OFMM Mission Discussions

[Arrowstar]

Dgatsoulis, are you suggesting that 22,664 km/s of dV is too low? It would seem to me that we could go anywhere in the solar system multiple times over on that.

 

[Pipcard]

There have been some concerns, in the trajectory office backroom. about the power that the Mars ship can yield. For reasons explained here, we have already seen that the total weight of the supplies that can be delivered to the James Cook cannot be more than 467 tons. If that was all fuel and the ship had nothing else onboard, it would give a dv of 127 km/s, which is a slight overkill.

But if we assume a 70 - 30 ratio (70% for air, food, water and equipment - 30% fuel) for the weight of the cargo, we get 140.1 tons of cargo that consists of fuel. Calculate in a factor of 0.85 for the weight of the containers and the fuel becomes 119.08 tons. So out of the 467 tons, 347,92 need to be added to the emptymass and 119.08 are the fuelmass. Let's add the weight of the two XR2s that will be docked and then make the calculation for the dv:

dv = ln((EmptyMass+FuelMass)/Emptymass)*exhaustV
EmptyMass=350000+347920+(2*16080)= 730080kg (the weight of the Cook + supplies for the trip + the weight of 2 empty XR2s).
FuelMass = 119080
ExhaustV=150000
This gives us a dv of 22664,026 m/s available at the TMI burn. (22,664 km/s)

We also need to keep in mind that at least 26 tons of fuel will be transfered to the XR2s when we arrive at Mars, lowering the available dv even more.

Dgatsoulis, are you suggesting that 22,664 km/s of dV is too low? It would seem to me that we could go anywhere in the solar system multiple times over on that.

22.664 km/s because some people don't use a comma for the decimal mark.

 

[Arrowstar]

Ah! Thanks. Okay, if that's our budget, I think we'll still be okay. Let me run some numbers and get back to everyone in the Traj Backroom thread.

EDIT: Dgatsoulis, can I get the units on each of those terms in your dV calculation?

EDIT2: Okay, with 30 tons of fuel going to the XR2s, I calculate that we'll have about 17 km/s of dV to play with. Still crunching numbers here...

 

[garyw]

Dgatsoulis, are you suggesting that 22,664 km/s of dV is too low? It would seem to me that we could go anywhere in the solar system multiple times over on that.

maybe - maybe not. How much dV to Mars and back?

how much from Mars Orbit to Mars Surface (times 5 for safety) - how much from Mars to Phobos?

that should give a rough starting figure for the fuel haul to the mars vessel.

 

[dgatsoulis]

Dgatsoulis, are you suggesting that 22,664 km/s of dV is too low? It would seem to me that we could go anywhere in the solar system multiple times over on that.

No, it's 22 point 664 km/s. I am used to using a comma for the decimal point.

EDIT:

Dgatsoulis, can I get the units on each of those terms in your dV calculation?

Mass is in Kg which cancel out in the division and exhaustV is in m/s. Thus the result is also in m/s.

---------- Post added at 08:29 PM ---------- Previous post was at 07:41 PM ----------

maybe - maybe not. How much dV to Mars and back?

how much from Mars Orbit to Mars Surface (times 5 for safety) - how much from Mars to Phobos?

that should give a rough starting figure for the fuel haul to the mars vessel.

For the free return trajectory we need roughly 12 km/s

5.6 km/s TMI burn
0.4 km/s MCCs + Mars approach Eq. adjustment
6,0 km/s MOI burn
--------
12,0 km/s total

Let's not forget the dv required for the lunar shakedown mission OFMM 19

3.15 km/s TLI burn
0.85 km/s LOI burn
0.90 km/s TEI burn
3.30 km/s EOI burn
------
8,20 km/s total

I'm expecting some guests now, so i won't be able to crunch the rest of the numbers untill later tonight.

