Going hella fast in-atmosphere

[flyingsandwich]

Having realised that my players will probably try to chase somebody in-world using their ship instead of a land vehicle, I wanted to see how long it'd take them to travel the 12km to their mark in their Thrust-1 ship.
Well, in space it'd take them just under 49 seconds, and so long as they didn't mess around with trivial things like braking, they'd reach speeds of over 1700km/h.
Which got me thinking: What's the common agreement on using a streamlined ship to go really fast in an Earth-like atmosphere? There must be a point where it'd just come apart from the stress, but I don't know what it is. Type A Free Trader, if it matters.

 

[Wil Mireu]

It's probably too much information, but this is an interesting read if you want to know how the Space Shuttle did it. http://www.nasa.gov/centers/johnson/pdf/584730main_Wings-ch4d-pgs226-241.pdf (the re-entry velocity profile is at page 11)

 

[ShawnDriscoll]

There must be a point where it'd just come apart from the stress, but I don't know what it is.

Who is the role-player in your group?

 

[Wil Mireu]

There must be a point where it'd just come apart from the stress, but I don't know what it is.

Who is the role-player in your group?

What does that have to do with anything?

 

[GypsyComet]

Going by non-Mongoose material, a "Streamlined" hull like a Beowulf tops out at around 1000kph in standard atmosphere. It'll turn into a wild ride above that, but starships are built to be sturdy and take a lot of heat, even the "unarmored" civies (who dive gas giants to save a buck).

 

[flyingsandwich]

1000kph seems good. I ended up finding the thread where they discussed it extensively, and I'm a little ashamed to admit my eyes glazed over when they started getting really into it.

Who is the role-player in your group?

If that's a comment about how I'm focusing too much on what's realistic instead of thinking about what's fun, know that I'm a relatively new GM and often find myself getting caught in minutiae like this. I like to give my players some consistency, I suppose, and I think finding out just how fast their ship can fly in atmo is quite cool.

 

[ShawnDriscoll]

Who is the role-player in your group?

If that's a comment about how I'm focusing too much on what's realistic instead of thinking about what's fun, know that I'm a relatively new GM and often find myself getting caught in minutiae like this. I like to give my players some consistency, I suppose, and I think finding out just how fast their ship can fly in atmo is quite cool.

So no one then.

 

[Wil Mireu]

So no one then.

And who are you to judge, exactly? Not everyone likes throwing a bit of realism out of the window just for the sake of "roleplaying" or "the story". Different groups want different things out of the games they play, and nobody here has the right to sneer at anyone for doing that. And the OP certainly doesn't have to justify himself, his group, or his interest in the subject to you either.

 

[flyingsandwich]

So no one then.

You're not serious, are you? If this sort of stuff didn't matter in the Traveller system, then page 145 of the core rulebook would have a section called 'fuck it, do what seems dramatically appropriate'. In reality, it's called 'Travel Times' because Traveller is a fairly crunchy system. We're all roleplayers above all, and I'd like more than one sentence from you on why internal consistency is a bad thing in a scifi roleplaying game.

 

[dragoner]

You're not serious, are you? If this sort of stuff didn't matter in the Traveller system, then page 145 of the core rulebook would have a section called '**** it, do what seems dramatically appropriate'. In reality, it's called 'Travel Times' because Traveller is a fairly crunchy system. We're all roleplayers above all, and I'd like more than one sentence from you on why internal consistency is a bad thing in a scifi roleplaying game.

He is kind of a turkey, don't let him get you down, there are plenty of helpful and friendly people here. Definitely if you like science, you will see many discussions of it here because of the Traveller community's focus on hard sci-fi.

Cheers,
Rob

 

[Captain Jonah]

Are they going to be coming down from orbit or taking off from the planet, one is going to be building up speed to catch the target, the other is going to be about slowing down enough to catch them.

If you are going up then anywhere around the listed 1,000 Kph is useable, plus its in the rule book so it has the advantage that you can point it out because that's what the rules say.

Coming down from orbit is a very different case, you can fly down carefully using your drive to keep your speed down and use the same speed limits or you can belly dive into the atmosphere and have that fun mach 24 ride down as a ball of fire.

The Free and Far trader or fat trader are all good designs for atmospheric belly flops.

Somethings to bear in mind.

You can push your speed above the listed by turning yourself into a missile but turning or any kind of course change then becomes interesting, even a streamlined ship at speeds above its rating will run into problems if it tries to turn and catches the airflow at the wrong angle.

Also slowing down, if you are chasing something slow and it stops as you reach it you will fly straight past at high speed and they can just turn around and fly the other way.

Something else, a starship is simply not going to be as maneuverable as an airfoil design in an atmosphere. You can catch that air car of aircraft but it can fly rings around you.

 

[flyingsandwich]

The target will be a crashed air/raft, 12km away from their position. So they'd take off, fly there, and land. Of course, there are creatures waiting in the forest for them that will get lonely/hungry if they do that, so I'll just emphasise that the canopy is dense as shit for miles around when they land in the hole that their fugitive made. Still, this might come up again so I'm curious.

Your reply's been pretty helpful, thanks. I think I'll go with a practical max of 1000km/h, plus or minus 500 in a thin/dense atmosphere. Let's say they can double that but will require Difficult Pilot/Flyer checks to change direction. Takes them about 40 seconds to reach top speed, 1 minute for 'holy shit whoa' speed.
As for decelerating, how does .25 for a dense atmosphere sound, .5 for normal, and .75 for thin? Simple but consistent.

