Dive Drive

Dive-Drive Engines are a type of starship engine that enable efficient intrastellar travel across Tau Elpis. The engine's basic principle involves subjecting a refined Thaumium crystal to a carefully modulated electric field. This exploits the metaplanar properties of the crystal and creates a nullfield bubble around the ship. Once given momentum and set on its course, the vessel is allowed to "sink" into lower dimensions . This allows it to cross a much shorter amount of space without breaching relativity.

Function

Dive-Drives envelop a vessel in a nullfield bubble, allowing it to submerge into a lower dimension where spacial distances scale geometrically. A ship must first build momentum in real-space, then initiate the drive to sink below the dimensional "surface". Once submerged, the vessel glides at its chosen "depth factor", its course set by the dive vector and unalterable. When the drive disengages, the ship surfaces back into real-space, where conventional thrusters decelerate and stabilize the ship's approach.
Navigation requires precision and skill, and a skilled navigator will know to combine classical orbitals, gravity wells, and dive-drive routing in her calculations. Ships depend on astrometric modules. Charts of dimensional currents and graviometric anomalies called sectoral rutters are also published to assist navigation.
During a dive, ships enter a state of comms and sensor silence, as subspace conditions scatter and disrupt electromagnetic signals. Observers in real-space may be able to spot the tell-tale red-shifted or blue-shifted ghost of a diving ship, thus inferring a ship's trajectory and real-space speed.
Diving is risky and requires a sturdy dive-drive assembly. Disrupting the bubble can cause metaplanar incursion, as can diving too deep, which effectively implodes the bubble and destroys anything within it. Each dive weakens the crystal, which must be replaced after a set amount of uses or time spent diving. Specialized weapons called interdictors also exist. Whilst usually too weak to outright implode a nullfield bubble, they will force a ship to surface back into real-space and may cause system damage.
A few conspiracy theories suggest that the lower quality of dive-drives in Tau Elpis (compared to the highly performant, FTL-capable drives of the Pyxis Globula) is behind the resurgence of Psionic Metadisease.
Homebrew gameplay article : Navigation & Charting a Course

Travel Speeds

Realspace travel speed

Out in space, the unit of choice for realspace travel is the Kilo-Knot (KKn) (also known as Klik or Klunk). The conversion is thus :
1000 KKn = 0.0017c
(for reference, 1 KKn = 0.514 km/s = 1150 mph)
Most ships have maximum orbital realspace speeds of about 20 KKn or 23000 mph. Nothing but a ship's energy reserves and the condition of its thrusters can stop it accelerating beyond that, but ship crews tend to slip into hyperdrive the moment their ships exits the nearby gravity exclusion zone, for the ship will also need to decelerate upon arrival.

Hyperspace travel speed

Due to the faster apparent speeds achieved in hyperspace, the favoured unit becomes the Dive Factor - which represents how far the ship dives, and thus how fast it appears to go in realspace terms. Hyperspeed is expressed as a ratio of V_f, the speed of a ship travelling at Dive Factor One (or 2550 KKn)

As a rule of thumb (if you must forget everything else) :
A ship at Dive Factor 1 will take one sideral day to cross one Astronomical Unit.
A ship at Dive Factor 0.5 will take twice longer.

This factor is quite handy, as most vessels travel at speeds approximating Warp Factor 0.5. A light ship, barely laden, with a fresh drive and the best technology available in Elpis will travel at a speed just slightly below Warp Factor 1.

Type of Ship	Dive Factor achievable
Skybarge	0.1 Vf (10x slower)
Heavily-laden cargo vessel	0.25 Vf (4x slower)
Frigate or heavier armoured vessel	0.4 Vf
Lightly-laden cargo vessel	0.5 Vf (2x slower)
Cruiser or lighter armoured vessel	0.75 Vf
Fast courier vessel - reference speed	1 Vf

Travel Times

Thus here are some example trips and the time it would take different ships - without any heroics, such as slingshot manouvers, gravometric anomalies and the like :

Journey	Distance (AU)	Travel time at Factor 0.5	Travel time at Factor 1.0
Elpis - Nevael	0.7	~ 33 hours	~ 16 hours
Elpis - Nufano	0.87	~ 40 hours	~ 20 hours
Elpis - Hakaria	0.92	~ 2 days	~ 23 hours
Elpis - Embassy	1	~ 2 days	~ 24 hours
Elpis - Aistanar	1	~ 2 days	~ 24 hours
Elpis - Planetfall	1.32	~ 2 days 12 hours	~ 31 hours
Elpis - Hestia	6.3	~ 2 weeks	~ 6 days
Elpis - Zindra	11.2	~ 3 weeks	~11 days
Elpis - Nyx	31.1	~ 2 months	~1 month
Elpis - Hepera	37.2	~ 2 months, 2 weeks	~1 month, 1 week
Embassy - Aistanar	2	~ 4 days	~2 days
Hakaria - Aistanar	0.1 to 1.92*	~ 4 hours to 4 days	~2 hours to 2 days
Nufano - Aistanar	0.13 to 1.87*	~ 4 hours to 4 days	~2 hours to 2 days
Aistanar - Hestia	5.3 to 7.3*	~ 10 days to 2 weeks	~5 to 8 days
Aistanar - Zindra	10.2 to 12.2*	~ 3 to 4 weeks	~10 to 13 days

* - As bodies shift throughout their orbital cycle, the distance between planets can change dramatically, which has impacts on local economies, astrometrics, etc.

Maths for rough estimates

For the more formula minded, this is what the math looks like :
An ideal ship will travel at a Warp Factor of 1 and thus achieve this apparent realspace speed :
1V_f = 2550 KKn = 0.0044c (0.44% of the speed of light)
Many ships will travel at a Warp Factor of 0.5 - twice slower. Thus we can estimate their realspace apparent speed to be :
0.5V_f = 1275 KKn = 0.0022c
And a handy formula that roughly estimates the time required to cross from two points of the Elpis system, negating any external gravometric effects and assuming ideal momentum at the point of diving is :
Δ_t = AD * wV_f
where Δ_t is the travel time in days, AD is the realspace distance in astronomical units, and w is the Warp/Dive Factor, with 1V_f the Dive Constant, equivalent to 0.0044c or 2550 KKn.
In addition, the estimated distance between two points in space can be roughly calculated using Elpis as a reference star (so long as somebody knows the average, or even better true distance between Elpis and both points, it can be triangulated)
AD_min(A→B) ~ | AD_avg(Elpis→A) - AD_avg(Elpis→B) | _ADmax(A→B) ~ | AD_avg(Elpis→A) - AD_avg(Elpis→B) |
Thus resulting in this rough calculation :
Δt(A→B) = | AD_avg(E→A) ± AD_avg(E→B) | * wV_f
...or you can consult astrometrics tables and local astrometric condition , then simply have your onboard computer do it all for you as you plot your course and file your flight plan, with proper up-to-date data and all variables accounted for beyond the scope of this quick article.