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#51 2018-01-10 20:56:07

SpaceNut
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Re: Alternate BFR (Big Falcon Rocket)

The scale of falcon 9 is shifting the 2nd stage to the capsule side of the equation for what we will see on orbit for payload and fuels.
If payload to orbit functions like the falcon 9 then we can use full thrust to get more of the fuel to orbit still in the 2nd stage cargo or crewed version as residual fuel making it less for the refueling unit to deliver. Even if it is 2 refuelings per cargo or crew unit its still going to be cheaper than Nasa's mission with the SLS....

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#52 2018-01-11 03:42:58

elderflower
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Re: Alternate BFR (Big Falcon Rocket)

There might be an advantage in just attaching tanks to the outside of a ship in orbit, but the ship would need to be designed to take the loads, which might be huge, depending on the acceleration to which they would be subjected. Its a similar problem to Spacex' redesign of the core stage of Falcon Heavy to take the loads imposed by the boosters. It might be useful for missions to the outer Solar system bodies that don't have atmospheres such as Jovian moons where a large quantity of fuel could be required at the destination to enable the core vehicle to land and then return.

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#53 2018-01-11 21:22:50

SpaceNut
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Re: Alternate BFR (Big Falcon Rocket)

The next issue that of external tank once the ship is modifie for its use is boiloff of the propellant which is very fast depending on the fuel type. Also if the intent is to spend huge amounts of time traveling around from planet to planet then possibly adding the ION drive for the fuel and using that fuel tank for the extended travel distance. Sure its a joy ride but for some that still would be heavenly.

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#54 2018-01-11 21:58:21

Oldfart1939
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Re: Alternate BFR (Big Falcon Rocket)

I consider the rapid boil-off as a major issue yet to be adequately addressed in most mission architectures. This is another reason I've harped constantly on non-cryogenic propellants--other than LOX. Liq CH4 is semi-cryogenic, but still has tank size and pressurization issues as well. Liq H2 isn't even a starter in the deep space game. RP-1 has too high a melting point (some would say too high a freezing point, but that's just semantics). Aerozine 50 and UDMH are both candidates for deep space propellants.

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#55 2018-01-11 22:19:34

Void
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Re: Alternate BFR (Big Falcon Rocket)

With respect....
I have wondered how they deal with boil off in the first place, as apparently they intend to land on Mars using fuel that they bring with them.

I do know that they super cool both the Methane and the Oxygen, so that it will occupy less volume and so they can pack more in the tanks in the first place.

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#56 2018-01-11 22:43:42

RobertDyck
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Re: Alternate BFR (Big Falcon Rocket)

Oldfart1939, I have to disagree. Short term LCH4/LOX, it has better Isp and can be handled in space. Yes, it requires pressure, but that pressure is either the same as or marginally higher than the external tank of Shuttle. When I say "marginally higher" that means up to a maximum of twice the gauge pressure. Again, absolute pressure within the tank provides greater stress on the hull (gauge pressure) when in space because it doesn't have Earth's air pressure to compensate.

Long term there are some very interesting technologies. Gas core nuclear thermal rocket (GCNTR) has the promise to provide as much thrust as chemical propulsion, and high Isp of ion thrusters, and doesn't require any electricity. No electricity means no heavy reactor. It appears the be the best of all worlds. The catch is exhaust is highly contaminated with fission fragments, that means extreme radioactive waste. Testing for development is an issue, that's why no prototype has ever been built. Some here on New Mars have suggested the ideal place to develop it would be a permanent human base on the Moon. Could be, but shipping manufacturing and testing equipment to the Moon is expensive. And we already have a problem with Planetary Protection people saying they don't want humans on Mars at all, they may complain about rocket exhaust on the Moon.

Another very interesting technology is micro-fusion thruster. I got a paper from a NASA website. If memory serves I got it from the website of the Glenn Research Center, but the paper credits Marshall Space Flight Center. It uses multiple plasma injectors to converge on a point within an exhaust nozzle. They start with a deuterium-tritium plasma target, each stream has deuterium plasma at its head, then hydrogen plasma behind it. Injectors are MagnetoPlasmaDynamic (MPD). It's a pulsed fusion system.

