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#26 2022-02-22 19:01:50

SpaceNut
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Posts: 29,310

Re: Large Ship Chemical Propulsion

unless the tug lands on mars we need it brought to it while its in orbit around mars.

The big thing is we need to find mass saving for the large ship to cut the propellant requirements back into the realm of plausible....

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#27 2022-02-22 19:30:03

tahanson43206
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Re: Large Ship Chemical Propulsion

For SpaceNut re #26

Thanks for taking up (and continuing) the flow of this discussion.

I'm trying to find out (from GW Johnson) if his flight plan includes a Tug at Mars.

I am assuming a Tug at Earth to assist with departure, and I am assuming another at Earth to catch Large Ship on the return and ease it into orbit around Earth.

The Tug is intended to be a Deep Space vehicle and never to descend lower than LEO.

In my post #25, I am hoping GW Johnson will clarify how that tonnage of propellant will be used.

My goal is to obtain a reliable total of propellant that would be needed to provide a Large Ship flight from LEO to LMO and back to LEO.

I have not yet seen figures that I can understand.  I see figures, and they look as though they are heading in the right direction, but there are details that I ** think ** are missing, so I am not confident of the result.

The goal is to be able to size a nuclear reactor to provide the propellant.

This forum is fortunate to have a member who can compute the right size for a reactor, if he is provided the requirement.

Here is what I am looking for:

1) Propellant needed by Large Ship to reach Escape after Tug Release
2) Propellant needed by Large Ship to "dock" in LMO without a tug - I want Large Ship to accelerate at Mars to enter LMO by itself
3) Propellant needed by Large Ship to depart LMO ** without ** assistance of a tug
4) Propellant needed by Departure Tug to give Large Ship the push it needs, ** and ** to return safely to LEO
5) Propellant needed by Arrival Tug to reach Large Ship as it returns, and to slow Large Ship into a safe position in LEO
6) Propellant needed by tankers to place all that propellant into LEO for the three vessels

The sum of all those figures is the amount of propellant to be manufactured by a dedicated fission nuclear plant.

Thanks again for your interest in and support of this topic!

(th)

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#28 2022-02-22 19:37:37

SpaceNut
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Re: Large Ship Chemical Propulsion

repost

GW Johnson wrote:

Easiest thing for me to do is just send that spreadsheet to anybody who wants it.  You can put your own assumed numbers in,  and see the sensitivity of the results to them.  All the inputs are highlighted yellow.  Everything else is automatic.  It only models a single stage item doing all the burns from its propellant supply;  burns are Earth orbit departure,  Mars orbit arrival,  Mars orbit departure,  and Earth orbit arrival.  For low thrust propulsion,  you must factor these way up for long-burn gravity losses;  everybody else gets to use factor = 1.

mass ratio-effective dV = (factor for gravity & drag losses) x (orbital dV)  I used factor = 1 for all but ion,  I used factor = 2 for ion

mass ratio MR = exp(eff dV/Vex) where Vex, km/s = 9.8067*(Isp, sec)/1000

propellant mass fraction = 1 - 1/MR

inert mass fraction is an input (must justify appropriate values)

payload mass fraction = 1 - inert fraction - propellant fraction (only feasible if solid;  stop if negative)

payload is an input (doesn't really matter,  but 1000's of tons for any sort of realistic colonization mission)

ignition mass = payload / payload fraction

inert = ignition x inert fraction

propellant = ignition x propellant fraction

do the ignition / payload mass ratio;  you can scale any size from that ratio and any payload you want to deliver.

How much simpler can it be?

GW

GW Johnson wrote:

Tahanson 43206:

The easiest and best thing to do about the spreadsheet is a column of all the kinematic dV's that must be achieved.  That gets followed by a column of the appropriate factors reflecting gravity and drag losses,  or hover and divert requirements.  Their product is the mass ratio-effective delta-vees that you need to use the rocket equation.  MR req'd = exp(factor*dV/Vex). 

Be aware that Vex depends upon your engine selection for the burn,  and its Isp performance at expected backpressure conditions.  Vex = gc * Isp,  where gc is 9.80667 metric,  and 32.174 for US customary.  It does NOT depend upon where you are,  being only a units conversion factor.   These are all additional columns in the spreadsheet. 

