New Mars Forums

The first applications of Scramjets will be military.  Long range sonic to hypersonic missles of all sorts will almost certianly be the first applications of such a system.  The reasons are pretty simple.  Smaller engines for these missles are much easier to design and the potential military payoffs are enormous.  Scramjet missles could bring incredibly enhanced range and speed to missles of all sorts.  We aren't quite there yet, but my money is that scramjet missles and unmanned drones will dominate the next generation of aircraft.  Their introduction makeing our 5th generation fighters and BVR (beyond visual range) missles as obsolete as these fighters are making our current 3rd and 4th generation fighters obsolete.

I don't think such a situation is likely.  For one, a cyclers are likely to be in a signifigantly diffrent orbit than any direct transfer vehicle, regardless of their velocity.  The free return orbits they use are generaly diffrent than the one direct aproach vehicles have to fall back on because they are planning on free-returning both ways.

A fast transfer vehicle is likely to use a dramaticly diffrent orbit than a cycler.  And even if they were, by chance, in the same region of space, the diffrence in their velocities would likely be immense.  Slowing down to dock with the cycler would be a difficult manuver and would take a large amount of energy, it would probably make more sense to just tough it out to the destination.  But such a close encounter bettwen such vehicles would be highly unlikely.

Rember also that cycler orbits are generaly periodic.  You can't launch into an effeciant cycler orbit just anytime you want, they are only avaliable at certian time periods, so your cyclers (all of them) are all bound to be at their diffrent destinations at specific points in time, not strung out through space arriving and departing every month or so.

I think it is worthwhile to start at least thinking about the possibility of a Inter-orbit tug.  We are eventualy going to need such a vehicle.  Transport bettwen various orbits is a common enough task (that will hopefully become more common) that it will probably make sense to eventualy develop some sort of dedicated vehicle (probably robotic) to perform these tasks.  High impluse ion engines are fairly reusable (need to be stocked up with Argon/Xeon periodicly) and so it makes sense to consildate the engines need for changing orbits into one craft with higher efficency and reusability.

Currently I do not think the design costs are worth it.  Especialy as we begin to focus our space program on Moon/Mars and away from Earth orbit.  However, if we did have such a vehicle, building constructs such as the ISS would be much easier, AND it would allow us to have easier access to higher energy orbits (like GEO).  Furthermore, this project is of small enough scope that a smaller space program like that of Japan or the ESA could focus on it.  It would eventualy pay enormous dividends to sucha group who continued to focus on operations in Earth orbit.

Wow, I may not be the brightest crayon in the box, but even I saw the sarcasm in Clark's last post.  I think the appropriate phrase here is now "YHBT."

I'll leave aside the NTR or GCNR vrs. Cycler discussion as I think it is an apple to oranges comparision.  One is a propulsive technology, and one is propulsion technology and the other a mission profile/approach.  High-energy transit systems such as a GCNR are superior to practicaly everything else, which should come as no supprise.  However, we are still a long ways off from creating one.  A cycler could (in theory) be built with today's technology.

That said, I still think Cycler's lose vrs. a direct approach, regardless of the propulsive technology used.  As I (and others) have said before they key to this is delta-V, necessary for the trip, or rather the lack of savings in delta-V.  To rendevous with a cycler requires essentialy the same amount of delta-V to get you to Mars.  Actualy in most cases it is a little more.  And in most cases the trip is signifigantly longer, despite spending more delta-V.

Because to dock with the Cycler you must match it's velocity.  There is no other way.  The cycler is moving far to fast for any kind of "fly-by snagging" scheme to work.  This high speed redevous is slightly dangerous, and is a do or die situation.  You must dock with the cycler or die, those realy are your only alternatives.

But this isn't the biggest problem.  The biggest problem is that the Cycler realy doesn't save you that much effort in comparison to a direct approach.  The only to areas you save in are consumables and hab-mass.  Most direct approaches already use a ~95% enclosed LSS so switching to a 99% enclosed one on a cycler will save little mass.  A day shuttle can get by with a less massive hull than the crewed vehicle, but not dramaticly so.  The day shuttle needs the engines and fuel-tanks of essential the same size as our direct vehicle, and it may require some sort of aero-capture system as well.

So in short, a cycler is more expensive, more dangerous, and slower than a direct mission option, and offers little benifit in return.  A marginal saving in hull-mass and consumables.  Recycling the ITV on a direct mission makes much more sense.

