[Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

The only propellant for chemical rockets listed on SS1 p.21 is hydrogen-oxygen. While this combination may be the most efficient one in terms of mass, it is much less so in terms of volume. As a result, many rocket designers have chosen to use denser (if less efficient) propellants, sacrificing ΔV for reduced tankage requirements. (For more information on propellants, see the Rocket Propellants page(†) of the Rocket and Space Technology website.)

The ΔV and thrust provided by different propellants can be found by comparing their specify impulses (Isp). From the classic rocket equation ΔV = Isp × g × ln(Mp/Me), we can find the ΔV in miles/second of a single tank by multiplying the Isp by a constant equal to 21.8 × ln(20/19) / 3600 = 0.00031. Reversing this equation, the Isp of standard hydrogen-oxygen propellant is 0.15/0.00031= approx. 484 seconds. I've noticed that changes in ΔV vary inversely with those of thrust; therefore, the acceleration provided by a chemical rocket using a different propellant would be 3×484/Isp = 1,452/Isp Gs. The endurance of the tank would be Isp squared times 0.00031×3600/(1452×21.8) = 0.000035 seconds.

Example: According to page 13 of "Mars Direct: A Simple, Robust, and Cost Effective Architecture for the Space Exploration Initiative" by Robert M. Zubrin (Copyright © 1991 by Martin Marietta Corp.)"the optimum oxygen to methane combustion mixture ratio is about 3.5:1, as this provides for a specific impulse of 373 s" Thus, ΔV/tank for an O2/CH4 rocket would be 373×0.00031=0.11563 mps, with its thrust being 1,452/373=3.893 Gs. A tank of propellant is consumed every 4.9 seconds. A Mars Ascent Vehicle would require at least 13.5 tanks of propellant just to reach LMO (low Mars orbit, ΔV=2.48 mps).(Note that the mixture ratio is by volume, not mass. This will be important later.)

Calculating the cost of the propellant is a little trickier. Oxidizer/fuel mixture ratios are usually stated in terms of volume, not mass. The price per ton of propellant is (R×Mo×Po+Mf×Pf)/(R×Mo+Mf) with R being the volume mixture ratio and Mo/Mf and Po/Pf being the molecular weights and price/ton of oxidizer/fuel respectively.

Example: The price of liquid methane is $500 per ton (SS7 p.21). I tried in vain to locate a 4E price for liquid oxygen, so I had to calculate it from 3E sources. I got $18.99/ton from GURPS Transhuman Space and $20.83/ton from GURPS Vehicles 2nd Edition, so I'm splitting the difference and pegging it a $20/ton. Thus the price O2/CH4 propellent is (3.5×32×20+16×500)/(3.5×32+16)=$80/ton.

† NOTE: Isp on this page are for sea level, not vacuum. This can make a significant difference.

EDIT: I realized that I could deduce the 4E price of LOX from LH2/LOX rocket fuel. With Rocket fuel at $800, Hydrogen at $2,000 (SS1 p.46) and the ratio of oxygen to hydrogen by weight in water being 8:1, the 4E price of LOX would be ($800×9-$2,000)/8=$650/ton. Thus the price O2/CH4 propellant would be (3.5×32×650+16×500)/(3.5×32+16)=$631.25/ton.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

Cool! Have you given any thought to peroxide engines? They require about 90% hydrogen peroxide to operate but look pretty feasible (except for the current limitation that the US government has acquired the only company that makes 90% H2O2 and isn't allowing anyone else to make it, last I heard). One of the X-Prize contestants was developing their rocket engine around this until they hit that snag, and was making good progress until they hit the limit of the commercially available 70% peroxide.

Alternate Chemical Rocket Fuel
 

Thought I read somewhere that some bright person figured out (c. 2000) how to make rocket fuel from old truck tires.

While it was very inefficient in terms of specific thrust per unit of mass it was very efficient in terms of specific thrust per unit of money.

Should be on line somewhere.

Re: Alternate Chemical Rocket Fuel
 

In this context "specific" means "per unit mass".