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#57 2018-01-11 22:45:41

Void
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Re: Alternate BFR (Big Falcon Rocket)

Elderflower said:
Quote:

There might be an advantage in just attaching tanks to the outside of a ship in orbit, but the ship would need to be designed to take the loads, which might be huge, depending on the acceleration to which they would be subjected. Its a similar problem to Spacex' redesign of the core stage of Falcon Heavy to take the loads imposed by the boosters. It might be useful for missions to the outer Solar system bodies that don't have atmospheres such as Jovian moons where a large quantity of fuel could be required at the destination to enable the core vehicle to land and then return.

I don't think you can imagine how pleased I am that you addressed the issue of external tanks.  I am very pleased.  Thank you.

I have thought about it myself, and think that rather than mounting it on the hull of the ship, instead you would have a frame that is bonded to the bottom of the ship, where the engines are, but of course not allowing the engines to burn the frame.  So, it would be a cylinder frame which would surround the ship, and which would have discrete tanks included in it for Oxygen and Methane.  The whole frame should be able to be disconnected from the BFR itself, and the frame with tanks left in orbit.

I don't know what the thrust loads will be for a Hohmann transfer.  I suspect that for a ballistic capture you could go nice and easy.  Maybe just run one or two engines for a long time.  But maybe I don't know what is true in this regard?  Things to learn, not enough information.

But thanks again Elderflower for at least looking at it.

Last edited by Void (2018-01-11 22:46:42)

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#58 2018-01-12 03:01:18

elderflower
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Re: Alternate BFR (Big Falcon Rocket)

Return propellant, Spacenut. ISRU isn't going to work in 30 days.

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#59 2018-01-12 10:58:31

Oldfart1939
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Re: Alternate BFR (Big Falcon Rocket)

Robert-
Thanks for your comment but we have to then agree to disagree. When the size of tankage is considered, methane doesn't come across as looking that much better than UDMH. The Isp (vac) for CH4 is ~ 385 (couldn't find a primary reference on that). Isp (vac) for UDMH is 363. The density of (liq) CH4 is 0.79, and that of UDMH is 0.96, and of Aerozine 50, 1.00.

Musk repeatedly states that he believes his new Raptor engine may achieve an Isp (vac) of 383. I'm sorry, but I don't have an Isp (vac) for Aerozine 50, but if we do a little hand-waving and extrapolation, the Isp (vac) should be approximately the same as methane with 20% smaller tanks required.

Isp is a useful value, but the mass density of fuel is another huge consideration, one which completely rules out  H2 (liq) from consideration. I prefer to look at the density-corrected Isp, which is Id, when doing the evaluation.

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#60 2018-01-12 21:40:44

GW Johnson
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Re: Alternate BFR (Big Falcon Rocket)

I looked up Aerozine 50 in the internet.  It is a 50-50 blend of hydrazine and UDMH.  My ancient P&W handbook lists its specific gravity as 0.898.  It also lists hydrazine's sp.gr = 1.004,  and UDMH sp.gr = 0.786.  For reference,  MMH sp.gr is 0.874,  almost indistinguishable from Aerozine 50. 

My ancient P&W handbook has hydrazine and UDMH listed with LOX oxidizer,  but not MMH or Aerozine 50.  Sea level expansion at 100% nozzle efficiency and Pc=1000 psia,  hydrazine LOX has Isp=313 s,  and UDMH-LOX 310 s.  For vacuum with 40:1 expansion ratio,  100% nozzle efficiency,  and only 100 psia Pc,  hydrazine-LOX lists as Isp = 367 s,  and UDMH as 364 s. 

What I found in Wikipedia about Aerozine 50 is quoted below.  Spacex likes MMH.  The similarity between the two is quite high. 

GW

Aerozine 50 is a 50/50 mix by weight of hydrazine and unsymmetrical dimethylhydrazine (UDMH).[1][2] Originally developed in the late 1950s by Aerojet General Corporation as a storable, high-energy, hypergolic fuel for the Titan II ICBM rocket engines. Aerozine continues in wide use as a rocket fuel, typically with dinitrogen tetroxide as the oxidizer, with which it is hypergolic. Aerozine 50 is more stable than hydrazine alone, and has a higher density and boiling point than UDMH alone.

By cutting straight hydrazine, hydrazine's inconveniently high freezing point of 2 °C is lowered through freezing point depression. In addition, UDMH is a more stable molecule; this reduces the chances of straight hydrazine decomposing unexpectedly, increasing safety and allowing the blend to be used as a coolant in regeneratively cooled engines.