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As for the Cycler as Space-colony idea, this may have some merit.  But it totaly changes the equation, the cycler becomes the destination, not a way-point or means of transportation.  If space-colonies make sense anywhere, then having one that cycles back and forth bettwen Earth and Mars might be attractive as well.

But a cycler as a military vehcile is stupid.  What good is a military vessle that only approachs it's target once (for a very short time period) every 2 years or so?

I've been looking for this metric for a while.  If correct, it means that SSME are a solid bargin.  ~150k to reuse and 20 odd launches each?  That's a steal compared to disposable engines.

I can understand the argument that it takes a slightly "bent" personality to want to go to Mars.  But the kind of lunacy we are talking about here is realy on another scale all together.  I mean Rick Dobson accused me of being a secrent agent for the CIA/NSA or something (which I know proudly proclaim in my sig).  Looking back over some of his later posts, it's quite obvious that the guy is not firing on all cylinders.  There are other examples I've seen around here as well, none as blatent as Rick, but ones that make me worry about the mental health of the posters.

It's A-Okay in my book to be a little crazy.  But when you start to suspect that members of this board are part of the secret police trying to suppres your "vision" for space, that's when I start to get a little bit worried.

While I agree it was an unfortunate accident, I don't think this can be called a minor injury.  Apparently he suffered a heart-attack due to pelts being lodged close to his heart.

Of course, if you are willing to take a face-full of bird shot to prove it doesn't hurt, I'm sure that can be arranged

To me the simplest solution is to simply use CO2 as the inert buffer you may need.  This can be take from the atmosphere or re-used from the products of combustion.  CO2 only breaks down at realtivly high temperatures, higher than you should see in a well operating engine.  And so, it should play little to no part in the reaction.

In many ways a Martian internal combustion engine may well be the most efficent ICE ever designed.  The lack of nitogen in the chamber removes the single biggest cause of incomplete combustion and will increase efficency greatly.  The compression and effective mixing of the gasses in an ICE should allow virtual all of the fuel and oxygen to be combusted.  The resulting CO2 and H2O is then vented untill the pressure in the cylinder is at the appropriate levels, and then the cycle begins again.  Further, since liquid oxygen and fuel can essential be added in whatever amount is needed, the engine may be have an even greater specific power then conventional engines.

As for this systems implementation, air-independent engines are old hat.  The Russians were using them in the Quebec class as far back as 1956.  Off the shelf systems of various types are avaliable today.

Actualy, that is pretty much exactly what I am saying.  The reactions involved are functionaly identical.  To produce so much energy, you burn a certian amount of fuel in combination with a certian amount of oxygen.  For methane this ratio is 1:2 stoichiometricly or 1:4 in terms of mass.  EXACTLY the same as it is for a fuel cell.  It may suprise you to find that most ICE operate at nearly stoichiometric ratios, which makes sense, since this is the most efficent ratio to run them at.  So oxygen is consumed at basicaly the same rate per-unit of fuel as it is in a fuel cell.

Your calcuation is simply bogus.  If we were to burn 5,000L of oxygen a minute we would have an engine producing 1.7MW of energy, that's like 2200 horsepower! A we bit excessive for a martian rover, thats more like a jet engine.  No 2.5L engine could possible produce energy at this rate, the cylinders would melt from the heat!  Your crazy engine is chugging through fuel (methane) at ~2kg a minute!  Which again, is more like an air-craft than a car.

But don't take my word for it.  I'll run throught the math with you, so you can see it for yourself.

2.5L of oxygen consumption per revolution doesn't tell us much, since the actual amount of oxygen (mass/moles whatever) could varry greatly depending upon the actual conditions (temperature and pressure) it is under.  In this case, however we are probably talking about oxygen close enough to standard temperature and pressure (STP, 273K, 100kPa) for goverment work, which is convient since 1L of any gas at STP is equal to 22.4mols so:

2.5L O2/rev * 1molO2/22.4L = .11 mol 02/rev

Now, the equation I gave earlier [CH4+2O2->CO2+H2O+891kJ] not only tells us how much oxygen is consumed in relation to methane, but how much energy this reaction produces per unit.  So, we can calculate the amount of energy thusly:

.11 mol O2/rev * 1 mol reaction/2 mol O2 * 891kJ/mol reaction = 50kJ/rev

Now, since we know how much energy is produced every revolution, we can figure out how much energy the energy the engine produces.

50kJ/rev * 2000rev/min * 1min/60sec = 1700kJ/sec = 1.7MW

The process I just went through is a simple stochiometric analysist of your situation, and shows just how silly it realy is.  Stochiometry is the bane of many an early chem-student, but you should learn to love it, because it provides such a powerfull tool for analyisis of many situations.  As you can see, all the units properly cancel out (a good way to double check) and every ratio I multiply bye is actualy equal to 1, so we can be sure our number has not actualy changed.

