The Chemical Rocket Third Imperium

[atpollard]

Except it's not truth.

We can, presently, build high delta-V orbit-to-orbit ships capable of Mars in a few (3-4) weeks with payloads in the 20 ton range, and venus in 2-3 weeks. O.01G, fission powered high-end ion thrusters.

Problems:
1 - they violate the Space Treaty. No Fission reactors allowed.
2 - they are orbit-to-orbit ONLY, as they're essentially sheets of ion thrusters with a crew/cargo capsue.
3 - It's still weeks (but at least it's not months)

Let's look at closest approach: 75.3e9 m 37.75e9 per half, and 1/100 G
D=0.5AT²
37.75e9=0.5•0.1T²
20*37.75e9=20*0.5*0.1t²
7.75e11=t²
880340=t
244.5 hours to midpoint
2.9 weeks orbit to orbit

NTRs are known tech, and quite workable, too - higher G, lower delta-V, but still well above H2/O2 rockets

Have you seen the analysis of NTR vs HydroLox for a Mars Rocket ...

Just in case anyone was interested, here is some data on NTR vs LH2/LOX rocket performance from Selenian Boondocks

... it suggests that the benefit is marginal at best. My only point is that it will work with Chemical Rockets just as easily. I was aware of about 8-9 weeks to mars (still 2 months). I read that the problem with speed is the fuel needed for braking at the other end. Diminishing returns sort of thing.


I mean, heck, we could certainly embrace this whole hog and require FTL drives to launch from beyond the Gas Giants so that getting from a world to the Jump Point is an adventure in itself with transfers at multiple inhabited stations and fuel outposts and who knows what else, plus travelling with interesting strangers that could be an adventure all on its own. Some little 0.1G or 0.01G or 0.001G drive just thrusts forever to transfer you from the outer system to the inner system where you can catch a few short transfers to the Mainworld.

The Solar System takes over the job of the old Traveller subsector as the place where the action happens and then FTL becomes an epic leap to a whole new 'system' full of places and encounters to spend months adventuring in.

 

[aramis]

You wouldn't need the nuclear plant for such a vessel, just a lot of solar panels. No treaty violation, but very expansive in terms of missions to get the panels into orbit and then the expense of assembly - astronaut time doesn't come cheap either.

Solar doesn't have the energy density per plant mass to get 0.01G; neither does RTG. Fission does.

 

[mike wightman]

Take the same ion engines you were going to hook up to your fission plant - replace fission plant with several square km of solar cells, you can power the same bank of ion engines.
What is more the solar cells will be a lot less massive than your fission reactor.
Solar provides a lot more power per kg than a fission reactor plus shielding.

 

[Brandon C]

Solar provides a lot more power per kg than a fission reactor plus shielding.

It also becomes less efficient as you move farther from the sun.

 

[aramis]

Take the same ion engines you were going to hook up to your fission plant - replace fission plant with several square km of solar cells, you can power the same bank of ion engines.
What is more the solar cells will be a lot less massive than your fission reactor.
Solar provides a lot more power per kg than a fission reactor plus shielding.

Shielding isn't needed to the same level on a spacecraft as it is on the ground. And, according to NASA, solar panels don't have the same output per KG when stressed for acceleration.

Large reactors are much less shielding needed, and the current NASA 500 W/kg is for very low thrusts - as in ~1N on a 6 ton ship... ≤1 mN/kW.

@Arthur - Your data is obsolescent.

NASA has an NTR with an ISP of over 850,
(pdf, NTRS.nasa.gov). Nozzle thrust on it is 157kN on a drive of 2822 kg; T/W 55.6 while at ISP 850.

Winchell Chung lists ISP 500 for H2/O2 and T/W of 0.841 [URL="http://www.projectrho.com/public_html/rocket/enginelist.php](Atomic Rockets www.projectrho.com)[/url]; note that lower ISP's can be set, to generate much higher T/W; the F1 gets 230 or so with a T/W of 94 or so, for example.

 

[McPerth]

While thinking on this universo and comparing with 2300AD setting, I thought that anohter aspect of this universe, where no gravitic exists and so STL propulsión is through chemical thrusters would be the usual use of microjumps for in system transport, so making the STL speed mearly irrelevant for most non.military ships.

