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# Strategy for cheap satellite launch

## Question

Here are the contract specs:

(have a thermometer and an acceleratometer)
Ap: 30,390,457m
Pe: 30,278,764m
inc: 90deg
LAN: 195.3deg

If it were an equatorial (or near) inclination, I know the below would work out good. But it being a Polar orbit, normally the thing to do would be to launch to the North straight away. I have no idea of how to (precisely, or even vaguely) calculate the delta V requirements of a satellite launch. All I know for now is, the approx dV to get to LKO; to get to the Mun; and to get to Minmus. But things in between, not so sure of. So, the idea is:

1. Launch to LKO, try get inclination 0.0deg on the way up (to save fuel later setting it; or to mean I can aim for the Mun at any point rather than restricted to ascending/descending node).
2. Aim at the Mun, but don't aim for a low Pe, instead look to achieve a sufficient gravity assist to give a large eccentric Kerbin orbit upon Mun escape. As large as possible.
3. Once at Ap, perform the inclination change and also burn pro/retro as needed to 'set' the Pe to 30Mm
4. At the Pe, perform a retro burn to set the Ap to 30Mm

I (think I) know the 'cost' in dV of launching Polar - you lose the benefit of the eastwards launch, by the speed of an object (that object being the launch pad of KSC) on the surface of Kerbin - I'd need to look up the number. And I think 2-4 of the above is basically a bi-elliptic transfer, albeit the initial elliptical orbit powered by (the gravity of) the Mun, instead of rocket power. But its all a bit relative?

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1 hour ago, paul_c said:

I (think I) know the 'cost' in dV of launching Polar - you lose the benefit of the eastwards launch, by the speed of an object (that object being the launch pad of KSC) on the surface of Kerbin - I'd need to look up the number.

The dV advantage of launching with the rotational speed of Kerbin is rather small (<200m/s), the biggest advantage of a launch site at the equator is that you can launch into any orbit (even a Geo- or Kerbin-stationary orbit!) without doing a plane change!

So to get into a polar orbit you just launch straight North or South. To get into a specific polar orbit you need to launch at the right time, when the launchpad is in the plane of the target orbit (that's what a launch window is!). In KSP I don't know (or don't bother) to calculate the exact time for the launch, but estimate it by looking at the map screen, rotating the view so that the target orbit is displayed as a line, and then looking when the KSC is on that line. (You should also check if you need to launch North or South from that position before launch.) I don't match the orbit 100% but good enough that the plane change is not too expensive.

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Are you trying for a polar Kerbin orbit? Or Mun?

If it’s Kerbin, a gravity assist from the Mun will cost more dV than it saves without question. You can get a pretty decent gravity assist from the Mun but it won’t be in the correct plane and having to perform a plane change will nullify that.

Your best bet for staying cheap is to make your probe as small and lightweight as possible. The smallest probe core you have and the bare minimum equipment to keep it running. Then build your launch vehicle utilizing good staging (modest SRB burn time combined with a semi-efficient sustainer first stage make for a relatively cheap launch vehicle if done right) and well-executed gravity turn. Then like Hans said, go for a straight polar launch when the KSC crosses into the launch window, and bingo.

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Its a Kerbin orbit. I've come up with a better idea - I've stripped down the satellite to the bare minimum. However, building a rocket cheap/small to just about do it is proving a bit more tricky. I tried SSTO (to save on coupling, another engine, etc) but it didn't work. And then I tried a more conventional 3 stage (1st to about 10000m, 2nd to low orbit, 3rd on the sat itself) but that proved to be twitchy to drive so although I can get it into space, getting it nicely onto a low orbit is proving troublesome.

So.....since the satellite is only about 300kg I'll piggyback it onto the next big rocket going to the Mun. I'll need to do some deft switching of control to juggle the both; or I could just detach it at any point in between the Mun mid-course alignment burn and the Mun retro capture burn, to send it onto an eccentric Kerbin orbit. I will do it earlier than later, then do a tweak of some kind at some point, so it doesn't keep re-encountering the Mun and throwing it off or possibly crashing it into Mun or Kerbin.

