New Mars Forums

A laser primed fusion engine running on beamed power offers little thrust advantage over the typical lightcraft scheme.

If a laser-primed fusion engine does not achieve breakeven (produce more power than is required to keep it running in the first place), then the best it can do in terms of efficiency is less than double what you would expect of a lightcraft using the same power input, and its power output can be treated in the same way. That?s assuming it didn?t actually require ten times the power input of a lightcraft for the same thrust, which is a distinct possibility.  Plus, what size are the required engines likely to be, even with remote beamed power?  If it weighs too much more than a lightcraft, it may not be able to lift itself.

Without breakeven and self-sustained fusion to give an advantage over any other form of beamed power, one would do just as well to forgo fusion and build a larger lightcraft with a bigger ground station.  Now, fusion does yield higher exhaust velocities, which allow smaller mass ratios.  However, you could just make a lightcraft run hotter for the same power and get nearly the same advantage.  Without breakeven, fusion requires greater complexity without yielding any dramatic improvement in performance.

If you don?t have breakeven with beamed power, all you?re doing is running a hotter lightcraft.

Now, if a fusion engine with power plant could be self-contained aboard the craft and still lift itself, that would confer operational flexibility, making it worthwhile even if it couldn?t achieve breakeven.  It?s also important to note that any beamed power engines become more complex past a certain range of operating temperatures, because then you?re dealing with plasma exhausts. 

CME

A review of various proposals for advanced propulsion systems quickly reveals a disturbing common characteristic.  Although many of these systems appear promising for interplanetary transportation, few or none can even lift their own weight in the surface gravity of Earth or Mars, and most couldn?t even get off of the surface of the moon.  This is because, although these advanced propulsion systems are capable of providing far more energy than traditional chemical rockets, they seem incapable of the same level of power output.  Getting from Earth orbit to Pluto requires a lot of energy, but the rate at which the power is supplied is ultimately irrelevant.  Getting from Earth?s surface to Earth orbit requires a great deal of power as well as energy.

Advanced propulsion systems just don?t seem to have it.  Nuclear thermal rockets (the only contender with chemical rockets ? in terms of thrust -- that ever actually had engines on the test stand) must operate close to their melting points to exceed the performance of chemical engines.  This gives them a much slimmer safety margin than conventional nuclear power plants, and, given the consequences of a melt-down, that makes them simply too dangerous.  I fear even the much touted nuclear fusion rocket (if we should ever get one) is unlikely to be able to both sustain a fusion reaction and provide enough thrust to lift itself at the same time.

Does that leave us stuck with chemical rockets forever?  Are there any alternatives for Earth launch?

CME

Hello Tom.

I apologize for the error in the Power formula.  The actual formula is P = F^2/2m', where m' is actually the rocket's reaction mass (kg/s), not its mass (kg).  The idea is that the power of the exhaust is equal to the power added to the rocket, but I did not keep track of which was which.   

I guess that means that I, too, have violated conservation of energy. 

CME

It's possible to get a ballpark estimate for how large a solar collector would need to be to provide power to a rocket/jet/spacecraft.  The craft needs to accelerate at a certain rate, and it takes power to accelerate.  The solar collector needs to provide more power than that.

To accelerate at 1G requires P=F^2/(2m) power, where P is power output, F is net force (specifically, the change in momentum), and m is the rocket mass.  That translates to 48W/kg to accelerate at this rate.

The solar radiation constant is about 2kW/m^2.  So, a 1m^2 collector should provide enough power to allow 41kg to hover in 1G, assuming 100% efficiency.  100% efficiency will never happen with a solar thermal rocket.  Ever.  10% overall efficiency for a solar rocket is generous, though reasonable, but would allow only 4kg to accelerate at 1G.  Increasing that to 2G's decreases the allowable mass to just 2kg/m^2. 

A blimp-collector with a collector cross-section of 500m^2 (20m diameter, >50m length, with the collector being half the lift cell) could conceivably be used to  heft a gross "lift-off" weight of 1000kg.