Now you need a weight statement,  and a strategy for how to allocate it across your mission.  There is the inert structural mass of your vehicle,  the dead-head payload it carries,  and the propellant mass that fits within its tanks.  These sum to its ignition mass.  Ignition minus propellant is the dry tanks mass.  PERIOD.  END OF ISSUE.

Usually,  there is a factored launch or departure burn (for each stage),  a mid course correction burn,  and some sort of landing or arrival burn.  Your spreadsheet should reflect that.  Same is true for any return to Earth.  If you refuel somehow at destination,  there are separate weight statements for the outbound and return trips.  Otherwise,  the propellant available for departure is the propellant remaining at landing,  less any evaporative loss correlations.  Don't forget to adjust payloads outbound and return,  as appropriate. 

Each burn has a factored dV that results in a mass ratio for that burn.  The end of burn vehicle mass is its ignition mass divided by the mass ratio required of that burn.  The propellant used is that burn's ignition mass minus its end-of-burn mass for that burn.  Propellant remaining after that burn is what you had to start with,  minus what you used on that burn.  If your propellant remaining goes negative,  your analysis is infeasible.  PERIOD.  END OF ISSUE.

That's pretty simple actually,  and can be tailored in a variety of ways for this-or-that mission.  Just put the right stuff in the appropriate cells for the spreadsheet. 

Any particular spreadsheet I have,  is rendered obsolete if one just does that.  And I have NO IDEA how to use "drop box" or any of that sort of thing.  I can hardly make email,  "exrocketman",  and the NewMars forums work at all.  There is NOTHING about any of this stuff that is in the least intuitive to me.  I simply don't,  and cannot,  think that way.

GW

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#29 2022-02-22 20:01:46

GW Johnson
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Re: Large Ship Chemical Propulsion

Tom re post 25:

The 15,000+ tons at Earth supports both the big ship and the departure tug,  plus the tug's return to LEO. 

The 13,000+ tons at Mars supports both the big ship and the departure tug,  plus the tug's return to LMO. 

For departure at Earth or Mars,  the tug takes you from low orbit speed to just under local escape speed,  then undocks from the big ship.  This puts both into an elongated elliptic orbit.  The big ship immediately fires its propulsion stage to get from just under to just over escape,  which "far" from the planet is just enough to be on the Hohmann transfer ellipse to the other planet.  The tug has to make another burn (at low total mass) to go from the elongated ellipse to low circular orbit. 

For arrivals at Earth or Mars,  the big ship fires its propulsion stage at pretty much the elongated ellipse periapsis point,  putting it onto the elongated ellipse parking orbit.  The tug is still in low circular orbit,  at much shorter period.  After the big ship makes a turn about the elongated ellipse,  the tug burns to enter that same ellipse,  and can then dock with the big ship.  The docked pair then make a second turn about the elongated ellipse,  with the tug firing at periapsis to bring the docked pair into low circular orbit. 

Arrival has a timing issue to resolve which departure does not.  That is why there are no turns about the elongated ellipse on departure,  but there are two upon arrival. 

The burns described for these maneuvers need to be high enough thrust to be "impulsive",  so that there are no gravity losses.  That would be vehicle accelerations exceeding at least 0.01 gee,  and perhaps exceeding 0.05 gee. 

GW


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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#30 2022-02-22 20:59:41

tahanson43206
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Re: Large Ship Chemical Propulsion

for GW Johnson re #29

Thank you for the detailed reply regarding the chemical propulsion option for Large Ship.

I am (was) a bit behind the curve, because I did not realize you were planning to use a Tug at Mars.

I like the idea, but simply did not have it available in my earlier attempt to follow the calculations.

It's late (local time) so I'll read your post again tomorrow.

(th)

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#31 2022-02-22 21:44:57

GW Johnson
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From: McGregor, Texas USA
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Re: Large Ship Chemical Propulsion

Tom: 

What I found and documented in the stuff you stored in the drop box was that chemical at half the Isp of NERVA (but comparable thrust) could not make the round trip single stage.  Both the plain chemical propulsion stage and the tug-assisted chemical stage required significant propellant launched into LMO as well as to LEO,  at around 20,000 tons each,  unless reduced a bit by tug assist at both ends of the trip.  The only difference was the strength of the gravity wells on how much propellant got consumed while launching those propellants.  NERVA needed nothing launched at Mars,  but about the same ridiculous quantity (20,000 tons) launched at Earth.  It could make the round trip single stage,  without tugs.

