Speed Limit

[kilemall]

While reading through Brilliant Lances as suggested, I was struck once again by the Speed Limit Problem.

To wit, a lot of what those active and passive sensor are doing is detecting space debris to avoid.

Some of those short range commercial sensors, 30,000 km, can hardly give one the time to dodge. That means you have to go slower in order to have that margin.

Speed and sureness of detection even to small cold pieces of rock and ice is one factor.

Another is G potential- if one has more potential velocity, the closer one may 'cut it' to execute an avoidance maneuver.

Armor should reduce or eliminate many classes of objects as a threat.

Finally, if the debris is small enough, it may be simply shot out of the way, a la the Asteroids video game- or demolitions, if one is Bruce Willis.

I find myself imagining that a standard software package running is Auto Evade and Anti-Missile, so objects are auto avoided and/or shot with ship's weapons even if the crew is not on duty or distracted.

Shielding of course is not an option, except maybe for some anti -micrometeorite range of objects.

TNE ships clearly have a fuel limit so they are naturally limited, but others aren't.

So just how fast can we safely go??

 

[aramis]

TNE has much slower ships, anyway, since it lacks reactionless thrust. The highest thruust I recall seeing was around 16 G-hours... 576 km/s, 2,073,600 km/h.
Commercial ships will use maybe 3 G-hours total 108 km/s... Which leaves plenty of time for a 2,000 km detect... assuming a man at the helm, 18 s is at least 10 s thrust, for 500m per G ...

A CT/MT type A or R can easily hit 160 G-hours (5,760 km/s), but in TNE, they cannot.

 

[AnotherDilbert]

So just how fast can we safely go??

I vaguely remember references to large fractions of c, perhaps 0,6-0,8c, but I can't remember where.

Dark Nebula says:

Starships, depending upon specific capabilities, may move through space at just below lightspeed using sublight movement...

Sublight movement involves interstellar cruising at 90% of the speed of light.

Even if you accelerate from one end of a system to the other, say 10 000 000 000 km, at 6G you will not reach 0,1c, so it's probably not normally a problem?

 

[Whipsnade]

A CT/MT type A or R can easily hit 160 G-hours (5,760 km/s), but in TNE, they cannot.

That's roughly 0.02c, Wil. Didn't you once calculate top speed based on hull thickness and come up with about the same number?

I seem to remember that. Then again, I seem to remember lots of stuff that never happened!

 

[kilemall]

That's roughly 0.02c, Wil. Didn't you once calculate top speed based on hull thickness and come up with about the same number?

I seem to remember that. Then again, I seem to remember lots of stuff that never happened!

I believe he did a run-through on interstellar dust density and impact rates at frac-C as part of looking at my scout cruisers, but I recall the upper limit for that being higher.

In this thread I am looking at the more mundane in-system ice and rock, where they aren't conveniently giving off multi-spectrum output like a ship and are going to require good old fashioned active radar/ladar detection cause they are cold.

Another big Space Dodge needing doing is solar flares.

But in that case it's not detection range + Gs - object vee - ship vee - (total kinetic joules vs. armor)= ship survival, since avoidance may be a lot tougher with much bigger much wider frac-C flares.

I expect most ships regularly venturing into close orbital zones of flare stars or even stable ones to have heavy armor and jump at the ready to escape the otherwise unavoidable Big Kahuna flare (assuming it's not near-C).

But for a regular old rock, a maneuver or an asteroid shot should suffice and can be done much faster then a jump. The question given detection rates/distances is at what speed does that become untenable?

 

[aramis]

That's roughly 0.02c, Wil. Didn't you once calculate top speed based on hull thickness and come up with about the same number?

I seem to remember that. Then again, I seem to remember lots of stuff that never happened!

I did do a run on that. I don't recall the specific results, nor can I find them, as that was 4 computers ago... but I remember about 1 G-Week and MT armor 40 being a good baseline for the penetration.

 

[aramis]

But for a regular old rock, a maneuver or an asteroid shot should suffice and can be done much faster then a jump. The question given detection rates/distances is at what speed does that become untenable?

The speed at which the reaction time is insufficient for the debris to be cleared.

For solid bodies, the needed range is the shortest distance to ensure the clearance of the obstruction.

So, let's say a 20m asteroid, and a ship 10m tall and abeam, 1G acceleration.

The minimum safe reaction time is that which allows movement of (20+10)/2 m off axis, plus a safety margin. My preferred safety margin is in kilometers, but for a reasonable 1 sigma, 20m should do.

So that means ((20+10)/2) + 20 = 35m distance needed to divert closest approach...

35m at 1G is

D=0.5AT²
35=0.5•10•T²
35=5•T²
35/5=5/5•T²
7=T²
√7=√T²
√7=T
2.645=T​

So, you need 2.6 sec of acceleration for a miss on that 20m chunk

On the 8km...

4025=0.5•10•T²
4025/5=T²
205=T²
14.317=T​

15 seconds needed.
Getting that 15 sec is a function of how fast you're going.
​

So... if you've got 10 G-hours down, 3600*10*10=360,000 m/s
Which means, a 15 sec needs to be 15 * 360 km...5400 km. Not even 1 BL hex. But, i you've 100 G-hours, ir'a 54,000 km, or just shy of two BL hexes.

The bigger the item, the earlier you need to detect it to avoid it, and the easier it is to detect at range.

 

[kilemall]

Probably easiest if we just all stuck to Km per second or KPS.

So part of this question is the maneuver side and parameters for avoiding X size rock yes, and of course higher G craft can avoid that much more readily with less time and/or a bigger rock.

