The Unknown Delta V Factor In Scifi Space Battles...

[Spacescifi]

Scenario. You have torchships. So does the enemy fleet you are fighting.

However once they have warped into your solar system you really have no idea how much propellant the enemy fleet has in their tanks left, and if you knew that you could plan your attacks to exploit it.

What you know: Classified intelligence from ten years ago shows that the enemy has torchships that can burn through their propellant at 1g in 4 hours. You currently have torchships that will burn all their propellant at 1g in 1 hour. They have 200 torchships, you have 188. They have FTL jumped near Mercury and are headed toward your fleet which is orbiting Mars. Enemy vessels are known to have and use Ravening Beam Of Death laser cannons that can burn through steel a light secomd out. Your vessels use mainly use macron (dust) cannon and missiles, as well as railguns.

What you don't know: If the enemy has vessels more advanced since your intel is a decade old.

What do you do?

A: "Hey guys, it's just a misunderstanding? Can't we all just... get along?"

B: "Let's get our most brillant minds and think up some solutions and win this thing!"

C: Attack and hope for the best .

 

Maim Question: Is the unknown delta V factor not a big deal, especially for long range combat?

 

Edited July 18, 2023 by Spacescifi

[Shpaget]

So you have 36 km/s of dv.

Since "near Mercury" is not really exact, lets have them landed on the surface. They'll need about 20 km/s to reach Mars (using Hohmann).

From the original 144, they still have 120+ km/s to run circles around your defenders (or use faster transfer, in which case you decide just how much they have left, or if you want their nav officer to fall asleep at the wheel and then just whisk by Mars without the ability to stop, ending up crashing into The Big Jup).

Edited July 18, 2023 by Shpaget

[linuxgurugamer]

1 hour ago, Shpaget said:

So you have 36 km/s of dv.

Since "near Mercury" is not really exact, lets have them landed on the surface. They'll need about 20 km/s to reach Mars (using Hohmann).

From the original 144, they still have 120+ km/s to run circles around your defenders (or use faster transfer, in which case you decide just how much they have left, or if you want their nav officer to fall asleep at the wheel and then just whisk by Mars without the ability to stop, ending up crashing into The Big Jup).

Where did you get those numbers from?  I don't see it in the post

[sevenperforce]

Ships in science fiction have such high specific impulse that fuel reserves are really never an issue unless they suddenly need to be a plot point.

If you want them to be a plot point, make them so.

[magnemoe]

i assume 1 g  for 1 hour=36 km/s, so 1 g for 4 is 144.

In this setting I would add more fuel / reaction mass tanks. You have no need for an 1 g trust then you start burning, this is more true for the attackers, the defenders can wait for the attackers.  
This might let the defenders get more missiles and perhaps some upgrades if Mars has cities. 

I would start shooting dust in their most optimal trajectories,  this will work like an minefield, do damage and force them to use fuel to avoid the dust, assuming  enemy is not armored against it. 
Enemy can do the same but as you would stay in an high polar orbit around Mars its trivial for you to change trajectory and you would do it regularly. 
Then enemy start to get closer and the chase is on, I assume this is an enemy raid to cripple your fleet, think Japans attack on Midway, if successful you can follow up with an invasion fleet and start attacking Mars. 
You want to stay well away from the enemy lasers so I would get closer to Mars then start moving away as enemy get danger close, I would assume they would chaise you so I would launch my missiles rearward towards them. 
If they burn for orbital injection this is your target.  Railguns are close range weapons compared to light second lasers unless stage zero for missiles. 
Most likely if the enemy have very good long range and heavy hitting lasers they also has even better close in weapon systems so unless your warheads was stealthy nuclear bomb powered x-ray lasers you loose the battle. 

Else I agree that dV margins should be higher, and with lower initial g load if you want lots of dV.
 

[FleshJeb]

1 hour ago, magnemoe said:

i assume 1 g  for 1 hour=36 km/s, so 1 g for 4 is 144.

I'll expand on that with v = at for those who don't know.

1 hour ago, magnemoe said:

I would start shooting dust in their most optimal trajectories,  this will work like an minefield, do damage and force them to use fuel to avoid the dust, assuming  enemy is not armored against it. 

That would be my solution as well. With the added advantage that we probably have several decimal places more precision on the local mu s due to centuries of observation.

For story purposes, the fleet commander also happens to be the solar system's champion billiards player.

[kerbiloid]

Abrams The Tank..

Power/mass ~20 kW/t= 20 W/kg.
Speed = 72 km/h = 20 m/s.

So, roughly, the opposing force/mass = (P/m)/v = 20 / 20 = 1 m/s².

Kinda, the tank turbojet produces T/W ~0.1 g.

Range 400+ km, let it be 432.

Duration = 432/72 = 6 h = 21 600 s.

Delta-V = 21 600 * 1 = 21.6 km/s.

 

Fuel capacity = 500 gal = 1 900 l ~= 1.5 t.

Total mass ~60 t.

21.6 = ISP*g * ln(60/(60-1.5))

ISP*g ~=850 km/s

ISP ~= 85 000 s.