 

[GypsyComet]

Ships have gravitic technology to compensate for external gravity and maneuver inertia, and while 1G is a good bit of acceleration in atmosphere it isn't excessive. Something the size of a 747 and the width of a large house (the Mongoose Type A is 80 feet wide) is going to create a heck of a lot of turbulence, though.

The default technology assumes ships can hover over a target, by the way. Your players may ask. It is really up to you, and whether they want to hang out in a highly visible position, but it is the normal assumption.

 

[Galadrion]

Okay, getting back to the original question (how fast can a G-drive vehicle go in-atmosphere?)...

Gotta make some allowances here. Gravatic drive comes in at TL9, and we're only at TL8, so there going to be an element of guesswork here. Today's materials technologies have designers considering approximately Mach 6 as a rule-of-thumb maximum for vehicle-size craft - faster than that, and any craft built lightly enough to fly would probably come apart from various types of flight-induced stresses. So call TL8's "practical speed limit" to be around 7500 kph, and you wouldn't be too far off to be useful - this is a game, after all, and not a simulation. Now, how much that can change with each TL is up to the individual GM, but anything from 500kph to 1000 kph per TL difference could be easily justified.

Of course, as has already been pointed out, how fast a ship can go and how fast it can go in a controlled manner are two different things. There, you're getting much more into the territory of GM opinion. Obviously, a vehicle designed for trans-atmospheric combat is going to be maneuverable at higher speeds than some tramp merchant; the question is how much higher - and that's up to the GM to decide.

Personally, I'd say a Type A is going to have similar flight characteristics to a Boeing 747. They're designed for similar roles, and while the Type A has better engines and materials going for it (TL12 materials and design techniques, after all), the Boeing is, overall, better prioritized for in-atmosphere performance (better shaped for streamlining, et cetera). A 747 typically operates at about Mach 0.85 - call it 900 kph for convenience - so that makes a reasonable estimate for a civilian, merchant-focused ship.

So what about more militant vehicles? Well, the MiG-25 is currently considered the fastest fighter, with an estimated top controlled speed of around 3500 kph, or a little over 3900 kph if you're willing to risk significant engine damage. The SR-71 can manage a little faster (Mach 3.3, 3550 kph), but is not designed as a fighter - no weapon systems in most variations. I would expect a military spacecraft designed to work in an atmosphere to perform at least this well at a similar tech level, and will probably improve a bit more than a civilian vehicle would as the tech level advances. I'd say that top speeds would improve probably anywhere from twenty-five to fifty percent faster than the GM decides a civilian vehicle does.

In short, this is very much the realm of GM fiat. Anything that the GM can justify well enough that the players don't walk out of the game is good enough to work.

 

[GypsyComet]

Note that a 747 is an "airframe" in Traveller parlance, while the Type A is, in most incarnations, merely "streamlined". The Type A is also equipped with a mere 1g drive. Airframe hulls with higher G drives will be going a lot faster than the Type A.

 

[Galadrion]

Valid points. On the other side of the issue, the Type A is is typically built to TL12 specifications (that's the default assumption for ships built in a shipyard at a class A starport, even if local infrastructure isn't up to that level), whereas the 747 was designed and mostly built at TL7, with some of the later variation actually built at TL8. Also, none of these vehicles are exactly built for pushing-the-envelope performance, since they're all essentially combined passenger/cargo carrier vehicles. The primary design focus is payload versus cost, not record-breaking transit times.

Bottom line, as it relates to the original issue: if you're looking to get there faster, don't go in a merchant vessel. But that'll probably get you there fast enough, in most cases.

Bottom line for GMs: wing it. If you pick a reasonable number, that's good enough. Pick something analogous from the real world, "fudge factor" it a little for technological advancement, and call it good. It's not like most of your players are going to come back to you at TL12 and say "The {insert ship class here} flies 73 kph faster in-atmosphere than you said it would! You were wrong! You cheated!" (Or if they do, you've got a long lifetime ahead of you to find a better class of players and/or friends...)

 

[AndrewW]

Bottom line for GMs: wing it. If you pick a reasonable number, that's good enough. Pick something analogous from the real world, "fudge factor" it a little for technological advancement, and call it good. It's not like most of your players are going to come back to you at TL12 and say "The {insert ship class here} flies 73 kph faster in-atmosphere than you said it would! You were wrong! You cheated!" (Or if they do, you've got a long lifetime ahead of you to find a better class of players and/or friends...)

That's when you say the normal one does but the one you got has a little problem...

 

[locarno24]

As noted, 1,000 km/h is a nice ceiling.

However....as ever, note that this is your highest "advisable" speed. Going faster will require increasingly hellish pilot skill checks, but is not impossible.

 

[flyingsandwich]

Thanks, travellers. You've been great.

 

[phavoc]

Also keep in mind that your ship is powered by standard thrust, NOT an interia-less drive. So you won't stop on a dime. Traveller flight times are calculated with accelerating at full thrust half way to your target and then flipping and decelerating the rest of the way. In an atmosphere you'll have friction and aerodynamics to help slow you down, but you can't just turn off the drive and not expect to bleed off your inertia.

When the shuttle would come in from orbit it had to do a couple of maneuvers to bleed-off its velocity even though it's essentially a flying brick. Your free-trader will be in the same boat if it's moving at high velocity.