Of course this will make ships a Little more expensive (as J1 is a must), but, while in OTU this also means more space devoted to fuel (at least 10% of the ship), in this universe this will be reversed, as quite less fuel would be needed for rockets.

I'd evision most comercial ships as being unstreamlined, having Jump drives (just J1 for those used for in-system commerce) and enough fuel for the Power Plant and about 4-6 hours of thrust. They leave the Orbital Facilities with their load, accelerate just enough to leave the gravity well and jump to their destination, be it in system or in another one, and then use th thrusters to reach orbit again, where they would stay (at least) a week to cool off and perform comercial operations (unloading/loading passengrs and cargo/freight).

Even in many cases, they can have no thrusters, or less fuel for them, relying on tugs for orbit to jump point move

This wil llimit their trips to about 1/fourthnight, but, after all, I guess this sill not be slowerr than moving STL for most destinations (even Earth to Mars route), and they don't need to devote those large percentages to fuel as told in the OP, releasing the space for cargo (or passengers).

Again, this will be quite like 2300AD, where ships use their thrusters (if they have them at all) to leve orbit and gravity well, and use their FTL drives (that are in fact STL inside the 0.0001 G gravity well) for interplanetary/interstellar travel.

 

[Thot]

One important difference to 2300 AD is, though, that combat will take place without FTL help. 2300 AD ships (at least in the Mongoose PDF I bought) are frequently only equipped with a stutterwarp drive, which they also use in combat. With a jump drive, you get space combat that is less dominated by arbitrary conditions of a given fantasy FTL drive, and more by real world physics.

 

[mike wightman]

Missiles and slug throwers, railguns and lasers are possible but need a fair old power plant to provide the electricity, while the laser also has heat management issues (not to mention if build realistically the laser mirror will be several metres in radius and not have the range of Traveller laser weapons).

 

[aramis]

Missiles and slug throwers, railguns and lasers are possible put need a fair old power plant to provide the electricity, while the laser also has heat management issues (not to mention if build realistically the laser mirror will be several metres in radius and not have the range of Traveller laser weapons).

But it's also likely to be nearly 100% hit rate within it's focal range...
remember, we routinely hit a 3m target on the moon with a laser weekly, 1st shot every time, with a civilian laser.

Firing accuracy of ±10m at over 1 light-second.

 

[mike wightman]

The moon isn't evading nor is the Earth trying to avoid return fire , but I do agree laser range and accuracy will be an order of magnitude or two higher than slug throwers and rail guns (effective range measured in 10s of km at the most).

The biggest issue with laser weapons remains heat management.

 

[Chaos]

The moon isn't evading nor is the Earth trying to avoid return fire , but I do agree laser range and accuracy will be an order of magnitude or two higher than slug throwers and rail guns (effective range measured in 10s of km at the most).

The biggest issue with laser weapons remains heat management.

With "flight" times around 1 second and the sort of acceleration chemical rockets provide, evasion isn´t going to make much of a difference.

 

[Thot]

With "flight" times around 1 second and the sort of acceleration chemical rockets provide, evasion isn´t going to make much of a difference.

Let us not forget to mention that chemical rockets will have burn durations in the range of a few minutes only, and delta V will be mostly needed for regular navigation.

14,000 m/s delta V, which is about 90-95% of the ship's mass with common propellants, means 1400 seconds at 1 g.

 

[McPerth]

Given the low maneuverability and fragility of ships (any armored ship will be nearly inmobile), I'd expect combat be mostly with kinetic, long range, missiles and develop at verly ong ranges, lasers being used mostly as anti-missile point defenses.

Those missiles should attain high speeds soon, while levaing some fuel for menuvering and dodging, and ablative (or reflecting) anti-laser painting.

Rail guns will also be used as PD, with less accuracy, but also less affected by those anti-laser protections (any missile armored against them will be slower, due to increased weight).

Shrapnel eplosive missiles could also be effective, as (again) the low maneuverability and fragility of spaceships will make them vulnerable to it.

Bomb powered lasers could also be used (again ,as in 2300AD) if they are indeed posible. Not being (to say the least) an expert in the matter, I'm not sure if they are.

