Interplanetary SOP

1956 September , Illyrien Space Center, Prometheus Lounge

All right people, fill your coffee mugs and lets start this meeting. With the successful arrivals at Venus, and near arrival of the Mars probes., and the numerous lost rockets, its time we established a Standard Operating Procedure for future interplanetary launches. Further, we should take a look at what can be improved. I am sure you have some suggestions.

Launch and transfer burn

First thing which comes to mind is having sufficingย  TWR. We lost one rocket, when the burn leading up to its navigation node, accidentally slammed it into the atmosphere. The temporal length of Earths orbit is only about 90min, we simple can’t have burn times of 15-20min. Thus we need to ensure that TWR is at least in the 0.5 range.

We where rather successful with the transfer nodes from Earth, in most cases able to chart a part from their straight to Mars or Venus. Had it not been for the inclination issues, we might have been able to do a single transfer burn to either planet, as predicted by Koffman. All in all, pretty good.

Thirdly, we need to be very very accurate with those burns, in the first ones we where badly off, costing thousands of dV to correct. Unnecessary mistakes. Launching off pro or retrograde of Earths orbit has to be very accurate, not just generally correction. Indeed, we should aim for a apoapsis or a periapsis at the other planet.

Finally, we should do go for dV overkill, launching the probes with as much fuel as at all possible, to ensure that they have a reserve course adjustments. Especially inclination adjustments.

Mid-course adjustment

Next topic.Mid-course adjustments. As the Venus missions showed clearly, we need to have all future missions arrive as close to the planet as possible, since, due to the fickle nature of orbital mechanics, that apparently makes them much easier /cheaper to get into orbits, and to change to desired orbits. We should aim for arrival at 10.000km or less.

Fortunately, making such changes while underway is apparently unbelievably cheap, so it should be done. In addition, we need to check up on all IP missions at the least halfway there, to ensure that they still intercept the planets in question.

Finally, we need to add to the STOP to disable RCS after every manuever, otherwise it might try to stabilize in some way, which might spoil the planned intercepts. The nodeexec script has been updated to shut down RCS after all operations. Should save some fuel, as a bonus.

Design

As you are all painfully aware, we have lost previous missions due to problems with the center of mass being off alignment with the center of thrust. The Kepler 6’s probes where redesigned to take this into account, and thus behaved better, and cost less RCS fuel to balance on node execution. We need to keep this in mind, for the future. For the Kepler 6’es, we added a structural element at the top, and out all the science gear there, rather then one the sides of the tank, as we used to.

The Kepler probes where the last generation to have separate fuel and RCS fuel, and hopefully also the last time we are going to lose a probe to lack of RCS fuel, to initiate engines. Using the same fuel, especially the aerozine/nto mix has been pretty effective in our current set of Moon missions, and we should keep this in mind for the future.

Design changes

All right, I think we have those topics covered. Lets move on towards the next topic, alternative designs and major changes. I see two suggestions listed. Reaction wheel and Aerobreaking. So lets talk those.

The case for a reaction wheel is rather simple. We have two reasons for considering those. Stabilizing the probes, and to avoid unfortunate consequences of using rcs to change the direction of the probe. In some cases, this disrupts orbits and intercepts. Its expensive though, both in money, and in energy. Those reaction wheels are expensive to run. They are also quite week, so perhaps they would need to run for a long time? Well, a thing to consider.

Finally, an unusual option came up, when we where talking about the Venus and Mars missions, and that was the option of just using the target planets atmosphere to slow down probes. Risky, but rewarding. Potentially this will allow for huge fuel savings. A thing to consider.

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6 comments

  1. Wait, the RCS can use the same fuel as the engines? Now there’s an idea – I’m currently still using a separate RCS fuel tank, but going for the highest ISP one (since I tend to cram 500+ m/s in the RCS system as well)…

    Like

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