Maneuver Drive - How does it work?

[flykiller]

Is there a good reason why non-streamlined ships can't use contragrav to lift off from a planet with an atmosphere and then switch to their m-drives when in orbit?

nope. sounds like a great idea. you could make it part of the engineering section - 3% jump drives, 2% maneuver drives, 5% contragrav.

imtu maneuver drives are gravitic, and naturally oriented to a single direction, usually aft. they can be aimed down though, causing them to act like contragrav, at reduced effectiveness and increased strain which limits their time in this role. thus the streamlined M2 ship glides in, reorients its maneuver drives to slow and hover, then lands, before the drives overheat and fail. this works well for a ship that takes off and lands, but if it is to spend much time hovering in a gravity well then it will want a higher maneuver rating or contragrav.

 

[far-trader]

Originally posted by Shaira:
Hi all,

I've been thinking about manoeuvre drive and its relationship to contragrav, specifically with reference to streamlining on ships for takeoff and landing. Apologies if this has been done to death already - please point me to a good answer if there is one!

Here's the conundrum: the OTU says that non-streamlined ships can't land or takeoff on worlds with an atmosphere. However, the OTU also says that airrafts and other grav vehicles (and by implication grav-floating cities too - my assumption) can reach orbit "in a few hours" on worlds with an atmosphere. Is there a good reason why non-streamlined ships can't use contragrav to lift off from a planet with an atmosphere and then switch to their m-drives when in orbit?

My take on it...

Actually part of the answer is a critical assessment of what is actually said and implied.

True, Unstreamlined ships may not enter an atmosphere.

True, Air/Rafts (decidedly Unstreamlined) may make orbit in some hours (though atmosphere is not specifically ruled it is suggested by the fact that vacc-suits are recommended for such trips).

Nowhere does it say that an Air/Raft may deorbit and land on a world.

I think it's quite reasonable to presume that any vehicle, craft, or ship, regardless of size or configuration can make orbit. Maintaining orbit above an atmosphere and deorbiting safely into an atmosphere are entirely different beasts and only advisable for properly streamlined designs.

In MTU I take this to mean sure you may try to take an Air/Raft into orbit. It won't be easy, or safe. And your time there will be limited. Then you start to come down and burn up.

And sure you can attempt to deorbit your unstreamlined ship but parts of it are going to be destroyed (ripped off, burned through, etc.) and there is a good chance that damage may cascade and lead to a spectacular multi-million credit fireball and hundreds of kilometers of debris spread below.

In other words, it's not done routinely, but if the story calls for a heroic and/or desperate action I'll allow the attempt after making the players aware of the extreme hazard and risk.

 

[Andrew Boulton]

Landing unstreamlined ships is possible, but very dangerous because they don't have the necessary control systems. They're very vulnerable to winds, especially.

 

[flykiller]

Then you start to come down and burn up.

if one is moving at orbital velocity, sure. and the phrase "orbit" does imply an orbital velocity, sure. but with maneuver drives and contragrav it's not necessary to have an orbital velocity to achieve or descend from an orbital height.

 

[boomslang]

Originally posted by flykiller:
</font><blockquote>quote:</font><hr />Then you start to come down and burn up.

if one is moving at orbital velocity, sure. and the phrase "orbit" does imply an orbital velocity, sure. but with maneuver drives and contragrav it's not necessary to have an orbital velocity to achieve or descend from an orbital height. </font>[/QUOTE]Indeed; many players don't realize that there's no need for those stereotypical spectacular & dramatic fiery re-entries except under emergency circumstances. IMTU, we typically pull up into a near hover while still outside atmo, and then fly down at a graceful and sensible (subsonic, even) velocity to the LZ.

 

[far-trader]

I think there is every need for the typical hot re-entry in most cases. I don't see contra-grav alone allowing one to simply hover and descend vertically. Certainly that isn't the implied power available to the Air/Raft which takes hours to get to orbit. It appears to have to spend a lot of time climbing and I imagine it's due to a rapid drop off of performance as one gets farther from the gravity, dropping to near zero at LEO.

