Re entry in a grav vehicle

[Anonymous]

I don't recall this being discussed and this may be one of those questions that when written down, answers itself, if not, I'm hoping the hive mind can throw some light on it - this is, of course, all speculation as there are no grav vehicles, yet!

There is plenty of discussion of the vagaries of reaching orbit and interplanetary travel in grav vehicles but I don't recall any discussion of the physics of getting to and from orbit.

My knowledge of this in the real world is sparse at best, please forgive any glaring errors.

To orbit the planet you have to maintain a speed that varies with altitude above the surface. If your grav vehicle is immune to being dragged to the surface by gravity does it still need to maintain a speed relative to the planet or does it simply hover as the planet rotates below it?

Under what circumstances would a craft choose to not use its anti grav to maintain altitude and accelerate to a speed that will allow it to not re enter? Lack of fuel, damaged grav plates spring to mind but are there others?

On reentry does a grav vehicle that has not needed to be traveling at high speeds to maintain orbit, need a heat shield on reentry?

Or does it simply act like an elevator and descend at what ever speed the pilot chooses with minimal friction as the craft interacts with the atmosphere?

If so, as the craft descends thru the atmosphere and the rotation of the planet/atmosphere interacts with the craft, is the speed of rotation enough to generate heat that requires a heat shield or would a craft simply match the rotational speed of the planet as it descends?

 

[Lord High Munchkin]

I would assume that once it reaches altitude, it stays in position (i.e. at great, but fixed, rotational speed).

Going against the direction of spin might be an issue.

 

[simonh]

A grav vehicle could fly up to orbital altitude without actually accelerating into orbit, and hover under power. It could then descend back into the atmosphere under power and at low enough speeds not to experience re-entry heating.

The main reasons for wanting to accelerate to orbital velocity would be to be able to de-activate the grav drive for an extended stay, to drop off cargo/payload that you do want to be in orbit, or to rendezvous with another vehicle that is in orbit. They best way to do this would be to fly out of the atmosphere first, then once in vacuum accelerate up to orbital speed. If the vehicle can’t stand a ‘hot’ re-entry, to return to the surface you’d just reverse the manoeuvre. First decelerate from orbital velocity down to the rotational velocity of the planet surface below, using the grav drive to maintain altitude above the upper atmosphere, then descend into the atmosphere under power.

Note that the quickest way to get from one place on the planet's surface to another very distant point, might be to rise out of the atmosphere so your acceleration and top speed isn't restricted by drag. That doesn't necessarily mean you'd go into orbit though. You'd just go on a high ballistic trajectory, but it might turn out that the vehicle would actualy reach oribital velocity before decelerating and returning into the atmosphere.

Simon Hibbs

 

[Reynard]

Traveller has a wonderful technology in the form of gravitic physics. Before the discovery of gravitics, space and atmospheric locomotion depends on reaction momentum. Vehicles normally must move forward and are always affected by gravitational forces when operating at high altitude and orbital distances.

Gravitic vehicles, which includes space and star ships, have a lot more versatility since you work with gravity rather fighting it. Grav vehicles can adjust their velocities to below frictional heating when dropping towards a planetary body. Reaction drives often need to save fuel and make use of gravity but need shielding against friction and need HUGE flight paths to land.

The design of grav plates in conjunction with grav thrusters allow true three dimensional movement. That's why air rafts and G tanks don't need wings even though they can operate well above Nape of the Earth. I do believe the 'pop-up' maneuver is still done by military G vehicles and once only the realm of Harriers and helicopters. This also explains why most downports have landing pads alone or in addition to landing strips. Even the strips are more likely Short Take Off and Landing.

 

[dragoner]

Heat upon re-entry is due to friction to scrub off speed, LEO speed is 15,430 mph to 17,450 mph; a real need to slow down when coming back. These speeds a grav vehicle would need to orbit without it's anti-gravity turned on, basically going fast enough to fall around the circle.

As per other speeds, using the earth as an example, at the equator the earth rotates at about 1000 mph, and moves in it's own orbit around the sun at 67,000 mph.

 

[Anonymous]

Which brings up another interesting issues. Skimming gas giants for fuel, I don't recall stats printed in any Traveller book for surface G on GGs. Now I know that the term "surface" is rather loosely applied to GGs but looking up Jupiter's it's 23.1 m/s (from here: http://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html)

Roughly 2.5 times that of Earth. Can a Man-1 ship skim a GG with similar gravity? I'd say not but does any one "model" with this degree of accuracy?

 

[Reynard]

GG atmospheres are also way above the core. There must be enough to skim at safe distances. If we're going to bring up reality, remember how intense GG radiation emissions can be. Be thankful we play in a Traveller Universe.

That way we can... accept there was a Ahzanti High Lightning scenario involving dropping onto that cruiser type floating in a methane sea loaded with monsters on a gas giant!

Oh, MegaTraveller World Builder Handbook creates gas giant gravity.

 

[dragoner]

An M1 ship has 1G acceleration, no? That is 9.81 m/s^2, so with that being it's rate of acceleration, it would not necessarily be able to lift from the surface; with enough speed it could intersect or "skim" the atmosphere in an elliptical pattern.