Hydrazine may also be mixed with monomethyl hydrazine (MMH). Because MMH is slightly denser, net performance is increased slightly.

This type of fuel is mainly used for interplanetary probes and spacecraft propulsion because unlike other more common propellants like liquid oxygen or liquid hydrogen Aerozine 50 is liquid at room temperature and can be stored in liquid state without significant boil off, thus making it a storable propellant better suited for long term interplanetary missions. Aerozine 50 was largely used in ICBMs and in their derivative launchers such as the core stages of the Titan-II/III/IV rocket because an ICBM requires long term storage and launch on short notice; the rocket must be stored already fueled. This fuel was also used in ICBM heritage upper stages, such as the Delta II rocket. It was also used by the Apollo Lunar Module and the Service Propulsion System engine in the Apollo CSM. The Ariane 1 through Ariane 4 family used a related fuel, a mixture of 75% UDMH and 25% hydrazine hydrate called UH 25.

Aerozine is not used as a monopropellant. The extra stability conferred by the methyl groups affects reactivity and thrust.


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#61 2018-01-12 21:53:25

SpaceNut
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Re: Alternate BFR (Big Falcon Rocket)

As sort of put by elderflower that insitu isn't going to work in 30 days, so sending a supply would seem to be a good alternative for the first mission as a safety net and or for a scouting mission as well of 30 days.

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#62 2018-01-13 05:29:07

elderflower
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Re: Alternate BFR (Big Falcon Rocket)

Since we can only deliver about 150 te to Mars surface with each BFS we can't deliver enough to get even one back home, disregarding the difficulty of refuelling one ship from another. The situation will be worse with smaller tankers. I think this 30 day mission is another kite being flown by old space so they can get more subsidy and justify the SLS.

Last edited by elderflower (2018-01-13 05:32:25)

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#63 2018-01-13 11:00:52

Oldfart1939
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Re: Alternate BFR (Big Falcon Rocket)

I cannot recall where I heard the possibility of making only the LOX component on the Mars surface, but in terms of getting back to Earth by bringing only the fuel component along? It may have been in one of the panel discussions at one of the earlier Mars Society conferences. That was somewhat well received and thought worthy of future consideration. Methane manufacture must by necessity, wait until we have a confirmed supply of water and hence, a source of Hydrogen for the Sabatier reaction. This is why I've argued for the hydrazine derivative fuels; no storage or boil-off problems, and can be handled through hoses during the Martian days without freezing up or gelling in the lines while transferring from a tanker to the return flight spacecraft.

The ISRU concept is what has us here all abuzz with enthusiasm, but the seductive call of the Sabatier reaction for the BFR rocket is still over the event horizon.

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#64 2018-01-13 11:13:13

elderflower
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Re: Alternate BFR (Big Falcon Rocket)

The original proposition (it may have come from Zubrin) was to ship the Hydrogen to Mars and make fuel and Oxygen there. Hydrogen shipping is a bit difficult but methane, ammonia and pentaborane are much easier, not requiring extremely low temperatures and not giving very high boiloff rates. Any of these gives you a lot of Hydrogen for the manufacture of fuels such as ethylene, butene, propadiene or acetylene. The last of these gives the biggest bang for your imported Hydrogen, as I understand.

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#65 2018-01-13 12:43:26

SpaceNut
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Re: Alternate BFR (Big Falcon Rocket)

The easiest shipping of all is water sent ahead via ion drive and landed at the site selection of choice. Elderflower has some other excellent alternatives to getting the hydrogen to mars but that can wait if all we want is a toe hold to start mission going back to mars.
The tantilizing water found at the edge of cliff faces, is going to make it hard to test for unless it extends into the hillsides where they have been found. So where is the mission planning to drill?

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#66 2018-01-13 15:12:57

Oldfart1939
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Re: Alternate BFR (Big Falcon Rocket)

The original Zubrin/Baker mars Direct plan included bringing sufficient H2 along for manufacture of methane via Sabatier reaction. Unfortunately that requires larger tanks and insulation, thus adding to the weight penalty associated with H2.

Transportation by ion drive? Not even close on that one. Later, yes, but I guess I'm more interested in quick and simple solutions--KISS principle.