Contra-grav is described as negating nearly all local gravity from the equation for lift. Presumably the Air/Raft has additional grav modules to actually fly.

Even accepting the "settle into atmosphere vertically like a gently falling leaf" if that's how you allow contra-grav to work there's still high atmosphere winds to deal with, something that will ruin the day of an unstreamlined hull no matter how you slice it. Like dropping a leaf into a hurricane.

Space craft may be able to make straight in drops, if that's how their main thrusters are aligned or allowed to function. But most cannon designs aren't built that way. And again you have to deal with wind shear.

I also think there should be a size limit on safe deorbit. Landing a streamlined multikiloton starship seems inadvisable just from the surface area being exposed to upper atmosphereic winds as you descend. And forget even trying it if it's unstreamlined.

Or am I missing something?

 

[Anthony]

There are two major issues with how Traveller handles streamlining:

1) Ships are massive, dense, and have huge amounts of excess power. They aren't exactly dirigibles. Winds aren't going to be more than an irritant.
2) Weather when skimming a gas giant is going to be worse than anything you'll find on earth. If you can refuel from gas giants (canonically, everything but dispersed structures and asteroid hulls can) atmospheric landing isn't going to bother you.

The most likely issue that would prevent a lot of spaceships from landing is the lack of landing gear.

 

[Commander Drax]

IMTu hot de-orbits are a rarity - after all the modern day space shuttle necessitates a hot re-rntry only because it can't carry enough fuel to decelerate fully prior to atmospheric insertion - with an M-drive that's gravity based you've typically got unlimited fuel - thus you should be able to slow down to the point where heat build up is at least trivial or relatively minor.

MegaTraveller materials all supported the fact that air/rafts could routinely enter and leave atmosphere without any difficulty provided they were sealed - otherwise vacc suits would be needed. After all there's the Donosev Class Scout Surveyor (detailed in WBH) that routinely has a complement of 3 hurricance class air/rafts or 1 kankurer (I think that's how it's spelt) G-Carrier which is essentially a large and comfortable extended duration air/raft. Don't forget the Donosev was unstreamlined (in the MT rules) so could'nt enter or leave atmosphere itself.

On the subject of MT - vechicles of less than 20 tons displacement had grav modules - which depended on the local gravity field for thrust whilst starships and spacecraft above 20 tons could use either anti-grav thrusters (that also depended on the local gravity well) or the standard Thruster drive that didn't depend on local gravity to function - allowing it to be used even in deep (read empty hex) space!

Still there's a certain elegance to the FFS1 concept of all vehicles/vessels being supported by contra-grav lifters that simply negate 99 of gravitational attraction whilst a secondary drive such as Heplar provides the thrust - even with TNE rules - a vessel wouldn't need to conduct a hot re-entry unless it was desperately short of fuel - this would make a great emergency scenario!

 

[MR TEK]

Hey.

Remember, anyway you approch the planet, there will be a speed differntial betwen your orbit and the ground. The air/raft may not have to thrust to orbit in a few seconds, but it does have to reach the same velosity as a ship in a sytable orbit to dock with that ship.

Come straight in, and the grond is rotating benenith your feet. (the number bandied about is 1000 miles per hour. The erth has a circomfrance of 24000 miles, so yeah, I can by the ground moving 1000 across the face of the sphere)

That means that if you managed tapproch the planet in a straight line, never went to orbit and tryed to land, the ground would be moving at 1000 miles per hour relitive to your speed.

If in orbit, a low earth orbit is just a little over 24000 miles, but an orbital periode of 90 min, so maybe 18,000 miles per hour.

There is only one way to change velosity, and you can do it quickly which requires the ability to withstand larges differneces in velosity between your craft and the sourrounding air, or slowly which minimizes the air flow, and hence the structural requirements, but takes a very long time to or from orbit.

Settling like a leaf is not an option on a spinning planet. if you are staionary above a given spot you have to slow VERY quickly to stay above the same spot, including some hefty changes in velosity in atmospher, to slow from your orbital velosity to ground velosity.

