Interface rates for laser rockets

[Michael Thayne]

Some math that may be useful for designing relatively hard sci-fi settings.

Spaceships 2 gives the interface rate (cost of launching stuff into orbit on an Earth-sized planet) of $50,000 at TL9. This appears to be based on assuming something like the Conestoga-class heavy lift vehicle, and that such a thing is reuseable. It's not obvious from the description that the Conestoga is in fact reuseable, but the math checks out: fully fueling it costs $6,000/ton x 13 fuel tanks x 50 tons/tank, or $3.9M. It carries 80 tons of cargo, so that's $48,750.

Now what if we replace the HEDM rockets with laser rockets from Spaceships 7? Ablative plastic costs $80/ton, so fuel cost drops to $650/ton. Unfortunately, we'll need two 1TJ lasers to get it up there. As SM+15 major batteries, these will cost $15B each. If powered by fission reactors, those will cost $10B each. Spaceships 2 suggests the costs of running a spaceship—or, presumably, a laser launch facility—can be handwaved as 1.5% of purchase price per month, including bank payments. That's $750 million per month.

After that, the question is how many launches you can do per day. Spending the entire delta-V of the Conestoga can be done in just over 6 minutes. It can't be used 100% of the time, let's say you manage an average 5 launches per hour (perhaps more in peak hours, with it lying dormant in early morning). That's about $200K per launch, or $2,600 per ton. That's $3,250K with fuel costs. That's notably less than what Spaceships 2 lists at the price for a trip on on a TL9 space elevator, though less than it lists for a TL10 space elevator. Not sure what that means, since I'm not sure where the space elevator numbers come from, but if you don't like the TL10 number, the laser-lift number is good to know.

[Flyndaran]

Fission reactors seem like an odd thing to power it with.
I'd assume such facilities would be hooked up to the planetary grid with large batteries/capacitors to handle the short term enormous power required.

And also it would be easier to set up numerous small laser launchers to launch small ships into LEO.
Greater safety, back ups, easier to manage power requirements, and less usability as single ginormous weapons.

[AlexanderHowl]

There are political problems associated with having 1 TJ lasers with a range of 100,000 miles lying around and environmental problem associated with powering the laser and burning the plastic.

[Michael Thayne]

Originally Posted by Flyndaran

Fission reactors seem like an odd thing to power it with.
I'd assume such facilities would be hooked up to the planetary grid with large batteries/capacitors to handle the short term enormous power required.

At current grid electricity prices, powering those 1TJ lasers is sufficiently expensive that it may justify having your own power planet. On the other hand, a well-managed fission-powered grid might produce electricity that's much cheaper than grid power is today. Maybe use the same math as I used above, but cut the reactor price in half? That means the facilities cost is 80% of what I originally assumed, or a little over $2000. Make the price per person or ton of cargo $2700 once fuel costs are taken into account.

Originally Posted by AlexanderHowl

There are political problems associated with having 1 TJ lasers with a range of 100,000 miles lying around and environmental problem associated with powering the laser and burning the plastic.

Having 1TJ lasers lying around is indeed a big thing. Maybe the launch facilities would be government-controlled. The fission reactors are potentially a political problem, but given the way some countries today (like France) have embraced nuclear power, it seems like a possible future. And the discussions of laser lift propulsion seem to assume the plastic can be vaporized in a clean way, but maybe this is unrealistic—would be happy to have someone who knows more than I do weigh in.

[RyanW]

Right now, propellant costs are a fairly insignificant budget item in a space launch. The big cost for expendable vehicles is the hardware, and for reusable vehicles it is maintenance.

[AlexanderHowl]

Though the major maintenance issues deal with the delicate nature of the reuseable spacecraft. If you used something like the Sea Dragon, whose design had a hull of 8mm steel because of a large volume to area ratio, you would have much lower maintenence costs. Of course, you would need a much larger laser, since the Sea Dragon would be an SM+11 design.

[weby]

Note that a 1000 ton ship that is optimized by Spacehips rules(HEDM/Laser rocket) would be more like:
1 armor (streamlined)
1/3 smaller control
1 2/3 rocket engines (3.3g)
9 fuel =450 tons (for 6.3 mps)
8 cargo 400 tons.

So your fuel cost is only $90/ton lifted for the laser rocket and $6750/ton for HEDM and 2730/ton for a spaceplane using Nuclear Thermal Rocket Engines(3 engines so only 330 ton cargo)

As for energy:
1 TJ laser would use 200gw(25% efficiency 1 shot/20 seconds)

Electricity price alternative: one 1mwh at today's spot price is 29,78 eur, and industrial power transfer cost for one random provider is 2.38cents/kwh inclusive industrial power taxes=23,8 eur/mwh so one 6 minute launch you talk about is 1/10 of an hour so 20 000 times those mwh prices =1.07mil euro each=2.14mil eur for 2. So would result in a 4756 eur/ton electricity cost.