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#67 2018-01-13 17:28:37

SpaceNut
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Re: Alternate BFR (Big Falcon Rocket)

http://www.ulalaunch.com/uploads/docs/P … arture.pdf

Mars Orbit Mission starts on pg 24

0.1%/day Centaur boil - off rate achievable via already reviewed modifications for LH2/LOX
0.05%/day Reasonable near - term boil -off rate with passive thermal protection for LH2/LOX

http://www.ulalaunch.com/uploads/docs/P … arture.pdf

pg 10 has a table for duration and use of what modules in a mars long as well short stay profiles.

Analysis of Launch and Earth Departure Architectures for Near-Term Human Mars Missions

Earth departure using two 100 mt custom stages in tandem uses RL-10-derived LOX/LCH4 engines for the small TMI stage for the payload of 70 mt

Earth departure using both a 100 mt custom stage and the Ares V EDS uses RL-10B-2 LOX/LH2 engines for the small TMI stage for the payload of 70 mt

This is the size of mars direct tmi to mars.

Interplanetary Mission Design Handbook:Earth-to-Mars Mission Opportunities and Mars-to-Earth Return Opportunities 2009–2024

A STUDY OF CRYOGENIC PROPULSIVE STAGES FOR HUMAN EXPLORATION BEYOND LOW EARTH ORBIT

This document contains the sun shield to manage boiloff
Trajectories for Human Missions to Mars, Part 2:Low-Thrust Transfers

Exploration Requirements for Earth to Orbit

pg 12 describes the trajectory of mission optios and mission stays.

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#68 2018-01-13 18:54:27

Oldfart1939
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Re: Alternate BFR (Big Falcon Rocket)

I still suspect that Musk may have another BFR downsize based of exactly what we're discussing on this thread. How much LOX and CH4 can be produced from the resources available and the energy for doing these subsequent propellant production cycles. Building the BFR itself may be the easier problem!

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#69 2018-01-13 22:46:51

Void
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Re: Alternate BFR (Big Falcon Rocket)

So, you would consider any option other than simply having extra external tanks, filled with the BFR tanker?

And not bother with protection of biological isolation, per Earth>Mars or Mars>Earth?

Not care at all about the science that a sample from Phobos or Demos might provide?

Do you get the fact that fuel in orbit will be cheep relative to the present reality, if the BFR tanker comes into existence?

So how does that change everything?

You seem to be locked into a 1970's reality no matter what.

And yes again, can we ever talk about Ballistic capture, and the abundance of fuel, and as far as I can see, the ability to change all the rules about total mission time.

Or am I somehow not aware that I need medication?

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#70 2018-01-14 00:42:00

Oldfart1939
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Re: Alternate BFR (Big Falcon Rocket)

Void-

Seems as thought I'm one of the few willing to consider anything other than the Gospel according to Musk about the BFR. For starters, the project seems a bit too grandiose as a first jump from Falcon 9/Falcon 9 based Falcon Heavy. There are simply too many imponderables remaining to be answered before putting that many eggs (alright...dollars!) in one basket. You raise several interesting points, including the Planetary Protection policy which I think is a "red herring." My major concern is mitigation of the effects of null gravity. I need to go back to my old engineering textbooks and re-do my BFR (revised version). Will then answer at greater length.

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#71 2018-01-14 02:25:48

RobertDyck
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Re: Alternate BFR (Big Falcon Rocket)

Oldfart1939 wrote:

I cannot recall where I heard the possibility of making only the LOX component on the Mars surface, but in terms of getting back to Earth by bringing only the fuel component along? It may have been in one of the panel discussions at one of the earlier Mars Society conferences. That was somewhat well received and thought worthy of future consideration. Methane manufacture must by necessity, wait until we have a confirmed supply of water and hence, a source of Hydrogen for the Sabatier reaction. This is why I've argued for the hydrazine derivative fuels; no storage or boil-off problems, and can be handled through hoses during the Martian days without freezing up or gelling in the lines while transferring from a tanker to the return flight spacecraft.

The ISRU concept is what has us here all abuzz with enthusiasm, but the seductive call of the Sabatier reaction for the BFR rocket is still over the event horizon.

