-------- Original Message --------
 On January 27, 2018 3:27 AM, Tim <xxxxxx@little-possums.net> wrote:

>On Sat, Jan 27, 2018 at 04:03:42AM -0500, Caleuche wrote:
>>So, taking the same good ship Hamiltonian, this time in low orbit of
>> Home, and applying a deorbit impulse of 800 m/s, and modeled as a
>> conical starship of about 800 Dtons, with a height to radius ratio
>> of 10:1 with a frontal surface area of 328.6 m^2, and a drag
>> coefficient of 0.22 and a mass of 75 tons (that's very light, I only
>> modeled the outer hull, which has implications for the terminal
>> phase of reentry) the reentry looks something like this:
>>
> Is that assuming an uncontrolled entry?  I would expect most ships
> attempting such a maneuver to have at least some aerodynamic
> maneuvering capability, and choose their altitude (and consequently
> deceleration rate) during various stages of the procedure.

This entry was effectively uncontrolled. In actual game flights lift would be varied by pumping fuel or ballast around internally to shift the center of mass and modify the amount of lift being produced, matching the way that some semi-ballistic warheads are guided in terminal flight in the real world. In Traveller, the vast majority of entries are going to match those of an elevator slowly descending from space, and escape pods might not have the luxury of choosing a descent angle. In this particular run, though, the only player-controlled maneuver was allowed at the beginning of the flight.

>
> Also yes, 75 tonnes is very light.  800 dton ships in Traveller often
> have masses more like 7500 tonnes.  The main effect would be that peak
> deceleration would occur on the order of 30-40 km lower in the
> atmosphere.  That's somewhat unfortunate given that the current
> profile has the ship still moving with more than half its initial
> speed at those altitudes.  They will want a shallower angle of descent
> (or active deceleration with thrusters), lest substantial lithobraking
> occur.

Yes, I realized how important the mass was when already running the simulation and didn't want to restart it. I have built up a Traveller ship modeler for a few basic shapes (cone, sphere and cylinder) for several types of hull material which then has internal fuel storage, heavy machinery and decking generated which builds up the mass to something more like what it should be and computes moments of inertia while its at it. Also, should we ever get a ship-to-ship railgun hit in actual gameplay (never happened yet) I can plot the flight of the slug through the ship and estimate damage. I gave the deorbit problem to my 16-year-old, which was basically "your ship is in orbit, engines will fail shortly and you have time for one programmed maneuver applied as an impulse", wanting enough deceleration to allow atmospheric capture but not so much that the decent would be too steep and burn through the hull material. It should have been a shallower decent angle.

If the problem becomes something like "you need to deorbit and hit a specific point on the surface", steeper decent angles might be more useful as the shallower angles allow more time with varying thrust being applied by friction.

I should have realized that 75t was abnormally unmassive given the ship and that the ship builder was broken, and the changes in Cd from supersonic and hypersonic flight are significant and so far unmodeled. It was a flawed run, but what better way to spend a Friday evening?

>
>
>>I can also imagine situations in which starships with 1 or 2 g
>> acceleration could get themselves in trouble fuel skimming a large
>> enough gas giant.
>>
> Given the thermodynamics involved, meteoric gas giant skimming seems
> pretty ridiculous even for Traveller technology anyway.

Though Traveller 5 rules do cover hypersonic flight, and don't really say at what speeds or altitudes that fuel capture is actually occurring at, but rules and simulations covering meteoric reentry are probably mostly going to be useful for either actual meteors or the remnants of battles impacting world.