Electric could reduce the quantity of propellant required in LEO to something less ridiculous at 6-7000 tons for the round trip without tugs,  because of its far-higher high Isp,  despite the penalties for being very non-impulsive in thrust levels.  The travel time is longer,  due to the spiraling required for escape and capture.  Travel time is bad enough for Hohmann transfer.  Faster trajectories require significantly more propellant.  The spiraling trajectories rule out the use of tugs. 

Gas core nuclear Isp varies considerably with the exact cycle.  I figured 8000 tons of propellant for the round trip with the nuclear light bulb at 1300 s,  less for a regenerative open cycle at 2500 s,  and less still for a radiator-cooled open cycle at 6000 s,  all without tugs.  These get you down near or under 1 ton of propellant per ton of dead-head payload.  None of these are currently deployable technologies.  All require considerable development with no guarantee of success.  The nuclear light bulb has the most experimentation behind it,  but still a whole lot less than will be needed.

The nuclear explosion drive shows the highest potential at well under 0.2 tons "propellant" per ton of dead-head payload (around 650 tons propellant to push 5000 tons of dead-head).  In this case "propellant tonnage" is the mass of fission devices that are both shaped charges and have significant reaction mass that is to be vaporized and flung against the blast plate on the shock absorbers.  This is 1957-vintage fission technology.  We know it would work,  but after the Starfish Prime space nuclear explosion test of 1962,  we know EMP is a big risk. 

Actually,  I have known for many decades that the nuclear explosion drive is by far the highest propulsive potential that we humans know,  in a technology that we know we could make work.  In 10,000+ ton vessel masses,  it is near 10,000 s Isp,  and you have to work hard to hold the vessel acceleration levels down to 2-4 gees.  Isp suffers if you try to use it for small vessels (down under 5000 s at under 5000 tons).  I don't understand why that it true,  but that is what Freeman Dyson and the rest found while working on it for General Atomics in the mid to late 1950's.  I read his son's book about those efforts.  The data is in there.  That was USAF's old Project Orion. 

You build ships like that of 2 inch steel plate,  similar to how ocean-going vessels are built.  The forces are horrendous.  So is the radiation. 

GW

Last edited by GW Johnson (2022-02-22 21:49:37)


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

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#32 2022-02-23 07:33:17

tahanson43206
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Re: Large Ship Chemical Propulsion

For GW Johnson ... re Post #31 with multiple topic content

First, thank you for continuing to support this set of initiatives on behalf of Large Ship!

I have quoted your post in the Atomic Propulsion topic, and have offered a summary of Calliban's recent work on a mini-Atomic-putt-putt.

In this topic, I am hoping to enlist your continued assistance with the chemical only propulsion concept for Large Ship.

We MUST have this work in the archive, as a baseline for comparison with better systems.  It is as though the engineers were chafing at the bit to use steam engines in the ocean going vessels of the late 1800's, and management was insisting on cost estimates for a wind powered system.

***
I was taken aback by your forward looking innovation of placing a tug at both ends of a flight to Mars.   I had not stretched my thinking that far, and appreciate your taking that mental leap.

However, the request I have here is for figures for a flight that is entirely sourced from Earth.

If a funder .... let's say a Sovereign Nation, decides to build a Large Ship to project it's image as a Great Nation, then there would be NO infrastructure at the destination.  The Sovereign Nation might want to visit Venus, or it might want to visit the Asteroid Belt.

There will be no infrastructure available at those destinations, so I'm asking for estimates of the masses of chemical propellants needed for a flight to Mars in particular without infrastructure available. 

Here is what I'm imagining, with expectations of Departure Tugs and Arrival Tugs at Earth.

1) Ship is in LMO ready for departure: How much propellant is on board for the trip including course corrections?
2) Ship is approaching Mars and will be making a soft "docking" in orbit around Mars: How much propellant is on board?
3) Ship has just been released from Departure Tug, and is about to achieve Escape velocity: How much propellant is on board?
4) Ship is in LEO and is waiting for Departure Tug: How much propellant is on board? (Expecting same as 3 but covering all bases)
***
5) Departure Tug is loaded and ready for mission, to include push Large Ship and return to LEO: How much propellant is on board?
6) Arrival Tug is loaded and ready for mission, to catch Large Ship and return both to LEO: How much propellant is on board?
***
7) All propellant is manufactured and stored on Earth: how much propellant is needed to place 1-6 into orbit?