 

[Adam Dray]

The moon isn't evading nor is the Earth trying to avoid return fire ,

I maintain that evasion in space combat is overrated. You have a current velocity, which is very predictable. You have a max-delta-G that is very predictable. You can very easily calculate a sort of 3D heat map shaped like a curved cone that predicts where the ship will be next.

Add a behavior model to the prediction with a little machine learning -- entirely within our grasp in 2017, let alone the far future -- and you can probably still hit your target every freaking time, with just about any weapon.

That might not be fun for game play, so all the usual caveats apply, but if you're going to bring up evasion as a "realism" item, I feel like it's my duty as a space-gaming grognard to point out the flaws. :coffeesip:

 

[Thot]

More opportunities to use those sandcasters then.

 

[Adam Dray]

I'm willing to accept a handwave, but sandcasters are also made of handwavium.

See "Sandcasters" on Atomic Rockets: Warship Defenses. Stuff like:

Put simply, a layer of sand is no more effective at stopping a laser beam than a similar areal density of monolithic armor (in fact, it's a bit less effective due to structural issues); you can simply shoot holes in a cloud of sand, just like you can shoot holes in armor. As such, why spend X tons of your mass budget on temporary armor when you can just spend the same X tons on permanent armor?

In addition, a cloud of sand:

a) needs to be somewhat larger than the ship it shields (reducing areal density, and thus armor value)

b) cannot maneuver if the parent ship maneuvers (so if you deploy sand, you're stuck in your current position)

c) without some form of containment will simply disperse in a time frame that's comparable to the deployment time (if the cloud can cover the entire ship in 10 seconds, after 20 seconds it will have expanded to twice the size of the ship, reducing protection by a factor of 4. You can improve this time somewhat by using multiple projectors)​

If you're already in a setting with reaction mass and important solid-to-liquid mass ratios for thrusters, the point about mass budget is especially salient.

 

[mike wightman]

I maintain that evasion in space combat is overrated. You have a current velocity, which is very predictable. You have a max-delta-G that is very predictable. You can very easily calculate a sort of 3D heat map shaped like a curved cone that predicts where the ship will be next.

Yup, you can very easily predict the volume of space a ship can be in within the time it takes your weapon to get there - now where in that probability map do you aim for?
The centre? The front? The back? Unless your weapons can saturate that entire probability cone as you call it then you stand a chance of missing.
There will be a point at which you can not miss, but I think manned ships in a setting like this will start shooting at each other where there is very little chance of a hit in the hope of getting lucky, since the first hill is likely to be the winner.
The other tactic is to launch AKVs and missiles in the hope of saturating the point defence of the enemy, but every AKV and missile has mass...

Add a behavior model to the prediction with a little machine learning -- entirely within our grasp in 2017, let alone the far future -- and you can probably still hit your target every freaking time, with just about any weapon.

Not until they are in the autokill zone - outside it you roll the dice...

 

[Adam Dray]

What kind of distances are we talking about? How many light-seconds away?

Because, really, even at a million km (3 light-seconds), the probability cone is pretty freaking small.

 

[aramis]

Yup, you can very easily predict the volume of space a ship can be in within the time it takes your weapon to get there - now where in that probability map do you aim for?
The centre? The front? The back? Unless your weapons can saturate that entire probability cone as you call it then you stand a chance of missing.
There will be a point at which you can not miss, but I think manned ships in a setting like this will start shooting at each other where there is very little chance of a hit in the hope of getting lucky, since the first hill is likely to be the winner.
The other tactic is to launch AKVs and missiles in the hope of saturating the point defence of the enemy, but every AKV and missile has mass...

Not until they are in the autokill zone - outside it you roll the dice...

At reasonable laser ranges (0.25 LS or less), a 6G ship can only change position 7.5m. How figured

D=0.5*A(2L)²
D= distance in meters moved from predicted
L=distance in light seconds.
A=Acceleration in m/s²
T=2L as your data is as old as your laser fire will be.​

 

[mike wightman]

At 5kps it will take a railgun projectile a while to get to 3ls - no way will you be able to predict where the target will be in 2.3days time...