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

For the record: that's the US version of the Rocket Equation, in which specific impulse is given in seconds owing to an error with units, and has to be multiplied by g₀ before it can be used. The classic rocket equation as derived by Tsiolkovsky and used by everyone who is not stuck in a unit system in which "pounds" might be mass or might be force, is:

Δv = v(exhaust) × ln(Mp/Me)

Specific impulse is equal to effective exhaust velocity. Impulse is in units of force-times-time; divide by mass and you get units of velocity, not units of time.

Re: Alternate Chemical Rocket Fuel
 

You can make rocket fuel from salami. Or rather, salami is rocket fuel.

Not really GREAT rocket fuel, but it's rocket fuel. Nobody specified "great" :)

Re: Alternate Chemical Rocket Fuel
 

Being lactose intolerant, ice cream is rocket fuel to me. Not great to anyone within 2 city blocks of course.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

Actually, the equation is international. Isp is measured in seconds, so multiplying it by an acceleration (in any velocity units/second) gets a velocity.

v(exhaust) = Isp × g

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

I suspect not, as it seems to not be SI-compliant.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

Generally, tanks, pumps and engines of the same size would be more likely to have the same volume flow rate rather than mass flow rate, which would mean thrust is proportional to density x Isp, - this matters quite a bit because pretty much everything is denser than hydrox, and therefore gets better thrust despite lower Isps - and endurance is unchanged. I crunched through a lot of numbers for GURPS Vehicles at one point - http://www.gurpsnet.org/Archive/Vehi...struction/V2ad for chapters 2 and 7.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

Isp (specific impulse) is not measured in seconds in the SI, only in US customary units.

I is impulse, measured in Newton-seconds, a.k.a. kg-m-s^-1. "Specific" means "per unit mass", so you divide by kg and get specific impulse (Newton-seconds per kilogram) in metres per second. Or in US Customary units, impulse is in pounds-force seconds (or poundal-seconds) and to get specific impulse you divide by pounds-mass (or slugs). You only get seconds if you divide pounds-force by pounds-mass and get a dimensionless number instead of an acceleration. You can't even do that in the Système International. Specific impulse works out identical to the effective exhaust speed.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

If it's labeled Isp, it's commonly measured in Seconds even under SI. If instead it's labeled as Ve, it's not measured in Seconds, despite the fact that these two names are measuring exactly the same thing.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

That is bizarre, freakish, and bound to lead to confusion. Also, not what I was taught in my physics courses.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

Yeah, well, common usage fairly often uses physically annoying notations. Seconds should really be G-seconds, but it's a useful enough notation and commonly understood.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

But of course specific impulse isn't much used in physics. In spaceflight literature it is by and large quoted in "seconds" even in modern papers where everything is in metric units. In aeronautical engineering, it's almost always quoted in Ns/kg - and never reduced to meters/second, because in fact nothing is moving at anything like that "velocity", which could conceivably exceed the speed of light for a big enough rotor and sufficiently efficient engine.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

You have no idea how fun explaining this to a classroom full of engineering students was.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

I used to be an engineering student, once.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

Surely, by conservation of momentum, the exhaust must be doing so relative to the outlet.

I'm going to have to call for a citation on that one, because it will completely overturn my understanding of the limits of rocketry.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

Which explains why in Spaceships all the alternate reaction masses mentioned divide the delta-V by the thrust multiplier. The fuel tanks are constrained by mass, not volume, so fuel density is not the issue. (Not too realistic, I know.)

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

No. That's true if the thrust is provided by reaction to exhausting the fuel, but that's not actually the definition of specific impulse, which is the thrust of the engine divided by the weight of the fuel it burns. If you move a very large volume of air very slowly you can get lots of thrust for very little energy (and hence fuel) use. Turbofan specific impulses can break 100,000 Ns/kg, helicopter rotors routinely reach millions. It's more common to quote these as the reciprocal ("thrust specific fuel consumption") and multiply by 3600 to get them in per hour but its the same thing.

Re: [Spaceships] Homebrew: Alternate Chemical Rocket Fuel
 

We're talking about Tsiolkovski's Rocket Equation: it doesn't apply to turbofans and air screws, where delta-v is not even a useful concept.