It should be noted that you could also make hydrogen cheaper with a on location power plant and electrolysis unit. Basic electrolysis+compression is say 235 MJ/kg ready hydrogen so one of your 10B/200GW reactors would make 0.85 tons/second or 163.4 loads/day and using a 30 day month that is $150 million for 4900 loads or $93 ton fuel cost/ton to orbit.

So if you approve of nuklear drives a NTR seems like a good choice... if you do not like flying nukes but like nuclear plants then laser rocket is your choice, without on site electric supply the price difference to HEDM is pretty low and you lose a lot of flexibility with the laser.

Basically if you allow NTR and use on site electrolysis the cost of orbital travel is trivial(less than long distance air freight today)

As example A space plane like that would cost $95.2mil (3 NTR ram rocket, smaller control, 9 fuel tank, light alloy armor, winged) so at 1.5% it would be $1.428m/month or $47.6k/day or $144/ton if you do one liftoff/day, but at more launches it drops, so you should reach a total cost of <$200/ton with fuel, few launches a day for vehicle cost+loading/unloading costs, crew salaries and such.

[ericthered]

When using the lasers as weapons, remember they'll be limited in what they can point at. They'll be blocked by the horizon, and you don't build them with the infrastructure to to cart them around to battlefields. I'd argue that current rockets (at their heart a military technology) are better weapons, at least for ground targets.

They do make decent weapons against targets in space, I'll grant you, but you can't do so subtly.

[Michael Thayne]

Originally Posted by AlexanderHowl

Though the major maintenance issues deal with the delicate nature of the reuseable spacecraft. If you used something like the Sea Dragon, whose design had a hull of 8mm steel because of a large volume to area ratio, you would have much lower maintenence costs. Of course, you would need a much larger laser, since the Sea Dragon would be an SM+11 design.

Yeah. Maybe not wanting reusable craft to be too flimsy is the source of the apparently over-armored Spaceships 2 designs?

Originally Posted by weby

Note that a 1000 ton ship that is optimized by Spacehips rules(HEDM/Laser rocket) would be more like:
1 armor (streamlined)
1/3 smaller control
1 2/3 rocket engines (3.3g)
9 fuel =450 tons (for 6.3 mps)
8 cargo 400 tons.

So your fuel cost is only $90/ton lifted for the laser rocket and $6750/ton for HEDM and 2730/ton for a spaceplane using Nuclear Thermal Rocket Engines(3 engines so only 330 ton cargo)

As for energy:
1 TJ laser would use 200gw(25% efficiency 1 shot/20 seconds)

Electricity price alternative: one 1mwh at today's spot price is 29,78 eur, and industrial power transfer cost for one random provider is 2.38cents/kwh inclusive industrial power taxes=23,8 eur/mwh so one 6 minute launch you talk about is 1/10 of an hour so 20 000 times those mwh prices =1.07mil euro each=2.14mil eur for 2. So would result in a 4756 eur/ton electricity cost.

You have a point about real-world electricity costs. I think you can get prices like the ones suggested in my posts if you assume that in the future we somehow solve the problem of chronic cost overruns that currently afflict attempts to build nuclear power plants.

It should be noted that you could also make hydrogen cheaper with a on location power plant and electrolysis unit. Basic electrolysis+compression is say 235 MJ/kg ready hydrogen so one of your 10B/200GW reactors would make 0.85 tons/second or 163.4 loads/day and using a 30 day month that is $150 million for 4900 loads or $93 ton fuel cost/ton to orbit.

So if you approve of nuklear drives a NTR seems like a good choice... if you do not like flying nukes but like nuclear plants then laser rocket is your choice, without on site electric supply the price difference to HEDM is pretty low and you lose a lot of flexibility with the laser.

Basically if you allow NTR and use on site electrolysis the cost of orbital travel is trivial(less than long distance air freight today)

As example A space plane like that would cost $95.2mil (3 NTR ram rocket, smaller control, 9 fuel tank, light alloy armor, winged) so at 1.5% it would be $1.428m/month or $47.6k/day or $144/ton if you do one liftoff/day, but at more launches it drops, so you should reach a total cost of <$200/ton with fuel, few launches a day for vehicle cost+loading/unloading costs, crew salaries and such.

NTR does seem to allow much cheaper interface. I wonder if such designs realistically account for gravity drag, though. I'm not sure what the most realistic formula is, however. Pyramid #3/79 had an article that tried to give better formulas, but the formula for liftoff appears to include 1 mps delta-V required per atmosphere of pressure, which I'm pretty sure isn't right.

[Michael Thayne]

Note: my hunch is that the Pyramid 3/79 is close to accurate if you ditch the term for atmospheric pressure. That would mean, however, that a three-engine NTR would need close to 15 mps delta-V to get off Earth's surface. This is barely possible for an unstreamlined craft that only carries its pilot (if you use the Smaller Systems rule to use a small control room), but that's probably only appropriate for lifting off a coincidentally Earth-sized planet with no atmosphere.