Oh so that's the problem. Let me explain. Before Robert Zubrin and David Baker, NASA had studied the idea of ISPP. They came up with a way to make LOX and carbon monoxide (CO) from Mars atmosphere. The problem is CO/LOX is a very poor propellant. I could list all it's short-comings, but let's just say it sucks. Robert Zubrin came up with idea of bringing hydrogen from Earth, use a Sabatier reactor to convert H2 + CO2 → CH4 + H2O. And of course electrolysis to break H2O → O2 + H2; recycle H2. I could put in the numbers, but you get the idea. Robert Zubrin pointed out that methane rocket engines already existed at that time, and their performance was good. The result of this reaction was not enough O2 to balance needs of the rocket engine, so his solution was to use the system NASA had already developed to produce O2 and CO. Keep the O2, release CO back into Mars atmosphere. After all, CO is a natural component of Mars atmosphere. Robert Zubrin explained there would not be any boil-off of H2 on Mars, because the Sabatier would consume H2 at a rate greater than boil-off. All H2 gas resulting from boiling LH2 would be used by the Sabatier.

You'll notice the ISPP Precursor Experiment on Mars 2001 lander was designed to simply harvest CO2 from Mars atmosphere, then break it into O2 and CO. Justification is that is a necessary component for Robert Zubrin's idea. However, that's actually the equipment that NASA developed themselves before Robert Zubrin. And MOXIE proposed for Mars 2020 is that same system. "Not Invented Here" is alive and well.

One of my computer science professors at university called it "Not Invented By Me": NIBM. He was a big fan of IBM equipment, so accused all other computer manufacturers of that. His acronym was "N" plus "IBM". I don't want to support one monopolistic manufacturer so let's stick to "Not Invented Here".

Years after Mars Direct, ISS was built. America didn't have a working recycling life support system. Mercury, Gemini, Apollo, Skylab, and Shuttle all used bottled oxygen and removed CO2 with lithium-hydroxide. That sorbent is not regenerable, so LiOH canisters have to be replaced. Apollo and Shuttle used fuel cells to produce electricity, which produce water as a byproduct. That water was used as drinking water. Waste water wasn't recycled. Russia had developed a recycling life support system for Salyut 7, used on Mir, and intended for use on Mir2. Mir2 wasn't launched before the Soviet Union collapsed, so modules built for Mir2 were used for the Russian side of ISS. That included the recycling life support system. NASA learned how Russia did it. They used electrolysis to generate O2, recovered water from urine collection system and cabin dehumidifier. That water was purified to be drinkable, and supplied the electrolysis system. However, that system only recycles half of O2 breathed by astronauts. They had to supply water from Earth to keep the station working. It reduced launch mass vs supplying O2, but still wasn't good enough. So NASA noticed the Sabatier from Robert Zubrin's mission plan for Mars. Robert Zubrin didn't invent the Sabatier, in fact Robert Zubrin himself often gave credit to Paul Sabatier for doing so in the first decade of the 20th century. As Robert Zubrin put it, this is technology from the gas-light era. But NASA used it to improve efficiency of life support on ISS. This has demonstrated Sabatier in space.

Methane does not have to wait for a confirmed water supply on Mars. In fact it's intended for the first human mission. Mars Direct brings hydrogen feed stock from Earth.

So this is why you argue for old, obsolete propellant from the 1960s? This issue of density is addressed by modern materials. In 1990 Mars Direct included lithium-aluminum alloy. That was proposed for both the pressure hull of habitat, and propellant tanks. The brand name in Dr. Zubrin's books was "Weldalite". Now we have carbon fibre composite, even lighter. So larger propellant tanks for marginal difference in propellant density is not an issue.

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#72 2018-01-14 10:23:49

SpaceNut
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Re: Alternate BFR (Big Falcon Rocket)

Searching for who, what and why I see  and hopefully others will chime in as well as I am not the expert on all of these. I pulled up 4 topics which we have disussed Phobos or Demos....

VOID wrote:

So, you would consider any option other than simply having extra external tanks, filled with the BFR tanker?

Rocket equation explains that more mass to leave earth means we need to fire engines longer to push a give payload to mars at the desired speed. So a rocket that is already optimized to do that when an external tank is added to it means a recalculation must happen. That means that if not all the added mass is compensated for leaves us with either to much speed for aerocapture and requires burning even more fuels to slow for it or we have less payload than first calculated getting to mars. More fuel that is not used is either for later return use if we enter orbit and can be used for other things if the optimization was for orbit to orbit use of the extra fuel.

VOID wrote:

And not bother with protection of biological isolation, per Earth>Mars or Mars>Earth?

Space suits and isolation chambers or boxes to examine the samples and besides meteors from earth and mars have crossed paths already. So its even harder to prove or disprove where any life we find will have come from.