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5 hours ago, paul_c said:

1. Launch to LKO, try get inclination 0.0deg on the way up (to save fuel later setting it; or to mean I can aim for the Mun at any point rather than restricted to ascending/descending node).
2. Aim at the Mun, but don't aim for a low Pe, instead look to achieve a sufficient gravity assist to give a large eccentric Kerbin orbit upon Mun escape. As large as possible.
3. Once at Ap, perform the inclination change and also burn pro/retro as needed to 'set' the Pe to 30Mm
4. At the Pe, perform a retro burn to set the Ap to 30Mm

This is not a bad plan, in this case, since the target orbit is so high.  If you're in an extremely eccentric orbit with an Ap waaaaay up high past Minmus, and a Pe down by Kerbin, then your orbital velocity at that point will be so low that the plane change will be very cheap.  Also cheap at that point to raise your Pe.  And then doing a burn at Pe to drop your Ap will also be cheap, since your Pe is still really high.

(This would not be a good plan if the target polar orbit was much lower, like a couple thousand km, because then #3 and/or #4 would be more expensive.)

The alternate approach would be to just launch directly to a polar orbit.  You'd launch just slightly to the west of due north.  (The slight westward bearing is to compensate for Kerbin's rotation.  If you actually launched due north, you'd end up in an orbit that is almost-but-not-quite polar and would need to do plane correction later.)

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2 minutes ago, Snark said:

This is not a bad plan, in this case, since the target orbit is so high.  If you're in an extremely eccentric orbit with an Ap waaaaay up high past Minmus, and a Pe down by Kerbin, then your orbital velocity at that point will be so low that the plane change will be very cheap.  Also cheap at that point to raise your Pe.  And then doing a burn at Pe to drop your Ap will also be cheap, since your Pe is still really high.

(This would not be a good plan if the target polar orbit was much lower, like a couple thousand km, because then #3 and/or #4 would be more expensive.)

The alternate approach would be to just launch directly to a polar orbit.  You'd launch just slightly to the west of due north.  (The slight westward bearing is to compensate for Kerbin's rotation.  If you actually launched due north, you'd end up in an orbit that is almost-but-not-quite polar and would need to do plane correction later.)

It's intresting. And what if we want to make small satellite launch using solar pannels?

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27 minutes ago, Snark said:

This is not a bad plan, in this case, since the target orbit is so high.

You're right, that orbit is between the orbits of the Mun and Minmus, I hadn't considered that in my other answer. Now the proposed strategy makes sense.

Once caveat that this has is that setting up the Mun gravity assist is going to be really tricky. Because you'll need to get into an orbit where the AP is on (or at least close to) the plane of the target orbit. (And, yes, settling for a lower AP that is closer to the target plane is probably better than a higher AP farther away from that plane.)

I personally probably wouldn't bother with a gravity assist and launch directly into an orbit that is close to the target orbit. A few hundred m/s of dV for a satellite are cheap.

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Yes, its one of those funny assignments - I don't know if anything like that would be asked for in real life. I'm flying the mission now, and its a bit brain-wracking managing the both at once (the small Polar sat has 1 panel for cost/weight saving, the Mun lander has fixed panels covering 4 sides because extended ones could be damaged in a rough landing). Fortunately I stuck the upgrade aerial on the probe, since its 38Mm away at the moment. Didn't want to risk it on the Mun lander so it has 4x basic ones, fortunately I have 3 relays orbiting Mun but I need to keep track of it, especially since I chose to launch at new moon. I could always park it and wait for full moon though, to play it safe. So far, its certainly been a "cheap" launch:

Inclination change, 90% of the way to Polar: 181dV
Extra burn needed on Mun lander to lower its Pe to 15km: 55dV

I still need to jiggle around the orbit the hard work is done now, its just tweaks.

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1 hour ago, Regular everyday said:

It's intresting. And what if we want to make small satellite launch using solar pannels?

Do you mean using solar panels to provide the power (every satellite contract I've seen has a requirement to generate electricity) or something else? Normally I've put 1 or 2 (depending on equipment installed and loads) fold-out panels. In this case, I was going ultra-cheap so used the small flat panel and made a mental note to keep it facing the sun.

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Phew! The other aspect of the trip, landing on the Mun, has been done. It was an unmanned basic lander but with a service bay and some scientific instruments  - and a big fuel tank. I need to decide if I'll risk another T/O and landing on the Mun or just come home.