The grand total is the amount of propellant needed for a Large Ship flight to Mars and back, without infrastructure in place at Mars.

That grand total is the amount of propellant to be manufactured on Earth by a suitably sized fission reactor.

***
I would like to see an outcome from this process of at least one of your NewMars associates able to replicate your work, using your tools.

At the moment we appear not to have anyone capable of achieving this particular set of skills, but that does not mean we must give up.  We have an active and ongoing recruiting campaign, and you will have an opportunity to (try to) enlist helpers from the NSS community.

Let's all try to keep our spirits up.  We have embarked upon a perilous and difficult journey, but we are sharing the failures and successes via this medium.

(th)

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#33 2022-02-23 08:01:21

Calliban
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Re: Large Ship Chemical Propulsion

Another propulsion option, which would have propellant mass requirement somewhere between chemical propulsion and ion-electric, is a mass driver (linear electric motor).  The main difference is that propellant is really just reaction mass.  You could pick up regolith on the Martian moons.  It would be accelerated in buckets within the mass driver and spat out the end to produce thrust.  The motor itself, would be large and would have substantial mass.  The saving grace is that propellant isn't something you need to manufacture and lift into orbit.  At least not at the Martian end.  Ideally, we would take on enough reaction mass from Phobos for a whole round trip to Earth and back.  That means no propellant lifts are needed at the Earth end of the trip.  But that may demand more ISP than the mass driver is capable of.  It is worth looking into I would suggest.

Last edited by Calliban (2022-02-23 08:06:06)


"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

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#34 2022-02-23 13:13:26

GW Johnson
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Re: Large Ship Chemical Propulsion

If I understand what Tom really wants,  I need to refigure the Big Ship with chemical propulsion assisted by a tug at Earth but not at Mars.  Is that correct? 

Be aware that that requirement may be still beyond the reach of chemical in the form of LOX-LCH4.  I don't yet know. 

Also be aware that the tug-assisted arrival burn at Earth is a single-point-of-failure inherent in this mission architecture,  same as the arrival burn if not tug-assisted.  There is no way that I am aware of for a tug to go out and capture a big ship whose propulsion stage engines have failed. Even if they knew ahead of time.

GW

Last edited by GW Johnson (2022-02-23 13:16:43)


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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#35 2022-02-23 14:00:12

tahanson43206
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Re: Large Ship Chemical Propulsion

For GW Johnson re #34

I confess to a (modest) state of elevated interest in your work, as we consider pushing the envelope of what may be possible.

We (humans) DO NOT have atomic anything to propel a vessel of ANY kind at the moment.

Like the sail ship funders of old, I hear the engineers talking about those exciting new steam engines, and I'm sure they are going to be useful some day. But that day has not arrived.  We have to ship goods and people across the Atlantic, and we know how to do that.

In this case, we have an opportunity to discover if it is feasible to mount an expedition containing 1060 people and all their supplies for a two year "flight" to visit Mars and return, using nothing but chemical power for propulsion, and accepting boost at Earth.

You have questioned the feasibility of an Arrival Tug matching orbit with Large Ship upon return.

I am extrapolating from the work you have already done to show the feasibility of a Departure Tug.

It appears that just as you got ahead of me by imagining a set of Tugs at Mars, now I am ahead of you by imagining an arrival tug to catch Large Ship as it approaches from Mars, attaching securely, and decelerating the vessel so it eases into a desirable orbit in LEO.

In order for this to work, what I am imagining is a Space Tug sent on a giant ellipse, similar to the one planned for Departure Tug, but designed to put Arrival Tug precisely where it needs to be to match orbit with Large Ship.

We have already discussed a missed connection, but it is worth repeating, because others in the forum may wish to contribute to refinement of the concept.

If the Large Ship misses the connection with Arrival Tug, it becomes an Aldrin Cycler.  The miss will occur in plenty of time for radio transmissions to alert the mission planners on Earth, and they will (hopefully) be ready and able to launch intercept missions to offload the passengers who wish to disembark, and to resupply the crew for the long flight as an Aldrin Cycler.  Eventually the ship will return to Earth, and whatever bugs interfered with the success of the Arrival connection will have been resolved.

Nothing will be lost, except the use of the ship for two years.  The miss might even be turned into a junket for those who might like to descend into the inner Solar System for an Earth year or two.

