level and for landing


As I tried to indicate, it's a huge difference if you have a failure on a managed glidepath or you just fall down.

If three Shuttle engines fail at 5000 ft/s inertial speed, you're in deep shit and will probably die. If three Shuttle engines fail at 18.000 ft/s inertial speed, you'll not even notice you're on an emergency landing, it'll all look regular. And that in spite of the fact that you need to shed a lot more energy while entering the atmosphere.

Why?

Because in the first case there's nothing that keeps you up - you'll fall from 120 km altitude right down into the lower atmosphere - that's a good 90 km free-fall.

Whereas in the second case, there's a sizable centrifugal force that lifts you initially - you'll never even free-fall! As you hit the atmosphere, you'll be fast, so lift will come early in the thin part of the atmosphere to replace the centrifugal force as you decelerate. There'll be in fact too much lift - you're in serious danger of bouncing back into space if you do nothing!

From the first case, you need a thruster capable of 3 g or more working to abort from halfway int the trajectory back into space. In the second case, some 0.3 g is enough to push you back out from halfway down.

Gravity devices must be extremely reliable, even in a not so reliable world.


Why? That'd be a first.

Navigation in space is one of the interesting parts, and sadly it is the part that won't be possible to simulate in FG, because we would need a 3D representation of the galaxy with several 100 billion stars.


Well, that requirement is unlikely to be met by any computer program existing.

So we could make few jumps, each shorter than the previous one, to reach our target planet.


To discover that you're at rest and Earth moves with 30 km/s... The navigation problem is to match position and velocity, not position.

If you argue like this, you should insist on getting a parachute when you enter an airliner, and wear a helmet when you drive a car!


If pilots of some airline would habitually insist to do bad weather approaches by the seat of their pants rather than ILS, I'd probably not fly it. And if parachutes would help in air travel (or helmets in cars) they'd be mandated by regulators, but the fact of the matter is they do not.

Whereas real-world airliner operation has multiple redundancy procedures (as explained above) and does not rely on 'seat of pants' or 'single piece of equipment must be working' and is hence a very safe mode of travel.

Anyway, it's clear you're set how this thing is to work, and if that's what you enjoy, then that's what you should do. Personally I can't think of any context in which this makes sense, and I'm much more interested in the problem 'if you had such and such equipment, how would you really organize space travel?' - but since that's not what you're interested in, I guess we'll just leave it here, there's little I can contribute to your vision of space travel.
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