The little satellite's contract is also fulfilled now, here's the total dV spent:

Inclination change, 90% of the way to Polar: 181dV
(Extra burn needed on Mun lander to lower its Pe to 15km: 55dV)
Raise Pe: 144dV
Inclination change #2: 66dV
Pe adjustment (down! I should have remembered, an inclination change tends to circularise the orbit...): 9dV

TOTAL 415dV (could/might have been 397dV)

Of course, there's the extra rocket fuel and size for its additional weight piggybacking - in total this satellite weighed 520kg. I had loads of fuel too, I could have probably 1/8 filled the tank (instead of 3/4 filling the small doughnut one) and gotten its weight down a bit more. But I had no idea what the requirements would have been.

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1 hour ago, paul_c said:

Yes, its one of those funny assignments - I don't know if anything like that would be asked for in real life.

PAS-22 was a case in which a satellite that was supposed to be launched into a geosynchronous orbit had a booster failure that made it impossible to reach the target orbit.  The original owners scrapped the mission, the insurers declared it a total loss, and they sold all interest in it to another company that eventually decided to push farther to the moon and let a gravity assist adjust the orbit so that it could still reach geosynchronicity even with its reduced capability.

It wasn't exactly asked for, but it did happen.

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ETA the other issue was CofG when flying stage 2, I had to have a few goes. First time it did a loop the loop, I recovered but it was too late, I squeaked into low orbit but with a bunch of fuel not there. So I had to spend a bit longer rearranging where everything fitted together to get a nicely flying rocket too.

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2 hours ago, paul_c said:

Phew! The other aspect of the trip, landing on the Mun, has been done.

Well, you had your reasons to combine both missions. And it seems you had a good time figuring out how to make it happen.

However, satellites to Kerbin's orbit are surprisingly cheap if kept to the essentials. E.g https://kerbalx.com/Spricigo/SL37D

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I've looked at combining missions where they fit nicely together. About 70% of the time I do the career stuff, the other time its my own personal curiousity.

Here's an interesting one....is it possible to get a gravity assist off the Mun the "wrong way", ie approach it from further forwards then, figuratively speaking, "turn right" (if looking from above N pole)? I've never done it like that but I don't know if its simply not possible physically, or its just that the normal direction of entering the Mun occurs say 95% of the time.

The reason being, I have this:

Ap: 14,661,033m
Pe: 3,393,304m
Inclination: 180deg
Argument of Periapsis: 357deg

I don't fancy the fuel bill for going "the wrong way" then going uphill A LOT!

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8 hours ago, paul_c said:

Here's an interesting one....is it possible to get a gravity assist off the Mun the "wrong way",

Hmmm... the Mun's orbital velocity is 543 m/s, so you have to enter the Mun's SOI with a relative velocity (to the Mun) larger than that to be able to get into a retrograde orbit at all. (Assuming that you can exit the Mun's SOI perfectly retrograde.) So my personal verdict is: Possible? Yes. Better than directly launching retrograde? No.

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I managed an insertion at 127deg for Minmus, there was a satellite contract for 300 x 360km, 180deg inclination. However, obviously the speeds at Minmus are a different kettle of fish. The strategy was to do a trans-Minmus burn to aim directly at the moon (no Pe shown), then do a normal/antinormal at the midpoint. Of course, if you lose comms at the midpoint.....

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Just had another one with a distant orbit of Kerbin, this time they asked for 24x16Mm and inclination 11.2deg. I aimed for the Mun, swung by on gravity assist and had a Kerbin orbit of 75x12 (ie beyond Minmus). I was also able to line up the Ap to not too far from the AN/DN of the desired orbit. This meant that doing the plane change cost buttons (about 24dV). Then it was a case of raising the Pe while far out at my 75Mm Ap (I was worried about comms signal, but it worked out okay), then burning retro to complete the contract by setting the desired Ap.

Basically a bi-elliptic transfer but with a Mun assist to save many dV from its first 2/3 burns.

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I now have these two around Kerbin:

As can be seen, they are very different. One is a polar inclination and the other is an elliptical orbit in and out of Minmus's radius. How would you (or wouldn't you) launch both satellites on one rocket?