 

[Arrowstar]

For the free return trajectory we need roughly 12 km/s

5.6 km/s TMI burn
0.4 km/s MCCs + Mars approach Eq. adjustment
6,0 km/s MOI burn
--------
12,0 km/s total

Let's not forget the dv required for the lunar shakedown mission OFMM 19

3.15 km/s TLI burn
0.85 km/s LOI burn
0.90 km/s TEI burn
3.30 km/s EOI burn
------
8,20 km/s total

I'm expecting some guests now, so i won't be able to crunch the rest of the numbers untill later tonight.

The numbers for the lunar shakedown seem fine, but I think we can pare down the Mars run a bit. First, if done right, I think we can leave Earth with closer to 4.1 km/s: the Cook just needs to be in the proper parking orbit. Here's my analysis:

The optimal departure from EARTH occurs at 5/13/2018 13:57:45 (C3=7.7497 km^2/s^2)        
The optimal arrival at MARS occurs at 12/8/2018 5:0:45 (arrival velocity=2.9747 km/s)     
The optimal trip duration is 208.6271 days.

And looking at departing from Earth (300x300 km orbit, 51.6 deg inc, RAAN 180 deg):

Computing Departure Burn...Done!                                                   
Burn Delta-V is 4.1397 km/s.                                                       
Burn should be performed at true anomaly of 13.9601 degrees of pre-departure orbit.

So the departure and MOI can be done with ~7.1 km/s, I believe. I'll look at return numbers later tonight.

 

[dgatsoulis]

The numbers for the lunar shakedown seem fine, but I think we can pare down the Mars run a bit. First, if done right, I think we can leave Earth with closer to 4.1 km/s: the Cook just needs to be in the proper parking orbit. Here's my analysis:

The optimal departure from EARTH occurs at 5/13/2018 13:57:45 (C3=7.7497 km^2/s^2)        
The optimal arrival at MARS occurs at 12/8/2018 5:0:45 (arrival velocity=2.9747 km/s)     
The optimal trip duration is 208.6271 days.

And looking at departing from Earth (300x300 km orbit, 51.6 deg inc, RAAN 180 deg):

Computing Departure Burn...Done!                                                   
Burn Delta-V is 4.1397 km/s.                                                       
Burn should be performed at true anomaly of 13.9601 degrees of pre-departure orbit.

So the departure and MOI can be done with ~7.1 km/s, I believe. I'll look at return numbers later tonight.

Nice!:thumbup: Getting the TMI burn down to 4,1 km/s instead of 5,6 km/s is impressive. But we'll have to calculate the orbit that the Cook must have after shakedown mission and keep it there for sometime. I think it would be better if we get the ship back to equitorial orbit and make the TMI burn from there.

maybe - maybe not. How much dV to Mars and back?

how much from Mars Orbit to Mars Surface (times 5 for safety) - how much from Mars to Phobos?

that should give a rough starting figure for the fuel haul to the mars vessel.

LMO to surface for an XR2 (default settings) takes about 2 km/s of dv. (proper hover landing and correct use of aerobrake + airbrakes + 20% margin of error). Ofcourse, this changes according to the weight of the cargo.

Mars surface to LMO ~ 3,8 km/s (also depends on the cargo).

But i think we are starting to knittpick here and perhaps get into lengthy calculations and a debate that will stall the project.

150 tons of fuel for the James Cook (and 2x times that in supplies/landers) are more than enough to make the trip (Earth->Moon->Earth->Mars).
Sending fuel there ahead of the mission (about 120 tons of it, in the Quasar OFMM-20) is essential for our success and also gives us a reasonable margin of error.

I see no show-stoppers in the mission roster, so i say:
GO for launch OFMM-P1.


(Excuse my tone, i've had a couple of drinks too many tonight).

 

[garyw]

I see no show-stoppers in the mission roster, so i say:
GO for launch OFMM-P1.

Great work guys. Any dissent from other OFMM members? I know that there was some concern over using the James Cook so I want to make sure that everyone is happy before I post the first scenario.

 

[Pipcard]

Great work guys. Any dissent from other OFMM members? I know that there was some concern over using the James Cook so I want to make sure that everyone is happy before I post the first scenario.

Nope, I'm fine with the James Cook after learning that we don't need to use 100% fuel.

Also, can you please forgive me for asking too many questions about the XR2 Moon mission?