VOID wrote:

Not care at all about the science that a sample from Phobos or Demos might provide?

Science is a short stay mission due to the size of the moon and can be a pre-scout mission of sorts to test out the long duration exquipment. After that its about mining and using the moon for an artificial life support location system. Sure we can make it to both if the mission is designed for that but then, what?

VOID wrote:

Do you get the fact that fuel in orbit will be cheep relative to the present reality, if the BFR tanker comes into existence?

Cheap only when using the lowest priced system to deliver it but it still costs and it only aids when a fuel tank is empty unless that is the cargo to be transported as payload to a destination that is lacking of either energy or raw insitu materials to make.

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#73 2018-01-14 11:36:48

Oldfart1939
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Re: Alternate BFR (Big Falcon Rocket)

RobertDyck wrote:

.
Methane does not have to wait for a confirmed water supply on Mars. In fact it's intended for the first human mission. Mars Direct brings hydrogen feed stock from Earth.

So this is why you argue for old, obsolete propellant from the 1960s? This issue of density is addressed by modern materials. In 1990 Mars Direct included lithium-aluminum alloy. That was proposed for both the pressure hull of habitat, and propellant tanks. The brand name in Dr. Zubrin's books was "Weldalite". Now we have carbon fibre composite, even lighter. So larger propellant tanks for marginal difference in propellant density is not an issue.

Robert-You misunderstand what I'm suggesting. The long-term definitely needs production of methane via the Sabatier reaction, but for a first few missions--I suspect not. I suggested the Aerozine 50 or MMH, or UDMH as the most energy-dense fuels available; those not requiring both manufacture and liquefaction--then semi-cryogenic storage. I've read the whole Mars Direct architecture often enough that I could probably recite it in my sleep. What is needed to support the production of methane on Mars in addition to the H2 feedstock is the apparatus for collection of adequate CO2, then the Sabatier reactor, and finally the compressors and refrigeration systems for liquefaction, and appropriate storage facilities. This is also an energy-intensive process, requiring either a larger nuclear powerplant, or if Louis has his way, a monster solar array. What I'm offering is a somewhat "simplified" approach for the first few pioneering missions which would only require the Moxie units and apparatus for production of LOX.

In the long term view, methane IS a superior fuel in spite of the lower density. It results in less coking inside the engines and is roughly equivalent in Isp, but not Id.

The scale of the BFR really DOES require some prior "boots on the ground," just to pave the way for the future. The "pioneers" need to (1) find the water, (2) set up all the power generation facilities, and (3) start methane production in a MONITORED fashion. Then, and only then, will the BFR become reality.

To summarize: I believe in a Mars Direct pioneer type mission, but in a somewhat streamlined manner. Get the LOX manufacture running, which also enables the life support aspect of things. But bring the fuel for achieving Mars orbit rendezvous with the parked in orbit return ship with all the life support on board.

Last edited by Oldfart1939 (2018-01-14 12:00:29)

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#74 2018-01-14 12:00:52

SpaceNut
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Re: Alternate BFR (Big Falcon Rocket)

Ballistic capture for mars is where we point the craft to go into a location that is ahead of the planets path or what is also called gravitational capture.... https://en.wikipedia.org/wiki/Ballistic_capture

Ballistic Capture: New Method to Travel to Mars Cheaply, Easily and Safely

Traditionally, spacecrafts enter the orbit of a space body using Hohmann transfer. The conventional method involves a spacecraft, traveling at high speed, to hit the brakes hard and shoot retrorockets to enter the orbit.

Yes this expends more fuel mass to which we can not really afford to do as we want payload to mars.

The claims that ballistic capture can reduce fuel needs by 25 percent is due to going slow to mars ion drive to near match mars orbital speed...which is fine for cargo but not so good for humans.

Earth–Mars Transfers with Ballistic Capture
https://home.aero.polimi.it/topputo/dat … 2015-3.pdf

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#75 2018-01-14 12:03:44

Terraformer
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Re: Alternate BFR (Big Falcon Rocket)

You need to bring a lot less fuel if you leave your return ship in orbit and use a shuttle to reach it from Mars, compared to landing your ERV. It's what they did with Apollo.

As GW has said, if ISRU works out on that mission you can use it to visit a lot more sites. If it doesn't, then they'll have to wait.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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