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Old 01-02-2012, 03:16 AM   #76
dgatsoulis
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Yes the dv is 3313 m/s . I did the calculation again and also checked with BTC 2.0 . The result was the same.

As for the fuel ship, there are two ways to deal with it. Either place it on a similar free return trajectory or push the date of departure back to the second launch window. I recommend the later because of the dv expenditure, but it's up to Gary to decide.
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Old 01-02-2012, 03:20 AM   #77
River Crab
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Originally Posted by dgatsoulis View Post
 One thing I don't understand is why "the DSCS vehicles have 2017 km/s of ΔV available."
With an emptymass of 780kg , fuel 1497,54kg and an exhaust velocity of 3092 m/s , the available dv is 3313,25 m/s. Am I missing something here?
I'm sorry, poor wording on my part. This is not the mass of the fuel, rather, the mass of the entire vehicle when fueled. Here is the calculation.
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Old 01-02-2012, 03:44 AM   #78
dgatsoulis
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@River Crab

The calculation you posted is of course correct, but in the addon you have this:

Which gives a dv of 3313 m/s
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Old 01-02-2012, 03:51 AM   #79
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Originally Posted by dgatsoulis View Post
 As for the fuel ship, there are two ways to deal with it. Either place it on a similar free return trajectory or push the date of departure back to the second launch window. I recommend the later because of the dv expenditure, but it's up to Gary to decide.
We can push it back (and I've considered it), but if we do that, we're going to have fuel sitting in Mars orbit for a very long time. Typically I'd be worried about boil-off, but I suppose we can hand wave that away. The other consideration, of course, is that the 2015/2016 window is going to be pretty full if we add this launch then. I've been spacing flights three days apart, which means we'll have two solid weeks of launches. Can we sustain that launch rate (or should we just hand wave that away, too)?

Gary, your thoughts?
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Old 01-02-2012, 04:26 AM   #80
River Crab
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Originally Posted by dgatsoulis View Post
 @River Crab

The calculation you posted is of course correct, but in the addon you have this:
{image}
Which gives a dv of 3313 m/s
Oh my, I really smegged up there
Thanks for the catch, this is why the version number is less than 1.0. Will fix this ASAP.
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Old 01-03-2012, 02:39 PM   #81
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I've been lurking in the wings following the OFMM sporadically. Arrowstar, here are my comments on your Tech Memo:
  • Does 'DSCS' stand for 'Deep space communications satellite'? If so, saying 'DSCS Communication satellites' is redundant.
  • Add a note to state the altitude of the aerostationary orbit (17,000 km).
  • Are the three DSCSs to be spaced out 120° around the planet? If so, say so
  • Give launch windows (with a nominal T0 time) rather than exact launch times
  • Plan some nominal DSMs (deep space manoeuvres) for mid course corrections and give them nominal dates and times. Extra ones can be added in if the trajectory is way off, but better to have burns at expected times rather than willy-nilly. You can always have 'DSM2 not required' in the mission log if the course correction burn was not required due to excellent piloting.
  • Is there a name/designator for the vessel that'll carry the DSCSs rather than 'DSCS vehicle' - think of the way that NASA referred to Apollo CSM or Centaur booster for missions rather than 'vehicle'
  • Use terminology 'nominal' rather than 'preferred' when talking about orbits
    radius/altitude. That makes it sound like you'd planned the mission, rather than just letting the pilots wing it.
  • Give rough (or even accurate) timescales for orbital insertion timescales.
  • There seems to be some inconsistency in the final orbits of the DSCSs. You have said that they are aerostationary with an inclination of 45°. Did you mean aerosynchronous? Stationary orbits can only ever be of 0°inc
  • There is no plan in the memo for when and how to separate the 3 DSCSs per mission to insert them into their separate orbits.
  • OFMM 14-17:
  • Do not let the throttle-jockeys pick their launch times. All important mission times should be nominally selected by the trajectory boffins
  • OFMM 33 - is 300x300km a little low? Think of how often the ISS gets reboosted, which is at an altitude higher than this. Will there be missions to reboost the James Cook or will it use its own power to do this? Is it more fuel efficient to use a higher parking orbit and not have to continually reboost?


---------- Post added at 14:39 ---------- Previous post was at 14:23 ----------

Also, just noticed that there is some disparity between the final satellite positions for OFMM7 and 8. The main mission assignment page states that OFMM7 will place them at 0, 60, 120 and OFMM8 will do it at 180, 240, 300, whereas you state OFMM7 at 0, 120, 240 and OFMM8 at 60, 180, 300. I agree with this latter plan as a failure of any one mission will mean that you do not lose coverage for half the planet. Maybe the main mission assignment page should be updated?
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Old 01-03-2012, 03:45 PM   #82
Arrowstar
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Hi agentgonzo,

Thanks for the feedback! I will consider all your suggestions (and they look great) soon.
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Old 01-03-2012, 07:47 PM   #83
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It will not be possible to fully stop the satellites at Mars without aero-braking.
I needed more than 100 m/s for corrections, and ended up with an inclination of around 20° on my test flight with one of them.
I was still able to get into an aerostationary orbit, but had nearly no fuel left for station keeping. This was in the 2020s, maybe Mars was a bit closer than 2014...

And do we really need them with 45° inclination? If we don't take a landing site close to the poles 0° should be enough.
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Old 01-03-2012, 07:49 PM   #84
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 And do we really need them with 45° inclination? If we don't take a landing site close to the poles 0° should be enough.
I'd say that the main landing site should be somewhere on the equator or not far from it as it makes the whole thing easier. No reason that there couldn't be a special trip to one of the poles but for ease of everything the main landing should be equator based.

Agentgonzo - I'll update the main mission plan sometime this week.
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Old 01-03-2012, 09:34 PM   #85
Arrowstar
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 And do we really need them with 45° inclination? If we don't take a landing site close to the poles 0° should be enough.
I had no information on the science requirements of the mission when I wrote that, so I picked a value that could be globally useful. I'd be more than happy to work with an equatorial mission!
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Old 01-03-2012, 09:36 PM   #86
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Originally Posted by garyw View Post
 I'd say that the main landing site should be somewhere on the equator or not far from it as it makes the whole thing easier. No reason that there couldn't be a special trip to one of the poles but for ease of everything the main landing should be equator based.
This will mean that all your ground satellite dishes will need to be actively pointed throughout the day to point to the correct latitude for the satellite which will increase cost and mass for them, rather than bolting them to the rock and having them point at a fixed alt/azimuth.
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Old 01-03-2012, 09:37 PM   #87
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Fair enough - thats our first landing constraint then - +/- 10° of the equator.

---------- Post added at 21:37 ---------- Previous post was at 21:36 ----------

Quote:
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 This will mean that all your ground satellite dishes will need to be actively pointed throughout the day to point to the correct latitude for the satellite which will increase cost and mass for them, rather than bolting them to the rock and having them point at a fixed alt/azimuth.
Yes. Let's assume the team have the equipment available to do that.
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Old 01-03-2012, 09:37 PM   #88
Urwumpe
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The orbit inclination should be at least bigger than the landing site latitude, so we can ensure two opportunities per Mars day for a launch back to the mothership.
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Old 01-03-2012, 09:46 PM   #89
Arrowstar
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 Fair enough - thats our first landing constraint then - +/- 10° of the equator.
Great, we can use a 10 degree (equatorial) inclination for the DSCS vehicles, then.
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