To renew the request ....

Since the Arrival Tug concept appears to be the next item on the Critical Path, let's focus on that....

Is there a large ellipse orbit that allows the Arrival Tug to match orbit with the returning Large Ship?

I expect there is, but whether ** we ** (er, you?) can find it remains to be seen.

Thanks for accepting the opportunity to work on what appears to be bleeding-edge conceptualization.

(th)

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#36 2022-02-27 18:53:55

SpaceNut
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Re: Large Ship Chemical Propulsion

Tonight's GW drop box on orbital mechanics for the flight of the large ship has a space tug pushing the ship on a course to mars direct.

The next is set to a loop for gravity assist before firing the main ships engines on the large ship as an option to getting a 5000 mT ship to mars. Of course the space tug will need a level of fuel reducing what the large ship will need to finish getting on its way to mars.

It is a lot when you do not work with it all the time to understand.

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#37 2022-02-27 20:21:06

tahanson43206
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Re: Large Ship Chemical Propulsion

For SpaceNut ... thanks for taking a look at the draft document!  When I joined the meeting a few minutes ago, discussion seemed to be about propulsion in general, including the chemical concept.

Everyone seems to be in agreement that the Russians are unlikely to help with nuclear propulsion, at this point.

***
The ISS may be in trouble as well.

***
(th)

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#38 2022-02-27 22:08:22

SpaceNut
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Re: Large Ship Chemical Propulsion

It may also delay any nuclear power sources as well, even a smallish KRUSTY style unit as well from being used. We will just have to play the wait and see game for the ISS and other items.

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#39 2022-02-28 07:50:28

tahanson43206
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Re: Large Ship Chemical Propulsion

For SpaceNut (primarily - all welcome) ...

During last night's Zoom, Dr. Johnson revealed that he has added a fourth scenario to his plan for the Chemical Propulsion (Earth/Mars) document.

He is NOW adding a version that will use tugs at Mars, with the reasoning that the less intense gravity well provides an energy savings that might yield a less expensive propulsion solution than the other three scenarios.

I will now attempt to recover the other three scenarios from my memory ...

The first that comes to mind is the one I am pulling for: Tugs at Earth only, for Departure and Arrival
The second is the one that GW Johnson added: Tugs at both Earth and Mars so refueling at Mars
The third was (I'm drawing a blank)
However, the fourth is this new variation, with tugs only at Mars.

I am pulling for the Earth Tug Only version because there is no infrastructure in place a Mars, or any other destination for Large Ship in the Solar System.

It is (or rather, ** may ** be) feasible to outfit an expedition from Earth with a manifest of 1000 scientists and support staff to visit remote sites in the Solar System, using nothing but chemical propulsion.  That is the scenario I am pursuing, because it does not require invention ot new technology.  By the time Large Ship is ready for departure from LEO (several years from now) one or more of the advanced propulsion solutions should be ready.

There is an easy to understand comparision ... in the 1800's, steam engines were in rapid development.  My understanding is that the first experiments were carried out on rivers, and with demonstrated success, the engineers (and their funders) were brave enough to try adding steam to sailing vessels. The Great Eastern is an excellent example of the hybrid concept.

In today's world, the only technology we have available is chemical propulsion.  Other technologies look interesting, but at this point, they are no more than toys, suitable for demonstration on one ton probes with no passengers.  In order to secure funding for a ship the size of Large Ship, the design team must show that the ship can be safely moved between Earth and Mars using existing technology.  I am hoping that the work of GW Johnson will have this happy result.

If some bright young engineer (or even a middle aged one such as might be a member of this forum) invents an atomic powered propulsion system, then it would be installed on Large Ship (and the Tugs) with enthusiasm.

In the mean time, a steady, rock solid, methodical advance of the Large Ship enterprise is in motion.

(th)

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#40 2022-02-28 17:33:00

GW Johnson
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Re: Large Ship Chemical Propulsion

4 scenarios:  (1) baseline big ship with propulsion stage,  no tugs,  (2) first improvement big ship plus tugs at Earth (for departure and arrival),  (3) second improvement big ship plus tugs at both Earth and Mars (for departure and arrival,  both planets),  and (4) third improvement big ship plus tugs only at Mars (for arrival and departure). 