[magnemoe]

4 hours ago, FleshJeb said:

I'll expand on that with v = at for those who don't know.

That would be my solution as well. With the added advantage that we probably have several decimal places more precision on the local mu s due to centuries of observation.

For story purposes, the fleet commander also happens to be the solar system's champion billiards player.

LOL, don't expect the dust to bounce well, also the drop tanks would be excellent dust armor assuming its well subdivided. You want to move around anyway as you could get sneaky missiles or railgun shells. 

A lot like how you move in an sig-sag pattern if enemy submarines is in the area and you want the pattern to be random. Yes the sub could be lucky but mostly fast ships was hard to catch by subs in WW 2. 
It happened but rarely. Most of the blue ribbon level ocean liners ran un-escorted as few ships could keep up. 
 

[kerbiloid]

5 hours ago, magnemoe said:

how you move in an sig-sag pattern if enemy submarines is in the area and you want the pattern to be random.

Spoiler

 

[Spacescifi]

On 7/18/2023 at 8:36 AM, sevenperforce said:

Ships in science fiction have such high specific impulse that fuel reserves are really never an issue unless they suddenly need to be a plot point.

If you want them to be a plot point, make them so.

Thing is, that fundamentally changes the battle.

Probably makes it kind of one sided too.

Assuming the enemy fleet has competent command and running out of fuel is not an issue then there is almost no way in which they will lose against a foe who has limited fuel supply that will run out sooner than later in battle.

How does fuel no longer being an issue effect the battle?

1. Repeat strafing. The fleet could split up into multiple divisions or wings and laser anything in range at a light second out. This is an optimal strategy too since the defenders cannot really afford to chase down an enemy fleet with virtually unlimited fuel.

2. What this means is the enemy can do drive-by laser zapping all day long and at a range where the defenders are unlikely to hurt them.

 

In the scenario where both offender and attacker have both have virtually unlimited fuel then what that does to effect the battle depends on whether or not missiles also have unlimited fuel. If they do then space battleships become extinct like dinosaurs since missile swarms would outrun and overwhelm them as well as be cheaper. If only ships have unlimited fuel then what happens is fleets can chase each other indefinitely across the solar system, and spaceships also become massive RKVs that make moon bases nearly undefendable from attack as well as make leaving on a planet less safe.

[linuxgurugamer]

I suggest you take a look at the Honor Harrington series, by David Weber.  Lots of good info there, some of the books have good discussions about this subject

[Spacescifi]

39 minutes ago, linuxgurugamer said:

I suggest you take a look at the Honor Harrington series, by David Weber.  Lots of good info there, some of the books have good discussions about this subject

I know a little already... suffice to say he relies on pure fiction scifi shields to compensate... without which missile swarms could and would make all his battleships virtually obsolete and little more than missile buses.

[kerbiloid]

The offender should just run to the interceptor's base and be a kamikaze.

[linuxgurugamer]

11 hours ago, Spacescifi said:

I know a little already... suffice to say he relies on pure fiction scifi shields to compensate... without which missile swarms could and would make all his battleships virtually obsolete and little more than missile buses.

Ummm, you should read the whole series.  It's far future, the dV stuff, acceleration, etc are all vetted by a team of outsiders.  That team discusses various things about the storyline, including ficitonal elements like that.  Keep in mind that there is also pure fiction in most if not all science fiction.  After all, the term "Science Fiction" specifically says the word "fiction".

 

All good science fiction relies upon a "suspension of belief" in that there is always something which is pure fiction, whether it be light speed, shields, etc.   But in the early part of the series, they were doing single missle combat, and the shields weren't that effective, over time (the series is more than 25 years long, from the time the first book was printed to now), the technology in the series has improved due to R&D

[magnemoe]

On 7/21/2023 at 3:30 AM, Spacescifi said:

I know a little already... suffice to say he relies on pure fiction scifi shields to compensate... without which missile swarms could and would make all his battleships virtually obsolete and little more than missile buses.

The battleships are missile busses. Original design had the missile launched out of torpedo tube style launchers along the side, torpedo as you would reload. 
Later ships went for an bomb bay setup there you filled the core of the ship with missiles you simply released at the rear, launch pods then rotated towards enemy and the missiles flew away letting you release thousands of missiles at once. I assume they still had the tubes as you only drop the load for an major battle or you was massively outnumbered, a bit like ripple fire all you anti ship missiles on an warship today. 
The shields only partially protected the ships. limiting the angle of attack, think top an bottom, solution would be to rotate 90 degree before the missiles arrived  so your front face was shielded. 
Problem was that most missiles payload was bomb pulsed x-ray lasers who fired then flying past you. 

Defense was anti missile missiles and close in weapon systems probably lasers. 

it uses pure scifi drives capable of 100's of g both on ships and missiles. Another obvious issue is that at the start missiles burned until engine was out of fuel. In some degree the very high acceleration of the ships make intercepting them was more like intercepting an agile and fast fighter jet with long range anti air missiles. 
Its not like you aim torpedos ahead of an warship,