If you are moving in s traight line to the center of the planet then you will have to change velosity to "catch up" to the Ground,again, some of that would have to occure withing teh atmosphere. .

 

[Andrew Boulton]

In FF&S1, contragrav only negates mass, but in FF&S2 it also provides thrust.

 

[atpollard]

An air raft in a stable orbit at 1000 km altitude, could fly down to and altitude of 100 km (still above the level of atmospheric interference) and slowly decelerate to 100 kph. At this point, the air raft is simply flying at a leisurely speed and a very high altitude. It then just lands like any other grav vehicle or aircraft.

Given a reactionless drive and very long power duration, what would prevent this?

 

[MR TEK]

Exactly why an air/raft would take hours to and from orbit.

Any ship could reproduce the same manuver, with the same timeframe.

Hence, any ship structurally sound enough that it could stay in one pieceunder outside gravity, and not be torn apart by the winds that could occur in the atmosphere can successfully land given enough time to match the velosity of the ground.

Taking off is still hotly debated depending on the manuver drive "thrust" exceeding local gravity.

So allowing time to change velosity would be the determining factor.

A high acceleration ship, streamlinded and built for atmosphere would take minuets to and from orbit.

That deep space sqarish hull that lands once a year maybe, would take several hours and would be at the mercy of high wind conditions.

Also if it only had Man-1 it would be limited as to what worlds it could lanch itself back into space under it's own power.

 

[atpollard]

There is also a basic structural design difference between a ship designed to land and one designed to operate only in space (I think that is why streamlined hull shapes are generally more expensive than un-streamlined hull shapes in CT).

In space, the ship needs to be able to resist the linear forces in the direction of thrust (so the back of the ship needs to support the “weight” of everything in front of it at maximum acceleration) plus the rotational forces of the ship turning around it’s center of mass (so some of the members near the center can be weaker than some of the members near the outer edge).

When a ship lands on a planet, the bottom layer needs to be able to support the weight of all of the layers above it (this makes tail-sitters useful since “down” on the planet is also “back” in space and both require strengthening of the same members). Plus the obvious need for landing gear.

Landing an unstreamlined ship may be possible, but the ship will need to be designed to land or it may crush the lowest decks. (As a referee, I would eliminate any cost savings from the design of an unstreamlined landing-capable ship).

Hope that helps.

 

[Anthony]

For a ship with 1G of acceleration to slow from orbital velocity to matching velocity with the atmosphere takes about 100 seconds per point of planet size. At that point, it's a matter of what stable speed is in atmosphere. An unstreamlined ship is probably limited to around 100 kph at low altitudes (below, say, 20 km), which means it takes another 10-15 minutes to land.

 

[sarahnewton]

Hi all,

Wow - thanks for everyone's input! There's plenty of food for thought here - I'm plagiarising freely!

I must admit though I’m still thoroughly confused with trying to make sense of the OTU. I'm trying to keep my thinking within canon boundaries, where possible, so am taking it as a “fact” that only streamlined ships can land on worlds with an atmosphere, and trying to understand why that must be the case. Partially streamlined ships can skim the upper atmosphere; unstreamlined ships cannot enter the atmosphere at all.

The fact that unstreamlined ships CAN land on 1G worlds with no atmosphere (if you can find one…) probably means that they don’t crumple under their own weight & they probably do have landing gear which works. It also means that it’s the fact of the atmosphere that’s significant, rather than the gravity. Presumably this has to mean it’s the wind / friction / heating effects experienced by a starship attempting to land or take off which prevent unstreamlined ships from landing or taking off.

QUICK ASIDE:

I’m not sure if the old DGP Starship Operator’s Handbook ever made it into canon. Probably not. However, I did like the description (and limitations) of the thruster plate technology it gave. Namely, that thruster plates were directional; 100% in one direction (usually aft), 10% in the opposite direction (forward), and 25% laterally (including up and down). Plus, the fact that thrusters could be overpowered by up to 140% for extended periods, enabling a 1G drive to escape a 1G gravity well; also, that the thrusters could be overpowered by up to 400% for 5 mins or so, enabling a 1G drive to “hover” during takeoff or landing.