The reason why your reasoning is so silly is because the entire cylinder is not filled with oxygen.  Not on earth, and certianly not on Mars.  Instead, in modern fuel-injected ICE a tiny amount of fuel is injected to react with the realitivly small amount of oxygen in the air.  I really don't know how you came up with this method of calculating fuel consumption, but it is realy wrong-headed.

Again, you are simply not getting it.  If a turbine is designed to produce the same amount of energy as fuel-cell or an internal combustion engine it will consume it's fuel and oxygen at basiclly the same rate.  In fact, a turbine will probably do a little better than a fuel cell since they tend to be more efficent (the large ones at least).

::EDIT, Sorry didn't see this little part on GCRN's post:::

Hmm... I would have to look into the kinetics of the reaction, which are actualy probably pretty complex, especialy in an ICE where the volume and temperature change dramaticly (and thus the rate of reation).  Generaly speaking, combustion reactions are governed by the fuel mixture, not the method of reaction at high temperatures.  An ICE takes advantage of this by compressing the mixture to make the reaction happen faster.  So in this sense, changing the mixture wouldn't have that drastic an effect on the rate of reaction.

On the other hand, burning the fuel with a great deal of excess oxygen would be similar to increasing the compression ratio (at least in comparision to a terrestial engine), since the partial pressure of oxygen would be ALOT higher.  A higher compression ratio can lead to knocking, so it might be an issue.  Also, methane (and methanol) reacts at a much lower temperature than traditional hydrocarbons and so would cause more knocking.  The total lack of Nitrogen (and thus NO2 emissions) also probably changes things.

Honestly I don't realy know though.  I don't design engines for a living :-).  The issue is pretty complex, but it has been delt with before on submarines and the like.

The kind and thickness of insulation used can be highly variable, and is the primary factor in the equation given above.  Another way to look at this factor is to calculate  it's "R" value, which you may be familar with if you have ever delt with home insulation.  "R" values are in (K*m^2)/W and is calculated like this:

R = L/k
R = R Value, (K*m^2)/W
L = Length of insulation, m
k = thermal conductance (same as used in above) W/(K*m)

Alternativly, you can calculate the thermal conductance, U, which is the reciprocal of R.  U values are in W/(K*m^2) and the formula (obviously) is U=k/L=R^-1

This allows you to final the thermal transfer easily with variable values for insulation.  The equation changes to this:

Q = (A*dT)/R   OR
Q = U*A*dT

So you can figure out whatever insulation combination you would like.  The silicon aerogell I used for the habitat has a value of 5.9, while the suit had an R value of .04.  I picked high density plastics (like what is used in Tuperware and milk Cartons), because I thought it would be fairly durable.  It's insulating value is less than spectacular, however.  Lower density plastics do better, and styrafoam does a lot better, as does fiberglass.  All of course would be lighter.  Give me an idea what you think the insulation should consist of, and I'll figure it out from there.

Are there any objections for my calculations on the outpost btw?

If we stick with the insulation I used before (high density plastics, 2cm) that brings the heat loss up to 12.5kW.  Not signfigantly worse.

Hmm, taking my new figure of 12.5kW and adding in 50% efficancy, that brings us up to ~.4g of methane a second, which takes 1.2g of oxygen a second to combust with.  Which requires an intake of 10mL of Titan atmosphere a second.  All in all, this still seems easily acheivable to me.

Maybe tomorrow I will run the figures for boot meltage, but I can tell right now that it won't be signifigant.  It's going to take quite alot of energy to bring the ice up to the melting point, and even more to melt it.

::edit skipped a decimel place::

Okay, I figured out the necessary volume of Titan Air a methane combustion system would need to provide the necessary heat for our suit.  A pretty simple Ideal Gas Law problem actualy.

V=(nRT)/P
V = Volume
n = moles of gas = 1.25x10^-5 mol CH4= .2g CG4
R = Gas Constant = 8.314
T = Temperature = 100K
P = Pressure = 2336 Pa = Partial Pressure of Methane on Titan

Pluging everything in, we get a volume of 5mL.  So our combustion system needs to bring in just 5mL of Gas a second, which is easily achievable.  So all told, I think dealing with the cold weather on Titan will be pretty easy actualy.

Next, I'll calculate how long someone would have to stand still on the ice for it to melt underneath them.