 

[Arrowstar]

Nice!:thumbup: Getting the TMI burn down to 4,1 km/s instead of 5,6 km/s is impressive. But we'll have to calculate the orbit that the Cook must have after shakedown mission and keep it there for sometime. I think it would be better if we get the ship back to equatorial orbit and make the TMI burn from there.

Okay, here's a proposal: what if we start the Cook out in an equatorial orbit and then have it return to a higher inclination orbit after the shakedown? This way we're in the proper position for TLI, but we can also take advantage of the benefits that a higher inclination orbit can offer. Your thoughts on this would be appreciated.

I'll put together a trajectory memo that outlines everything we've discussed in the next few days after I get feedback on the above from you.

 

[MaverickSawyer]

Looking good from here. let's do this!

 

[Arrowstar]

Let's get started with the initial missions, I agree. That said, we do have a problem.

I'm putting together a dV budget for the Cook's Lunar Shakedown and Mars missions and, long story short, we're going to need to get another 10 km/s from somewhere or all we're doing is flying by Mars and out into space. Here's the breakdown:

--18.45 km/s of dV budget, total (taken from the original 22.6 km/s budget we had and subtracting some fuel for the XR2s)

--8.2 km/s for the lunar mission, all told

--3.64 km/s for TMI
--5.1 km/s for MOI (free return trajectory)
--2.32 km/s for TEI (if the Cook is parked in the correct orbit and leaves 2 weeks later, detailed later in that memo)
--8.42 km/s for EOI

The total lunar cost is 8.2 km/s, and the total Mars cost is 19.5 km/s. This is a deficit of about 9 km/s of dV.

I notice we're sending fuel to Mars in advance. How much of this can get transferred to the Cook? Can we use some of the space meant for O2 or other consumables and replace it with fuel? Can we resupply the Cook after the lunar shakedown?

Comments would be welcome.

EDIT: DV Budget is available here.

EDIT2: I think I can free up 1-2 km/s by getting rid of that free return trajectory. Thoughts?

 

[Pipcard]

Another refueling mission after the shakedown would be good.

 

[dgatsoulis]

Okay, here's a proposal: what if we start the Cook out in an equatorial orbit and then have it return to a higher inclination orbit after the shakedown? This way we're in the proper position for TLI, but we can also take advantage of the benefits that a higher inclination orbit can offer. Your thoughts on this would be appreciated.

Agreed, it's much more fuel efficient to be in a correct high inclination orbit when we return from the Moon.

Let's get started with the initial missions, I agree. That said, we do have a problem... total (taken from the original 22.6 km/s...8.2 km/s for the lunar mission...total Mars cost is 19.5 km/s...a deficit of about 9 km/s of dV...Can we use some of the space meant for O2 or other consumables and replace it with fuel?

Remember that the initial estimate of the dv budget (22,6 km/s), comes from an arbitrarily chosen ratio of 70% supplies - 30% fuel. We can easily change this to whatever we want, if we need a little bit of extra dv.
For example: If we go to 60% supplies - 40% fuel we get a dv of ~31 km/s and the same calculation for 50% supplies - 50% fuel gives us ~39,5 km/s.
Another thing to keep in mind is that as the weight of the ship decreases due to spent fuel, we will get more "umph" from the remaining fuel, increasing our dv by a little bit.

I notice we're sending fuel to Mars in advance. How much of this can get transferred to the Cook?

I'd say all of it if neccessary, but that's up to Gary to decide, if the Cook and the orbital tank will be docked for the entire time we are at Mars or if they will be at different orbits.

Can we resupply the Cook after the lunar shakedown?

I don't see any reason to change the roster, but I just started working on the Quasar mission, to see how much fuel it can actually deliver to Mars. I'll have the results sometime later today.

I think I can free up 1-2 km/s by getting rid of that free return trajectory. Thoughts?

I think that the free return trajectory is expensive but essential for getting the crew back, in case things go wrong. My recommendation is to stick with it.

 

[garyw]

Another refueling mission after the shakedown would be good.