Whether the above is canon or not, the attempt to limit the functions of thruster plates and contragrav modules is important. If contragrav simply “negates” the effects of gravity on an object, allowing it to float, that is a different beast than if it lets it fly willy-nilly in any direction and generally play fast & loose with acceleration / deceleration. Likewise, if thruster plates have to be oriented in a certain direction to provide full-G thrust, then that has implications for the direction a starship hull is pointed in when entering an atmosphere (if you want 1G acceleration, you have to enter the atmosphere backwards, ie aft-first, which pretty much renders your streamlining rather less useful).

END OF ASIDE:

So, back to wind / friction / heating during landing. If we assume that thruster plates and contragrav have clearly defined but limited effects, like for example the above, then any starship entering a rotating atmosphere is going to come up against the need to speed up or brake to match the atmospheric speed (likely to brake). Even if we assume contragrav and therefore the lifting effects of a streamlined hull are relatively unimportant, the starship is nevertheless going to have to stay stable within the atmosphere to be able to adequately manage its descent – hence the need for a streamlined hull structure. At its simplest, perhaps an unstreamlined hull simply is not capable of staying sufficiently stable during an atmospheric descent through the various braking and orientation manoeuvres required.

I admit I’m fishing around a bit here. I’ve probably watched ET too many times to not believe that an unstreamlined hull equipped with contragrav couldn’t land on a world with an atmosphere, no matter what the Traveller rules say. Perhaps there’s an issue with contragrav – it can’t be hull size restricted, cos you have floating cities. Perhaps there’s some reason I haven’t come across why ships can’t use contragrav? Because they clash with m-drives? Or maybe hull stability is the answer after all?

Hmm. Perhaps I need to go away and read up on some orbital mechanics…

Please keep the comments coming guys, it’s really helping (despite the ramblings above!).

Happy travelling,

Sarah

PS - apologies for the LONG post. Note to self: must get life

 

[flykiller]

(if you want 1G acceleration, you have to enter the atmosphere backwards, ie aft-first, which pretty much renders your streamlining rather less useful)

not if the hull-shape is sphere or spheroid.

traveller cannon is not coherent. it was slapped together haphazardly over a decade, with each individual piece tossed in not because it fit with the rest but, near as anyone can figure, mostly because it sounded cool. just take it as a starting point and use what makes sense to you and your players.

 

[sarahnewton]

"traveller cannon is not coherent. it was slapped together haphazardly over a decade, with each individual piece tossed in not because it fit with the rest but, near as anyone can figure, mostly because it sounded cool. just take it as a starting point and use what makes sense to you and your players."

Absolutely - and that's precisely what I've been doing for over a quarter of a century! However, I'm just interested in people's current take on the unstreamlined / manoeuvre / contragravity thing, and particularly whether any consensus has arisen out of the mishmash of contradictory sources out there. Personally, I'd like to come up with a reasonable explanation of why unstreamlined starships can't land on worlds with atmospheres - although I'm certainly able to function on a day-to-day basis without the answer

BTW - out of interest I was checking the TML for similar info, and found a thread from May 1996 trying to sort out this very question. Urk! Nice to know we're contributing to a great tradition...

 

[Commander Drax]

I'm pretty sure that DGP's starship operators manual is still considered canon, although I don't pay that much attention to it as science has moved on somewhat since its writing.

When you consider the fact that high speed re-entry needs to be done at a shallow angle then the distance through the atmosphere increases giving more time for the ship to be at risk from wind shear - also unstreamlined gravitic vehicles are limited to a maximum of 300 kph in atmosphere probably because they become difficult to control above these speeds.

Also if I remember correctly there was no airframe configuration in CT book 2, ships were either unstreamlined, or streamlined. I'm under the impression that the Airframe configuration used in later traveller releases was equivalent to streamlined in CT Other options were added later. Hence the resulting confusion.