Which suggests that higher G craft have a higher 'speed limit' since they can avoid more readily.

But the other part is the speed of detection, how fast and reliably X rock can be spotted.

I use the HG sizes as modifiers to detection with an additional smaller size for below 0-sized craft and objects, missile sized -3. I also assume rocks and ice to be passive, unless the object's ice is getting close to a star and triggering a gas tail.

Then detection is at 1000 second rolls for normal operations, 100 second phases if the ship is at alert.

Of course times all the ships in a fleet, so generally it's the lone ships at risk, enough overlapping eyes and systems to virtually guarantee no rock gets that close.

So, that's IMTU, what are the detection factors for small objects for the other systems?

 

[AnotherDilbert]

Probably easiest if we just all stuck to Km per second or KPS.

K = Kelvin, a measurement of temperature. k = kilo, means thousand. Capitalisation of SI-units are not random or optional.

You probably mean "km/s".

 

[Xerxeskingofking]

K = Kelvin, a measurement of temperature. k = kilo, means thousand. Capitalisation of SI-units are not random or optional.

You probably mean "km/s".

he might, but so many sources use "kps", with or without capitalisation, that its not unreasonable for him to use it as well.

 

[Whipsnade]

... but I remember about 1 G-Week and MT armor 40 being a good baseline for the penetration.

That rings a lot of bells, Wil. Especially the MT armor bit. Thanks.

A couple years back, I was toying around with the idea of "deep" systems. You know, the systems which have huge stellar 100D limits? There are a bunch in the Marches with straight 1gee, "accel-flip-decel", 100D, transit times measured in weeks. I was wondering if those trips would actually be longer than calculated because of "speed limits" discussed here.

It looks like in the case of systems like Trifuge and Stellatio the 100D trip would take longer because the "speed limit" would be violated while in the case of Judice or Heya the "speed limit" wouldn't be reached during the trip.

 

[kilemall]

Well, part of the reason I want to reasonably define this is convey a sense of 'space terrain', so when one is coming up on a higher density field of objects, like an asteroid belt or battlefield debris, one slows down or risks Ugly.

Also gives proper due to higher-G ships being able to make those maneuvers as Aramis illustrates.

 

[aramis]

That rings a lot of bells, Wil. Especially the MT armor bit. Thanks.

A couple years back, I was toying around with the idea of "deep" systems. You know, the systems which have huge stellar 100D limits? There are a bunch in the Marches with straight 1gee, "accel-flip-decel", 100D, transit times measured in weeks. I was wondering if those trips would actually be longer than calculated because of "speed limits" discussed here.

It looks like in the case of systems like Trifuge and Stellatio the 100D trip would take longer because the "speed limit" would be violated while in the case of Judice or Heya the "speed limit" wouldn't be reached during the trip.

Armor 40 is the default MT "«unarmored» civilian shipping"...
33 cm of steel or equivalent.

The MT Pen formula, worked out by Greg Porter, is in 3G3, version 1.0... unfortunately, the ebook is only version 1.1, which has TNE/T4 but not MT.

 

[whartung]

TNE has much slower ships, anyway, since it lacks reactionless thrust. The highest thruust I recall seeing was around 16 G-hours... 576 km/s, 2,073,600 km/h.
Commercial ships will use maybe 3 G-hours total 108 km/s... Which leaves plenty of time for a 2,000 km detect... assuming a man at the helm, 18 s is at least 10 s thrust, for 500m per G …

In TNE, the Scout has 80 G-Turns of fuel (not including Jump fuel), a G-Turn is a 30m turn at 1G, so a Scout has 40 hours of fuel.

The Far Trader has 48 G-Turns, Free Trader has 56, Gazelle has 112, and the SDB 112.

For an 8 diameter world, if they decided go Full Burn, at 1G, a ship will have burned 7-8 G-Turns of fuel to get to the 100D limit, which means they hit it in 3.5-4 hours.

Mind if they just puff one burn to get going, they'll hit 100D in about 12.5 hours. 8 times the fuel (round trip), 100 tons of fuel, scrubs ~16hrs off the trip. Mind, that also costs the ship 43kCr in extra fuel costs (@500cr/ton refined at the pump). So, is the 16hrs worth the 43k.

 

[kilemall]

In TNE, the Scout has 80 G-Turns of fuel (not including Jump fuel), a G-Turn is a 30m turn at 1G, so a Scout has 40 hours of fuel.

The Far Trader has 48 G-Turns, Free Trader has 56, Gazelle has 112, and the SDB 112.

For an 8 diameter world, if they decided go Full Burn, at 1G, a ship will have burned 7-8 G-Turns of fuel to get to the 100D limit, which means they hit it in 3.5-4 hours.

Mind if they just puff one burn to get going, they'll hit 100D in about 12.5 hours. 8 times the fuel (round trip), 100 tons of fuel, scrubs ~16hrs off the trip. Mind, that also costs the ship 43kCr in extra fuel costs (@500cr/ton refined at the pump). So, is the 16hrs worth the 43k.

Speaking to the economics, depends on what the terms of the cargo contract are, and/or if they are being paid extra for 'fast transit' in which case one would need to factor in higher G capable ships and what they can earn potentially.

Another reason might be to exit jumpspace at speed to make attack/boarding unprofitably dangerous for pirates lurking on the boundaries. A higher-G merchant can remain at speed longer before firing decel for eventual planetary arrival.

This brings up a side point that I expect is more a TNE situation then others- since limited Triplanetary-like burns are 'a thing' with TNE, wouldn't it be more economical to maintain separate tanks, use refined fuel for the jumps and unrefined for the reactor and reaction maneuver mass?