# How big is a VU?

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Okay, so I've seen it marked down that a Void Unit is a flexible unit of somewhere between 1,000KM and 10,000KM.  I'm trying to get a rough idea of how big it is for some maps I'm generating.... and this doesn't make any sense to me.

So... let's say that a VU is 10,000KM, and a ship has a speed of 8 (VU/30m).

Pluto is presumably in the outer gravity well of the sun, so if one were to warp into the solar system of holy Terra, they would be about that far out.  Pluto is 5,900,000,000 or so KM from the sun... which means that it would take that ship around 4 years to reach it....  which doesn't seem right to me since my understanding was that it took about 2 weeks to reach the inner system from warp exit.

Of course, it also doesn't make a whole lot of sense to me that these ships are travelling much faster than that. 45KM/sec is booking.

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The easiest way around this is to adjust the effective definition of the 'safe' jump location.  It's essentialy based on the distance from the Central Gravitational Body (star usualy) is low enough.  In my games that's usualy just a bit outside of where Jupiter's orbit falls (few days travel).  This is close enough in that the Ship can generaly get everywhere in a reasonable time, and keeps the point outside of the majority of the "usable" system.  This does mean they'll sometimes jump in with planetary bodies behind them, but that's never been a problem for me.

Although there's no rules support for it, I've also said that such a point repersents the 'earliest' its safe to make a warp jump.  Unskilled Navigators, like the ones used on Merchent Guilds, might travel farther out to make the transition easier.

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Well, with a ship speed of 8, if a VU was 10,000 KM - coming in at Jupiter (779,000,000 KM from the Sun) is still 202 days travel.

Earth is 150,000,000 KM from the Sun though, so if the planets were aligned just right you could get there in around 163 days...  Still quite a trek.

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Think of the speed ratings given to the ships as a Combat Speed and not cruising speeds. Or just try not to think about it that hard. That may be the best answer.

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Also note that outside combat, you might want to apply regular physics (where it helps you): Constant thrust does not equal constant velocity, but constantly increasing velocity - and when there's no reason to constantly make evasive maneuvres that would tear the ship apart at higher speeds, you can get quite a bite faster. [/pulling stuff out of my behind]

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Yeah, like most space travel, thinking doesn't work to well.  After all. the distance between Earth and Mars varies from 55 million km to 401 million km.  So yeah, speed is something best not thought about outside of combat.

Cifer's right about constant acceleration to.  These ships might be able to cruise in a direct line faster then they can move in combat.

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Well, I would normally be content not to think about it, except that I was trying to make maps of star systems.  I'm trying to figure out some sort of scale to measure the map on.  I figured I'd start with our solar system so that I could get a general idea and the numbers blew my mind.

I mean, if a VU is 10,000 KM then the sun would be 139 VU, and Earth would be just over 1, and Jupiter would be around 12.  I'm okay with that.  Makes good sense to me... but then I started trying to figure distances that would make sense.  I wanted to mark how far away from the star each of the planets were.  I figured the best unit of measure would be in VU because that would allow for calculations on how long it would take to get from one place to another.... and that's where getting baffled began.

I mean, how many VU would you say an outer planet in a system like ours should be?  If a VU is 10,000 KM then the answer is 590,637 VU.  If it takes 2 weeks for a speed 8 ship to reach the inner system from here then the answer is more like 5,376 VU.  That makes a VU 1,000,000 KM.... which is fine... but my god these ships are moving fast. 16,000,000 KM/Hr.  I mean... I guess that's reasonable since the speed of light is what, 108,000,000 KM/Hr or so?

Just means that these 5KM long ships are trucking around at around 1/10th light speed.

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Okay... so taking the "Don't think about it like that," approach:

If I were to label a map, how far away from the star should the planets in the solar zone be?  habitable zone?  outer zone?

Should they be marked in VU, or KM?

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I wouldn't even give a scale on the map.  Just say 'not drawn to scale' at the bottem.  Instead I'd just mark out system into six zones.  Solar, Mercurial, Inner Biosphere, Primary Biosphere, Outer Reaches and Deap Space.

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Yea, the VU speed for ships is only used for combat speeds. It's generally assumed that full cruising speed would be faster.

Also note that, generally, it's meant to take a while for you to travel out of the system for a safe jump. Myself... I would imagine any lag-range point would be safe to jump to, as it means that there is no gravity or other forces affecting the ship as it emerges from the warp. That way you can have safe jump points a lot further in-system, meaning that you don't have to trek all the way out of the system before jumping.

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There's no such thing as maximum cruise speed! All the ships have listed values of max sustainable acceleration ranging from 1.6 g (56.5 km/h/s) for the slowest transport and 5.6 g (198 km/h/s) for the fastest raider. It might not be much of a speed right after you break orbit, but once you get a few days acceleration in there you'll be going really fast.

In fact, some of those accelerations are sufficient for relativistic kinetic planetbusters, a sword class frigate accelerating for 60 days would be going at 0.763c, at 6 megatonnes  that's kinetic energy equivalent to ~70 million billion tonnes of TNT, or 70 quadrillion tonnes! Not that that has much to do with the topic, I just like big numbers.

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In fact, some of those accelerations are sufficient for relativistic kinetic planetbusters, a sword class frigate accelerating for 60 days would be going at 0.763c, at 6 megatonnes that's kinetic energy equivalent to ~70 million billion tonnes of TNT, or 70 quadrillion tonnes! Not that that has much to do with the topic, I just like big numbers.

Does that factor in the little oddities of acceleration at relativistic speeds?

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I thought it did, I was lazy and used wolfram alpha for the calculations and it seemed legit but... double checking it seems It's going to be moving slower, after 60 days at 4.5 g it's going to have a speed 0.643c, that almost cuts the energy in half, giving us a mere 40 quadrillion tonnes of TNT equivalent.

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Also keep in mind that ships can safely move at a speed of up to twice their Speed rating, using an Adjust Speed maneuver action in combat.

So, for example, Frigates have a speed rating of about 8, thus could theoretically move up to 15 VU in a single turn... and the space combat turns represent about 30 minutes or so.

Lunar cruisers, with a speed of 5, could move 9 VU in a single turn (ie 30 min)

Flank Speed, used in conjunction, can increase this even higher (at the risk of damaging the engines).

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Rogue Trader Core Rulebook page 312, describing a typical interstellar voyage: "[The] ship's speed reaches approximately one percent of light speed."

This is after incremental increases in its speed to avoid space debris.  Arriving at the jump point is said to take several weeks of travel.

Hope this helps...  A decade later.  :-/

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