All four scenarios are technically feasible,  but the total propellant quantities (what must be delivered to orbit plus all the propellant required to launch those deliveries to orbit) are enormous and the required manufacturing monthly rates also enormous.

The original study I did compared propellant requirements for multiple kinds of propulsion,  all on the baseline scenario (no tugs).  To it I appended a less accurate initial look at the first and second (but not the third) improvement scenarios,  all with chemical propulsion.  The revisit looks only at chemical propulsion with all 4 scenarios,  with much more accurate capture ellipse orbit data and dV data.

With non-impulsive low-thrust electric,  you cannot use tugs,  because those burns have to be impulsive,  and electric does not support that.  Electric is already flying at smaller sizes,  even iodine at really small sizes.  All it needs is a scale-up to really large sizes,  plus a power source large enough to do the job.   Very crudely a factor 5+ improvement over chemical.   Could be more,  depending upon the details.  Not all electric is the same.  Very promising "get started" option.

Solid core nuclear thermal is only about factor-2 better on Isp.  That gets you single stage round trip for about the same total propellant sent up,  all at Earth alone.  Not really enough of an improvement. 

Except that they as-yet do not exist,  the gas core nuclear thermal options provide performance equal to or better than electric,  at thrust levels high enough to be impulsive.  So,  you could also use gas core nuclear thermal tugs with it on the big ship to reduce the propellant requirement further. 

As for nuclear explosion propulsion,  we know it would work in its original fission form, although there is lot of development yet to be done, and some sort of EMP risk associated with it.  Modernized fusion,  laser-fusion,  or hybrid fission-fusion variants are on paper,  nothing yet exists.  This is very most certainly the most long-term promising propulsion that we know of,  with the highest Isp potential by far,  and also at very high thrust (certainly impulsive).  Although I doubt you'd really need them,  you could do nuclear explosion drive tugs with this.  Or gas core tugs. 

GW

Last edited by GW Johnson (2022-02-28 17:39:38)


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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#41 2022-02-28 18:51:02

tahanson43206
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Re: Large Ship Chemical Propulsion

For GW Johnson re #40

For all following ongoing development of propulsion scenarios for Large Ship ...

An updated version of the working document is available:

http://newmars.com/forums/viewtopic.php … 40#p191640

The fourth scenario (Mars Tugs) is added.

(th)

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#42 2022-02-28 20:22:26

SpaceNut
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Re: Large Ship Chemical Propulsion

The big thing is to find mass savings in design, build materials, furnishings e4ct.. that allow for a propulsion system that can be smaller.

To do so means we need to get more approximate mass estimates in section to make use of as a baseline.

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#43 2022-03-01 15:26:06

Calliban
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Posts: 3,573

Re: Large Ship Chemical Propulsion

Work commitments are preventing me from making more than a token contribution to this forum at present.

I believe that fusion-fission hybrid pulse propulsion is the most practical near term variant of the nuclear pulse propulsion concept.  Pure fusion runs into plasma instability problems that have so far prevented its practical application.  If it won't work in a stationary power plant here on Earth with no weight limits, there is slim possibility of it being applicable to a spacecraft propulsion system with strict mass limits anytime soon.  Pure fission would produce too much radioactivity to be acceptable and the quantities of enriched fissile materials required would make it expensive and politically difficult.  With a hybrid concept, we are looking at having just enough fissile material at the heart of the hydrogen fuel pellet to create a hot spot as compression shock waves pass through it.  Ideally, only a tiny proportion of total energy yield will come from fission.

I suspect there are a number of reasons why the hybrid approach has not been followed so far.  Under the comprehensive test ban treaty, it would be illegal.  The treaty is not yet fully ratified and hopefully, it never will be.  The generation of fission products, even in small quantities, would make the inside of the reaction chamber a beta gamma hazard.  So fusion facilities haven't been eagre to test the hybrid approach.  There are political issues as well.  It would be easy to level accusations of covert weapons development against such activities.  Fissile materials also have their own criticality and toxicity issues.  None the less, this would appear to be the only realistic means of achieving nuclear fusion in the time remaining, before resource shortages permanently shut off our ability to build high technology.