Why? Is there a shortfall of propellant from the shakedown? Do the supply missions not provide sufficent prop for shakedown + Mars?

I've no issues with adding another mission but I'd like to know where the numbers break down.


---------- Post added at 13:30 ---------- Previous post was at 13:27 ----------

I'd say all of it if neccessary, but that's up to Gary to decide, if the Cook and the orbital tank will be docked for the entire time we are at Mars or if they will be at different orbits.

No they won't be. The thought was, get to Mars, dock with the orbital propellant tank, empty it then leave it in a decaying Mars orbit whilst the Mars vessel moves to its Mars Mission orbit.

I don't see any reason to change the roster, but I just started working on the Quasar mission, to see how much fuel it can actually deliver to Mars. I'll have the results sometime later today.

Awesome, thanks. We need to spend prop to send prop to Mars. Is that prop better used at Mars or before we go to Mars? Personally, I think refueling over Mars is just more fun and adds another Mars mission. not exactly practical I know but hey.... :lol:

I think that the free return trajectory is expensive but essential for getting the crew back, in case things go wrong. My recommendation is to stick with it.

Agreed.

 

[Arrowstar]

Okay, based on that discussion, let's go with this:

We use the 60-40 supply/fuel ratio on the Cook. We keep the Mars fuel mission, because that does sound like fun. :lol: We'll also keep free return for the crewed Mars mission and return from the Moon into a higher inclination Earth orbit. I think that's about it. Thanks for the feedback!

I do need to know two things, now that I think of it: first, how long are we looking at staying at Mars, and second, what are the science requirements for the Mars mission orbit?

 

[dgatsoulis]

We need to spend prop to send prop to Mars. Is that prop better used at Mars or before we go to Mars? Personally, I think refueling over Mars is just more fun and adds another Mars mission. not exactly practical I know but hey.... :lol:

It's a fun/crazy mission, but (for novice pilots) it's easier to hit a target that's already in orbit around Mars, than getting into a precise orbit.
It would make sense if the Cook couldn't handle the weight of the fuel. Since it can, we can always change the OFMM 20 mission from fuel to equipment.

That said, i have some not so very good news about the Quasar mission. (Not bad, just not so very good). It's easy to get things to Mars, but if you want to get things arround Mars, it's much more difficult. I used the heavy 471 Quasar and started with 120 tons of cargo, working my way down, untill i could get it at a circular 500x500 kms orbit around Mars. After spending 3 hours on this, I couldn't get more than 22~25 tons of cargo around Mars. (I used the 2nd (March 7th 2016) window, since the 2018 one, will get the supplies there, after the crew arrives.
Not a show-stopper, just a slight hick-up. Recommendation: I'd send 20 tons of supplies in that mission, leaving more space for fuel on the Cook.

how long are we looking at staying at Mars?/QUOTE]

What's the window from Mars to Earth after 2018?

 

[MaverickSawyer]

:hmm: could we come up with an aeroshell for the fuel tanks and aerocapture? We would still need to burn propellant to circularize, but not nearly as much as would be used for a propulsive capture and circularization.
Granted, the aeroshell cuts into propellant mass, but is it worth the effort?

 

[garyw]

:hmm: could we come up with an aeroshell for the fuel taqnks and aerocapture?

I was thinking this might do the job of being an aeroshell well enough -> [ame="http://www.orbithangar.com/searchid.php?ID=435"]Paracone 1.11[/ame]

 

[MaverickSawyer]

It's a great idea, but how big are the tanks? and how do we attach that to the front of the tank? Seems like this would be an excellent addon, if someone wants to make a simple one with a docking port on the back...
However, that could spiral us into addon dev hell. No point in waiting for that. let's just rock and roll.

---------- Post added at 05:17 PM ---------- Previous post was at 05:11 PM ----------

What if we used the [ame="http://www.orbithangar.com/searchid.php?ID=4350"]Albatros[/ame] tanker? It already has a heat shield, and carries 25 tons of transferable fuel. plus, it has its own integral engines, with 22 tons of fuel, some of which we would wind up using for manuevers, but still more than enough for our purposes.