 

[dragoner]

GG atmospheres are also way above the core. There must be enough to skim at safe distances. If we're going to bring up reality, remember how intense GG radiation emissions can be. Be thankful we play in a Traveller Universe.

That way we can... accept there was a Ahzanti High Lightning scenario involving dropping onto that cruiser type floating in a methane sea loaded with monsters on a gas giant!

Oh, MegaTraveller World Builder Handbook creates gas giant gravity.

Yes, and I'm of the opinion, born through experience, that when faced with actual science, most players devolve to paralysis. Which is fine, but a reason why "Adventures in Science" type campaigns fail, I think there is a certain romanticism for it in the sci-fi community which when faced with it, is not so glamorous.

 

[sideranautae]

To orbit the planet you have to maintain a speed that varies with altitude above the surface. If your grav vehicle is immune to being dragged to the surface by gravity does it still need to maintain a speed relative to the planet or does it simply hover as the planet rotates below it?

No, you don't. If the grav drive can lift you to X altitude it can hold you there and lower you too.

 

[sideranautae]

Which brings up another interesting issues. Skimming gas giants for fuel, I don't recall stats printed in any Traveller book for surface G on GGs. Now I know that the term "surface" is rather loosely applied to GGs but looking up Jupiter's it's 23.1 m/s (from here: http://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html)

Roughly 2.5 times that of Earth. Can a Man-1 ship skim a GG with similar gravity? I'd say not but does any one "model" with this degree of accuracy?

If the ship stays at or above orbital velocity it can skim. It would skim the outer atmosphere at speed. A highly elliptical orbit skimming shallowly once per orbit.

 

[Anonymous]

Yup, that's what I thought, the ship "hits" the atmosphere at high speed in order to be able to escape which means it's going to need a heat shield.

Either that or travel further to a smaller GG with lower G.

 

[sideranautae]

Yup, that's what I thought, the ship "hits" the atmosphere at high speed in order to be able to escape which means it's going to need a heat shield.

Either that or travel further to a smaller GG with lower G.

Yep, momentum + accel carrying them through to the "other side". Trav ships that are set up for skimming would have their leading edges insulated. The hulls' built in ability to shrug off micro-meteors hitting at thousands of KM/sec means that the hulls wouldn't even notice that level of heat.

Example: Small craft with NO armor (normal hull) and a 6G MD. Trip from Earth to Saturn has a max velocity (and close to this during a significant % of trip) of 9,160 km/s. At that velocity when hitting something as small as 1/10 of a gram will generate energy equal to:

= 4,195 Mj
= 4.19528E+16 erg
= 4,195,280,000 newton-meter
= 3,976,358 BTU

which the hull must withstand and dissipate.

 

[dragoner]

Yup, that's what I thought, the ship "hits" the atmosphere at high speed in order to be able to escape which means it's going to need a heat shield.

Either that or travel further to a smaller GG with lower G.

The ship's hull would be considered a heat shield for intensive purposes, too much also happens, irradiation from cosmic rays, flying through a hot nebula, that would effect the crew without the hull being a near impermeable barrier. Ships in sci-fi have a tendency to be magic carpets, which is fine because it keeps the story going; I accept it on the basis of it's the future, and currently, interplanetary vehicles don't exist (the only spacecraft we have built being orbiters for manned stuff). We'll know more one day when we do it. It's not worse than say a "hover tank" for example, looking at concentrated thrust, weight vs lift, multiple times past what a commercial jet has. A high-decibel screaming, pavement splitting, sod throwing vehicle that acted like an air hockey puck when it fires its main armament, isn't very realistic either, but is a staple of sci-fi.

 

[Reynard]

We were amazed not that long ago when science created that heat bleeding ceramic our spacecraft get shielded with. Now its just part of the structure. What next step in commercial application will have the TL 8+ ship hulls combined with to routinely shrug off heat, minor radiation and micro particles without a fuss to its existence. That's why it's a pleasure we don't have to know EVERY tiny detail why Traveller works.

 

[dragoner]

That's why it's a pleasure we don't have to know EVERY tiny detail why Traveller works.

^^^ This. When talking about vehicles that have literally millions of intricate parts that are intrinsic to its function, it is easy to get buried under the various considerations taken. As well as the more detail one goes into game-wise, such as with design processes, the more likely the amount of errors to crop up, esp with a cascade effect.

 

[Anonymous]

we don't have to know EVERY tiny detail why Traveller works.

True. The original question came from visualising how ships made planet fall, filling the gaps where I can makes for a better game for me, I like the detail but yes, it can go too far.

 

[Reynard]

I understand that and also love to fill in the gaps. Just don't need this particular rule system bloated like a tick with minutia. One of the things that appeals to me about Traveller is the willingness of the community to add to the game as we've been doing on this tread.

 

[dragoner]

I think we all agree that spacecraft are cool. 8)

 

[sideranautae]

I think we all agree that spacecraft are cool. 8)

Oh yes! And, for me, the coolest design even if not logical is the Type S. Something about that one...