Given all that is going on in the world at present, the first application for any working fusion concept should be an Earth based power source.  Unless we can develop a practical energy alternative to fossil fuels soon, forget about going to Mars.  We are perilously close to another great depression.
https://www.zerohedge.com/news/2022-03- … rket-crash

Last edited by Calliban (2022-03-01 15:35:29)


"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

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#44 2022-03-01 18:18:53

SpaceNut
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Posts: 29,310

Re: Large Ship Chemical Propulsion

here is a paper written on the https://smartech.gatech.edu/bitstream/h … sAllowed=y
which gives the reason for the aerocapture of the large ship and that means we really need a modification of its baseline design.

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#45 2022-03-03 12:30:07

tahanson43206
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Registered: 2018-04-27
Posts: 18,142

Re: Large Ship Chemical Propulsion

This post is in topic Large Ship Chemical Propulsion...

I have just finished slowly studying the recent paper by GW Johnson, as published in the GW Johnson Postings topic.

I did not attempt to study the detail printed inside the spreadsheets, but assume they accurately back up the conclusions stated in the paper.

We have too few active members who have a background matched to this paper.

We have over 1000 registered members.

There might be one or two in that cohort who would appreciate the work that Dr. Johnson has done here, and might be able to provide both validation and useful feedback.

If there is someone not currently a member of NewMars who would like to comment upon this paper (or any of the others we have available) please look at the Recruiting topic.

For Dr. Johnson ... thank you for this impressive study of the chemical propulsion problem for Large Ship.

(th)

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#46 2022-03-03 20:07:15

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 29,310

Re: Large Ship Chemical Propulsion

From GW's paper I worked backwards to determine the fuel loading in the central hub needed for the journey after the tugs had done the initial push and made adjustments to the totally mass estimates.

The 5000 mT / 0.97 = 5000 mT + fuel
which is approximate 150 mT to 160 mt of LOX and LCH4 for the vacuum engines.

The real issue is getting the tug and fuel to mars orbit so that we can come back since there is no infrastructure at mars and the one that brings the large ship up to escape speed returns back to earth to be reused. The next issue for the mars tug is fuel boil off while it waits on orbit for use to go home. Sure the reload of the large ship is a challenge but no where near the requirement for the tug.

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#47 2022-03-04 12:53:35

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,615
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Re: Large Ship Chemical Propulsion

The R = 0.97 = Wp/Wtot applies to the propulsion stage by itself,  without the dead-head payload! 

You guess a propellant quantity Wp,  compute the corresponding stage inert mass Winert as (1-R)*Wp/R,  and add that stage inert to the dead-head mass to determine burnout mass Wbo = Wdeadhead + Winert for the rocket equation.  Rocket equation ignition mass is then Wig = Wbo + Wp,  and the mass ratio MR = Wign/Wbo. 

Use the exhaust velocity to find out with the rocket equation what your delivered dV is,  and compare that to the summed factored astronomical dV's that you need to cover in the burns (plural) which that weight statement and resulting MR is intended to cover.  Iterate your guesses for propellant Wp until your delivered dV matches the sum of factored astronomical dV's. 

There is no way not to iterate!  That's what the spreadsheet is for.

Direct from LEO,  it takes 21,110 metric tons of propellant inside that stage (at R = .97,  Isp = 380 s) to send that 5000 metric ton dead-head payload to Mars,  direct from LEO to LMO.  Propellant is utterly exhausted when it reaches LMO.  It has to be refueled on-orbit there,  or else the trip is a one-way suicide mission.  Sorry,  over 21,000 tons,  not a few hundred tons!!!  Switching to LOX-LH2 at 470 s or thereabouts makes a few-% difference,  but not a factor of 2!  Little or no effective difference,  really.

Assuming the same 5000 tons is to return home,  it takes ANOTHER 21,110 tons of propellant to refill the propulsion stage IN ORBIT AT MARS (!!!!) to return the dead-head payload to Earth orbit.  Propellant is exhausted after entry into LEO.  That's for min-energy Hohmann transfer,  including a small "kitty" for course corrections.  If you try to fly faster,  the astronomical dV's are higher,  and it takes a whole lot more propellant than that!

Those are just the numbers it takes to do the big ship with chemical propulsion.  Those numbers are simply in the tens of thousands of tons of propellant to be be delivered on orbit at both planets.  The ferry propellant to launch such quantities to orbit approaches a million tons,  divided between Earth and Mars.  (That division is unequal,  since the gravity wells are unequal strength.)

That's just the tyranny of the rocket equation.  It's physics no one can circumvent. 

