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Impaler

Member

From: South Hill, Virginia

Registered: 2012-05-14

Posts: 286

Re: Transfer ISRU propellents between vehicles

Ice mining off-world doesn't scare me.  We have mined coal and other minerals for centuries.  That's where you start.  But it takes several years to adapt the technologies so they will be available when we need them!  I don’t see that effort going on,  and that’s what bothers me.  When we go to Mars,  we’re going to need it,  sooner or later.  Probably sooner than we ever thought.

Ice mining that is overseen by live astronauts and which is producing ice for a base that is going to receives resupply regularly dose not scare me.  It's only trusting an autonomous vehicle to land at a virgin site and mine ice that is necessary to make propellent to get the vehicle back with that I don't trust.  The vehicle is far more likely then not to fail to get enough ice fast enough to get back to base in a good time frame, even if it eventually limped back years later I would count the vehicle as failing in it's role because a quick turn around is the goal.

NASA says they want to put men on Mars in the 2030's.  But they also actively promote the public fiction that a crowded-as-a-phone-booth and relatively-unshielded capsule is going to carry crews there in zero-gee,  for 8 months one-way trip time,  on a 2.5 year mission,  when even the best "astronaut food" lasts 18 months at most.  Right.  And none of the things listed above is being seriously funded now,  in order to be ready after 2030,  when we will need them.

So pardon me if I disbelieve NASA projections and PR,  and pardon me when I hold up my hand saying "the emperor has no clothes" when NASA is re-building moon trip hardware,  not building Mars trip or asteroid-trip hardware,  and yet calling it hardware for going to Mars.  The fact that they have to re-create the capability to send men to the moon tells you how far we have sunk in the last 40 years.

We both know congress is making NASA make vehicles for an Apollo-on-steroid mission and is starving technology development (heck they don't even have the moon lander to go with the capsule and rocket), NASA PR is indeed pretending these vehicles are more advanced then they are or that they push boundaries that they are not actually pushing.  I'd say congress is the Emperor here with no clothing and NASA is stuck carrying water for them, a more honest NASA would indeed tell the public that congress has a horrible plan and that the vehicles they have are for the moon, but that would be expecting super-human levels of career-ending heroism from bureaucrats.  We should be thankful that retired NASA people ARE so vocal about the current programs short-comings and have in the past actually diverted us away from even worse paths.   The only bit of truth that NASA's official PR is allowed to tell is in the time-frame of 2030's to get to Mars, we don't accept this on face-value of course, we determine that it is true from our own technical analysis first and to me it looks like a reasonable time-frame considering how much money the current program will suck-up.  Just because NASA PR has BS in it doesn't mean everything is BS.

That being said,  this thread title has to do with propellant transfers.  No one offered a single comment (!!!!) about my post #15 above,  where I suggested what we need is an adaptation of existing hydraulic quick-disconnect fitting technology,  combined with receptacle (housing-in-the-vehicle-shell) design to control and contain (actually manage) the inevitable,  inherent spillages at connect/disconnect operations.  And these aren’t what you fear!  Not with the technology I suggest.

What I and every other farmer do on the farm with hydraulic fluids,  hoses,  and fittings every day could be done as easily with dangerous gasoline,  using the exact same fittings,  although there's no need for that here on Earth.  So RP-1 or any other kerosene (and likely any storable liquid) could certainly be handled that exact same way.

I've always said I though surface propellent transfer was an easy thing to do, to the extend I didn't comment on quick-disconnect fitting it is because I agree with you that would be a good way to do it.  I often comment very lightly (maybe too lightly) on things I agree with so don't take this as a sign of disagreement, if I disagree trust me you will not hear the end of it.

Last edited by Impaler (2014-12-06 16:29:07)

Impaler

Member

From: South Hill, Virginia

Registered: 2012-05-14

Posts: 286

Re: Transfer ISRU propellents between vehicles

I'd now like to update my earlier equation to take into account Hydrogen (or other propellent mass feed-stocks) being added to the lander.  The figure of merit here is the mass-multiplier for the ISPP, for each unit of feed-stock mass how much propellent mass (including the fuel-oxidizer mix) is created.  For a system operating entirely on atmospheric inputs this multiplier can be considered infinite which just makes the feed-stock mass is eliminated and the equation would reduce to the earlier form.