How much could that get to orbit?  Well, a solar thermal rocket running on pure hydrogen can reach exhaust velocities as high as 3000m/s at just 60C in near vacuum.  This means that a solar thermal rocket is capable of reasonable performance _even if entirely constructed of coke bottle grade polyethylene_.  Using kevlar can raise the operating  temperature as high as 200C, allowing exhaust velocities of 4000m/s.  (For comparison, the space shuttle main engines operate between 4000m/s and 4500m/s using exotic ceramics.)  Using actual metal in its construction increases still further the temperature at which its engine can operate. 

To reach orbit with such a craft, it would be necessary to attain an operating temperature where the exhaust velocity of hydrogen was at least equal to orbital velocity.  That happens at 1600C, which can be withstood by titanium, stainless steel, etc.  Getting hydrogen that hot is problematic, though, and heating enough of it to power a rocket is even more so.

Hmm...

CME

PS: A useful formula for estimating exhaust velocity in vacuum is v = 250 * SQRT( T / W ) +/-10%, where v is the estimated exhaust velocity in m/s, T is the operating temperature in degrees kelvin, and W is the mean molecular weight of the exhaust.  Note that the error bar is fairly large for this calculation, because pressure, specific heats, etc. are neglected.   Note also that the estimate I gave for performance are on the conservative end of +/-10%.  In practice, actual performance can be pushed to higher levels.

If you can be assured of sufficient heat from an outside source, such as in a solar thermal rocket, you can crack methane or propane to yield a rocket exhaust that is mostly hydrogen and pure carbon.  This mixture is relatively light in terms of molecular weight.

Basically, the lighter the molecular weight of the exhaust, the more efficient a thermal rocket is going to be.  That's one reason why hydrogen is among the best possible rocket fuels -- nothing is lighter.  Electric rockets, like ion drives, can get even higher efficiencies, but they use other means than heat to accelerate their exhaust. 

CME

Hello Canth.

Indeed, if a better generating system is in place to produce electrical energy (or just heat) for a plasma rocket once it's in orbit, one should use that.  However, I don't see any Mars-Transit rated microwave beam stations around anywhere.  (A pity, because the lightcraft method of propulsion is _very_ cool.) 

In lieu of anything better, we're pretty much stuck with solar, nuclear, and chemical power. 

Sigh.

Fortunately, I don't think we have to chose just one.  Each has its own unique performance characteristics, and we benefit by being able to pick and chose between them for each mission.  And we're GOING to have more than one mission. 

CME

Hello All.

I should note that VASIMR and some other plasma rocket engines are so much more efficient than chemical rocket engines that if you had the fuel to run a chemical rocket engine, you would be better off burning it in a generator to make electricity for a plasma rocket instead.  That means that chemical power plants, though not as efficient as some other types, are also a possible alternative for powering interplanetary spacecraft.   

The dry weight of a chemical power plant is less than that of either a solar power plant or nuclear plant of the same power output.  Yes, nuclear plants produce a phenomenal amount of _energy_ (kWh of electricity) over the course of their entire operating lifetimes, but they're often incapable of matching the _power_ (kW, no hours) output of a chemical plant of the same dry mass.  That's because of a chemical plant's primary advantage over nuclear power plants: a chemical plant doesn't have to be in one piece at the end of the day.  You can run it until it just melts. 

True, there are spacecraft nuclear power plants on the drawing boards that can get us from Earth orbit to Mars in months.  But if you decide to do without that advantage, there are still chemical plants that can get you there, using VASIMR, with a single space shuttle launch.  The theoretical operating limit for a chemical plant could -- in theory -- allow for one capable of powering an SSTO plasma rocket from the Earth's surface (though you would have to rebuild it after the mission).  Nuclear fission can't touch that, because a nuclear power plant weighs so much compared to its actual power output.

Thanks for your time.

CME