It's also why we need something LOTS LOTS LOTS better than chemical by the time we are ready to fly ships this large.  Simple as that.

GW

Last edited by GW Johnson (2022-03-04 12:58:39)


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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#48 2022-03-04 14:37:46

tahanson43206
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Registered: 2018-04-27
Posts: 18,142

Re: Large Ship Chemical Propulsion

For all ... this post is reserved for observations after analysis of recent work by GW Johnson, including Post #47 here and other similar posts in other topics.

This post will flow off the Active list, and no one will see it after that, except for Mars_B4_Moon, who has demonstrated a remarkable tolerance for ancient posts in ancient topics, in his (to me remarkable) study of the NewMars archive.

For Mars_B4_Moon, since you ** will ** (without doubt) eventually see this post, I am planning to extrapolate from the work of GW Johnson.

As things stand, we appear to have a firm estimate of propellant needed to send a fully loaded (5000 MT) Large Ship home from Mars.

We also appear to have a factor that can be used (with modest expectation of validity) to compute propellant requirements for other masses.

In short, I am encouraged to think that the work of GW Johnson is sufficient to answer the question I've been asking.

For GW Johnson if you see this (which I admit is unlikely) ... Thank you ** very ** much for your work on the entire set of propulsion options for Large Ship. Your work is a major part of the transition of Large Ship from a science fiction fantasy to an intermediate form of concept with potential to actualization.

SearchTerm:Chemical propulsion for Large Ship
SearchTerm:Propulsion chemical for Large Ship
SearchTerm:Round trip chemical propulsion requirements for 5000 MT Large Ship

This is the link to the GW Johnson Postings post with link to the Propulsion study: http://newmars.com/forums/viewtopic.php … 29#p191729

(th)

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#49 2022-03-04 17:50:26

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 29,310

Re: Large Ship Chemical Propulsion

normally I would compare what I know to derive the estimates and that means the mass/ payload plus fuels for a starship to send 100 people and this is a factor of 11 approximate number of ships to move the crew.

Of course the starship to mars was to send 2 cargo ships to land on mars for support with food, water, equipment to make a return trip possible once refueling and restocking the food and other supplies. Mars cargo that is 200 to 400 for those 100 people factor to add in on the large ship cargo for surface use.

With the real problem being the return had nothing but hard work to restock once on mars for the return trip.

so 2200 to 4400 for the large ships crew sizing would equal the planned starship use.
One way use is 150 x 11 is 1650 mT which makes the sum min 3,850 mt and max 6,050 mT.

dry mass is 85 mT to 120 mT with a base of 100 mt to 200 mt of payload with a min amount of fuel being 1100 mT to 1200 mt.
So min mass is 185 mT with max is 329 mT to equal a dead mass of 5000 mT which makes min 2,035 mT with the max 3,619 mT for ship structure for the large ship.

adding 3,850 to 2,035 mt for the min is 5,885 mt and for max adding of 6,959 mT + 3,619 mT is 9,669 mT for the structure and payloads.

So fueled up 1100 x 11 min is 12,100 mT to max 13,200 mT to get that mass to mars with the cargo supplies and structure.

restock and refuel is the same for fuel 12,100 mT to 13,200 mT but for the food and water we only need the trip outs quantity to be brought up to the large ship for payload just 1,650 mT.

So we are high for the dead mass structure and cargo at the max end of the spectrum, which makes the fuel requirement even higher.

So if we shoot for the average we do not really impact the ships probable fuels but the other would be less than the dead head mass number.

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#50 2022-03-06 21:43:59

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 29,310

Re: Large Ship Chemical Propulsion

I am wondering how much fuel can be brought to orbit by a starship cargo to refuel a tug since we know that a fully loaded is 1200 mT plus more in a payload if it have tanks to bring it up to the waiting station tug.

I had found that it takes for a zero cargo playload just the 20 mt of consumed for a crewed 85mT starship that its only going to need 340 mT to get home so we must be able to bring quite a bit from mars back up to mars orbit for refueling.

Thinking about landing back on the mars surface from orbit would seemingly require less fuel since we would be empty of payload which was requiring 100 mT but we can leave that so we are guaranteed a safe re-landing of a starship back on its surface.

1200 mt - 440 mT for round trip leaves 760 mT minimum for refueling of a space tug.

That sure is a large order with how many trips we need say nothing to how long it will take to make the fuel.

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