Creating a new THS space drive

[mindbound]

Let's assume I wanted to add a new entry to the Transhuman Space's Space Drives Table, based on the data from an Atomic Rockets datasheet, for example this one.

Has anyone attempted to do something similar? I'd be helpful for any starting tips, esp. in the process of converting the figures to the units per space-based format of the THS. Thanks in advance.

[the-red-scare]

I’ve done it a million times.

You’ll need to come ready with three figures: thrust-to-weight (T/W) of your engine, specific impulse (Isp), and the density of your fuel/propellant in tons per 500 cf space.

For a Vehicles drive, T/W is just 1/Weight on the applicable table. Isp is 3600/(Fuel Usage * Fuel Weight). Density is Fuel Weight * 1.87.

Thrust is based on the thrust-to-weight ratio of the drive. Atomic Rockets generally provides this. You just multiple the T/W by 4 and you’re done.

If you’re making it up from scratch, my advice would be to think of the acceleration you want the vessel to be capable of in Gs, then multiply a factor based on how much of the total spacecraft mass to be taken up by engines. This is 4 divided by the ratio of engine to mass. Max ratio should probably be about 0.3, minimum around 0.02.

Example: I want my fusion rocket to get 0.005 G from an engine massing 1/20 of loaded mass. I multiply 0.005 by 4/0.05 to get 0.4. The THS thrust is therefore 0.4 tons.

Unless it’s a special case, cost will be 0.4 or 0.8, but obviously on the table it ranges from 0.2 to 4 so take your pick.

Output is probably 0, but nuclear/fusion/antimatter type rockets with hot exhausts can tap them for 1 or 2 MW if desired.

Power also would be 0, unless the drive runs on electricity. If it does, we’ll need to calculate it.

Force (F) = Thrust * 8900
Exhaust Velocity (Ve) = Isp * 9.81
Power (W) = F * Ve * 0.5

Divide by 1,000,000 to get megawatts. This assumes 100% efficiency, which Vehicles generally does, but you could multiply if by anywhere from 1.1 to 4 and still be plausible.

RMC is the hardest one. First you calculate the mass flow rate of the propellant.

Flow rate (kg/s) = F/Ve

Divide by 907 to get tons per second, then multiply by 3600 to get tons per hour.

Then divide the flow rate by the density to get RMC in spaces per hour. For anything with hydrogen propellant this is easy, since it’s 1 ton per space.

Example: the fusion rocket has a thrust of 0.4 and an Isp of 36,000.

F = 0.4 * 8900 = 3,560
Ve = 36,000 * 9.81 = 353,160
Flow rate = 3560 / 353160 = 0.01 kg/s
= 0.000011 tons/s
= 0.0397 tons/hr rounded to 0.04

And the lost column is ISP, which you already know.

[Fred Brackin]

Originally Posted by mindbound

Let's assume I wanted to add a new entry to the Transhuman Space's Space Drives Table, based on the data from an Atomic Rockets datasheet, for example this one.

.

That link doesn't have enough information and since this is only a concept the needed information likely doesn't exist. Specifically you need the mass of the engine itself and no one has built one.

Lacking that information and without that information showing that this drive type is radically better than other Transhuman Space pulsed fusion concept I'd just have to call it another form of pulsed fusion (at best).

[mindbound]

That link doesn't have enough information

Yes, perhaps the specific example I picked wasn't ideal.

That being said, many thanks for the worked example! I think I'll be able to continue from here.

Edit: A few questions here for the-red-scare.

Example: I want my fusion rocket to get 0.005 G from an engine massing 1/20 of loaded mass. I multiply 0.1 by 4/0.05 to get 0.4. The THS thrust is therefore 0.4 tons.

What is the 0.1 in the calculation, how does the 0.005 factor in there, and how do you get 0.4 from 0.1*4/0.05? I assume I have misunderstood the formula there.

Thrust is based on the thrust-to-weight ratio of the drive. Atomic Rockets generally provides this. You just multiple the T/W by 4 and you’re done.

Do I understand it correctly that by multiplying the TWR by 4 (presumably the mass per space of drive?) I'll get the THS' thrust per space of drive figure?

Thank you in advance!

[the-red-scare]

Originally Posted by mindbound

What is the 0.1 in the calculation, how does the 0.005 factor in there, and how do you get 0.4 from 0.1*4/0.05? I assume I have misunderstood the formula there.

The 0.1 is a mistake! Should be 0.005 because that’s the Gs I specified. I rewrote the example and that got left in by mistake. Fixing it now above.

Originally Posted by mindbound

Do I understand it correctly that by multiplying the TWR by 4 (presumably the mass per space of drive?) I'll get the THS' thrust per space of drive figure?

Yep, T/W is unitless, so multiplying by any weight gives you the thrust for an engine of that weight. All THS drives are 4 tons per space, so multiplying 4 by the T/W is the tons of thrust.

[the-red-scare]

Here’s an example from Atomic Rockets. All I’m using is the info from the Engine List.

ACMF (Antimatter-Catalyzed Micro-Fission/Fusion)
Thrust: 2.7
Mass: 4
Cost: 0.8
Output: 0
Power: 0
RMC: 0.005N
ISP: 13,500

Thrust is 4 * 0.680
F is 24,030
Ve is 132,300
Flow rate is 0.0154 kg/s
Or 0.061 tons/hour
RMC is 0.061/12

All pulse drives use generic “nuclear pellets” which are 12 tons per space and we don’t really have anything better to base density on. But as you can see, pretty easy!

[mindbound]

Originally Posted by the-red-scare

The 0.1 is a mistake! Should be 0.005 because that’s the Gs I specified. I rewrote the example and that got left in by mistake. Fixing it now above.

Yep, T/W is unitless, so multiplying by any weight gives you the thrust for an engine of that weight. All THS drives are 4 tons per space, so multiplying 4 by the T/W is the tons of thrust.

Makes sense, I was just checking if I hadn't accidentally misunderstood any of your points. Thanks for the correction w.r.t. the G numbers!