Because I'm considering a Decent-Hoop-Assent scenario the vehicles must land with twice the feedstock necessary to fill it's propellent tanks, and on the Hop stage it will be carrying 1 refill worth of feed-stock as payload, then finally on assent it will be out of feed-stock.  Fortunately it should be possible to simply take this extra feed-stock payload as a Delta-V hit to the Hop and travel a shorter distance across the Martian surface in the Hop.  I'll need to get more specific as to what kind of hop distance is achievable but it really need not be more then a hundred kilometers to be an effective site change.

Vt = 1 / (1 - Sf - Pf - (Pf / (Rp * Td)))

is modified to

Vt = 1 / (1 - Sf - Pf - (Pf / (Rp * Td) - (Pf / Pm)) for take off mass at the Hoop assuming we wanted the full Delta-V we had prior.

Vt = 1 / (1 - Sf - (Pf / (Rp * Td) - (Pf * 2 / Pm)) gives initial landing mass as we substitute double feedstock for the take off propellent.

Pm is propellent multiplier, Zubrin sites a 20:1 leverage for Ethylene using 2.6:1 Oxidizer to fuel ratio, and the ISP is comparable to Metho-Lox.  Given a 69& propellent fraction for the Metho-Lox and dividing my 20 yields a 3.45% mass fraction for Hydrogen.  For two trips that doubles to 7%, but this would be 7% of take off mass and we know take-off mass needs to be in the range of 5x-6x payload so Hydrogen for two trips is 35% to 42% of payload, not that bad actually. Putting these numbers in I get the formula.

1/(.9 - (.69/(60 * x)) - (.69 * 2 / 20))

I've prepared a graph sketch again and made a permalink, GRAPH

The Blue line is total take-off mass, green line is total propellent for takeoff, red is vehicle structure, yellow is feed-stock for two fueling.  Teal is the mass of the ISPP system, and the purple line is the sum of payload, structure, feedstock and ISPP to give the initial landed mass, all lines are relative to payload.  As you can see the structure and feedstock lines don't begin to go up substantially until propellent rates are below 20% per day and at propellent production above 30% per day their is little additional benefit in reduced landing mass.

Zubrin estimates a 0.6 production rate for the Etylene ISPP equipment, which means that power sources would need to be between equal to or double the mass of the ISPP equipment to remain within the viable window of >0.2 overall efficiency.  Power consumption is 225kw for 12 hours a day.  If equal to the ISPP equipment in mass that would be a power density of 275 w/kg, if half that it would be just 137 w/kg.  This is far beyond conventional solar cells but the high density it is actually not that far from the goals of things like the MEGA-ROSA http://www.dtic.mil/dtic/tr/fulltext/u2/a444956.pdf and other solar projects under development now, though they are aimed at in-space applications rather then Mars surface.  So power requirements may actually be achieved as a side benefit and make the rapidly refueling lander viable.

Lastly I haven't addressed H2 boil-off because under such rapid propellent production it should be possible to simply consume all the boiling off hydrogen into the Propellent production process.  The scenario calls for only 120 days of production before the lander returns to the orbital base station and receives new hydrogen at which point it immediately lands and begin making propellent.  Only the orbital systems single large tank needs to be designed to achieve zero or low boil-off, a significantly simpler challenge then putting the system on a small tank on a mass constrained lander that needs to operate on the martian surface.

Last edited by Impaler (2014-12-06 21:49:27)

SpaceNut

Administrator

From: New Hampshire

Registered: 2004-07-22

Posts: 29,431

Re: Transfer ISRU propellents between vehicles

This sort of important if we are living in the rocket that is getting the fuel....

Create fuel in the cargo units and transfer just before using the return vehicle just before lift off.