I was thinking - not sure if its too bonkers:

* Launch into a POLAR orbit, approx the Ap and matching the orange orbit. Then detach the satellite, then tweak its inclination if needs be and raise its Pe with its own engine/fuel. Adjust the Pe of the rocket/2nd sat (pink orbit) to 70+km.
* Wait for Minmus to come into the extended plane of the orbit, then burn prograde to extend the orbit out, intercept Minmus (not too worried about the Pe), burn retro to be captured in a wide Minmus orbit.
* Do the inclination change to -167deg (the contract is for a RETROGRADE orbit around Kerbin) while orbiting Minmus
* Burn prograde to leave Minmus and achieve a very large orbit in approx the right inclination in Kerbin.
* Do a (hopefully minor) inclination adjustment, far far away from Kerbin
* Adjust the Ap and Pe using Hofmann transfer.

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16 hours ago, paul_c said:

I was thinking - not sure if its too bonkers:
[...]

Should work if done right. I guess you mean that, but to be clear: I would not rely only on the Minmus gravity assist to get into the retrograde orbit, but do a significant burn at the Minmus PE to get retrograde. Otherwise you have to enter Minmus' SOI with too high a speed and I don't know if Minmus has enough mass to turn you around enough in that case.

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15 hours ago, paul_c said:

How would you (or wouldn't you) launch both satellites on one rocket?

0. Launch only one satellite capable of fulfilling both contracts

1. Launch into a equatorial low orbit.
2. Raise AP so it just touch the orange orbit
3. Warp to AP and match orange orbit* , wait for contract to complete.
4. Raise AP so it just touch the pink orbit
5. Warp to AP and match pink orbit*, wait for contract to complete.

Reasonable easy and cheap while not taking too much player time (a far more scarce resource)

*following a previously set maneuver node

17 hours ago, paul_c said:

Wait for Minmus

I have serious doubt you will be able to make any saving with your idea so, other than for the learning experience, I wouldn't bother.

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In the end I did it a different way. I launched them separately. The polar orbit was for a very small satellite so I was able to build a rocket small, with marginal performance, which also flew reasonably well (I've had trouble with small rockets not being that controllable and ending up with a mishap). It still took 3 attempts. First time the 2nd stage did a flip - I recovered but not enough deltaV left to get anywhere near the orbit. 2nd try the final stage on the satellite was just too low performance, but it only took an upgrade of the fuel bag from a sphere to the sausage, to get enough performance. So the third time it worked out good.

For the very large orbit, I tried a Mun assist but I muddled up my thinking. I entered an orbit in Mun then did an inclination change to retrograde, but on coming out of the Mun orbit later, it didn't come out as a retrograde Kerbin orbit. It might have done at Minmus, but not the Mun. Anyway, I was getting impatient by then so instead of trying from the start, I was able to get another Mun flyby to enlarge the orbit even more, then at a very large one, I did the inclination change from something like 20deg to 168deg and it wasn't massively costly; and I had enough fuel to alter the Ap and Pe to the contract.

I'm interested in these unusual encounters etc so I'll be on the lookout for another like this, and I'll see if a Minmus assist can in fact reverse an orbit (still not sure!) and also Mun gravity assists are cool and save a little fuel. I also did a bit of research and found the formula to properly calculate what a change in the Ap/Pe of an orbit costs in dV, but its a horribly complex formula so I might need to spend a day with a spreadsheet!!!

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20 hours ago, paul_c said:

I also did a bit of research and found the formula to properly calculate what a change in the Ap/Pe of an orbit costs in dV, but its a horribly complex formula so I might need to spend a day with a spreadsheet!!!

Nothing useful there? Assuming that you are actually interested avoiding doing the math yourself.

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I don't know if this is already a response but make your first stage SRB only. also most likely you need about half as many fins as you think you need. also make the most basic payload possible.

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5 hours ago, The_Stinky_Broccoli said:

I don't know if this is already a response but make your first stage SRB only.

Well, it was more like a implicit hint, however I even provided an example craft.

I prefer to be a bit cautious with that suggestion because the combination of very light craft (as satellites tend to be) with SRB only fist stages tend to result in less than optimal launch trajectories. But it is cheap nonetheless.

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I normally look at SRBs for the first 1000 deltaV or so, because I know its going straight up; but also have at least one liquid rocket motor which can gimbal, so I have control. Tweaking the burn rate of the SRBs (and/or throttling down for the first stage with SRBs on) so the TWR is about 1.2-1.3 seems to be good for efficiency too, due to less drag.

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