IMTU - I only apply the rules to non gravitic vessels, with fast burn short duration drives such as rockets at which point the rules make sense. To me streamling affects the time you can make the descent and landing in and the vessels performance and agility in atmosphere.

 

[TheEngineer]

Hi !

Originally posted by Shaira:
Hi all,

Wow - thanks for everyone's input! There's plenty of food for thought here - I'm plagiarising freely!

...

END OF ASIDE:

So, back to wind / friction / heating during landing. If we assume that thruster plates and contragrav have clearly defined but limited effects, like for example the above, then any starship entering a rotating atmosphere is going to come up against the need to speed up or brake to match the atmospheric speed (likely to brake). Even if we assume contragrav and therefore the lifting effects of a streamlined hull are relatively unimportant, the starship is nevertheless going to have to stay stable within the atmosphere to be able to adequately manage its descent – hence the need for a streamlined hull structure. At its simplest, perhaps an unstreamlined hull simply is not capable of staying sufficiently stable during an atmospheric descent through the various braking and orientation manoeuvres required.

I admit I’m fishing around a bit here. I’ve probably watched ET too many times to not believe that an unstreamlined hull equipped with contragrav couldn’t land on a world with an atmosphere, no matter what the Traveller rules say. Perhaps there’s an issue with contragrav – it can’t be hull size restricted, cos you have floating cities. Perhaps there’s some reason I haven’t come across why ships can’t use contragrav? Because they clash with m-drives? Or maybe hull stability is the answer after all?

Hmm. Perhaps I need to go away and read up on some orbital mechanics…

Please keep the comments coming guys, it’s really helping (despite the ramblings above!).

Happy travelling,

Well, at least the MT ruleset uses contra- or antigrav locomotion for TL9-TL10 ships.
So I would not differ between both locomotion types m-drive or contra-grav regarding the behaviour of a ship during the decent process.

IMTU typical spaceships do no "hot-reentries", which is somehow resembled in the MT rules by setting top atmo speed even for streamlined ships to 1000 kph.
Anyway, even a unstreamlined ship is able to move at max 300 kph according to the rules.

I strongly agree with the approach to say, that stability during atmo flight really is the important issue. Maybe those maneuver grav drives are pretty powerful, but their reaction time, meaning the time they need to be adjusted to other external situations is just very slow, so that they are hardly able to get along with the turbulences typically around in the atmo layers.
The flight behaviour of an unstreamlined vessel during atmo reentry would perhaps be like a small balloon loosing his air rapidly, because the direction of thrust is mainly defined and influenced by the environment...
Streamlining reduces the sensitivity regarding turbulences to an amount, which could be handled even with the grav maneuver drive technology, so that the ship could navigate.
Streamlining might also simply include, that instruments and sensoric stuff on the hull is attached retractable or at least somehow protected, so that they are not ripped apart by a 300 kph jetstream...

Hull stability might be indeed be another factor, amplifying the problem, as the unstreamlined vessel not only starts to tumble around, but perhaps also breaks into pieces .... quite bad.

Best regards,

TE

 

[Merxiless]

I just always assumed that an Unstreamlined ship would burn up, upon entering atmosphere.

I figured that for taking, off, the wind shears upon crossing the jet stream would be a factor in losing critical parts of the ship, like dishes, and antennae and so forth.

I don't typically worry about it too much..it's more for me like:

"The Gypsy Queen travels from the black of space to orbit. The curve of a planet is seen in the background...some time later, your party emerges from the extended ramp, blinking in the sunlight of the docking port."

I don't worry about the details, other than the scenes with characters in them.

For me the engines are: "That blue glow of the Millenium Falcon, as it blasts out of Mos Eisley"...whatever that is. If it's M-Drive, fine. Or Heplar, or whatever.

Since none of it is real, I don't worry about how it might work, and press on. If it get's characters to orbit, and other solar systems, well and good.