Returning a science module with marginal fuel strategy

[paul_c]

I'm designing a new/improved lander, mainly for the Mun, possibly for Ike and beyond later on.

I think I have this right....this is manned, so obviously my last stage is a cockpit/crew bit with a heatshield and a parachute. If I do this:

* Make my last-but-one stage a Science Jr module with a bunch of science instruments (eg temp, pressure etc), a parachute and a heatshield, and control these two last stages into a return trajectory to Kerbin with a periopsis of (for example) 69,000m - possibly keeping the liquid fuel engine and tanks (but jettisoning them at some point to save re-entry weight on the chute)

* then using the last gasps of monopropellant and a couple of RCS vents which were at the front anyway, lower my cockpit's periopsis to (say) 30,000m 

Will the cockpit come back in one piece okay, AND the science module possibly do another orbit or two of Kerbin, but each time lowering so eventually it comes down safely all the way? It can be recovered maybe in a week or two. I guess I can arrange to stage the science module's chute alongside its release, but it won't deploy until eg pressure 0.04 and altitude 5000 (so I can be sure to clear terrain)?

Also what's the value/purpose of the science storage box? In the above scenario, I am aiming to return the Science Jr & instruments & chute & HS & decoupler are 0.84ton all in. Could I simply attach a science storage box to my final stage, do a transfer before take off off of Mun, which would save weight and thus improve dV, then I can leave that 0.8ton behind me (I'll need a decoupler)

It seems the trend in KSP is to return a 'last stage' all-in or almost all in; but in real life, when they went to the moon they even used separate descent and ascent engines, but the ascent module of the LEM was really very small (but then had to dock).

[Wobbly Av8r]

I think the biggest problem you'll have with this plan is the game dynamic which only allows you to focus on one object that is operating below 70,000m at a time - if you're under 70km, you will be 'stuck' monitoring the progress of the focus vehicle and as soon as the other component is beyond the ~2.5 km distance from the focus vehicle - if it is within the atmosphere - it will vanish into the Kerbal Ether, n'er to been seen again...

The only way this kind of situation works out is when you can 'usher' each part of the vehicle from above 70,000m to the surface, or with a mod, AFIK.

[pmoffitt]

I find if I can get my periapsis down to something like 45km the air drag will eventually bring the vessel to landing on Kerbin.  It will need plenty of heat protection as it tries to burn up in the atmosphere.  It will take a few orbits but it works.  I have had to do this lately with several vessels in a new career mode game that just didn't have enough fuel left after leaving one of the moons.  You get so much more change of periapsis way out there at apoapsis  for less fuel burn.  Just hope they are lucky enough not to hit a mountain on the way down, I haven't done so yet.

The Experiment Storage Unit allows you to store all the Science Information you pick up in it and has an action to collect it all.  It will not store duplicates of the same experiment from the same biome.  I find it very handy and if you have a Scientist that can restore experiments, allows you to run them multiple times to gather even more science in a single launch.  It could also be used like you suggested so you could remove the experiments to save fuel / increase dV by reducing weight at some point.  But, as I find that I may be able to collect more science with them even during decent for landing, they usually come back with the vessel for me.  I may need to look at that again as keeping enough fuel to get back home has been an issue.

Getting rid of the extra weight prior to opening chutes would allow me to have less weight in chutes.  Reducing dry weight of the payload is the best way to reduce fuel needs.

Edited November 14, 2020 by pmoffitt

[paul_c]

Indeed, that's part of the problem, but also part of the fun. The challenge is you need to set the module off on its own trajectory, knowing in advance how the physics broadly works. Then possibly do some testing, for example send a same-spec module up into LKO, set it off then jump back to the tracking station to look at the performance of the 'debris'.

 

[Wobbly Av8r]

I'm not sure what your other limitations might be (cost? part availability?) but it really isn't so hard to include the Science Jr with your command pod for reentry - maybe put a Service Bay between the Science Jr and the heat shield if the low heat rating of the Science Jr is a problem, or something similarly more heat resistant. What's the issue of using another parachute or two?

[paul_c]

ETA (just seen other reply too) I'm now on Mk3 or whatever of my idea of what a lander might be like. Instead of lashing it together in an hour, I'm going to spend a number of days tweaking everything - engine performance, fuel liquid vs mono etc etc.

A good summary to the approach might be "only take with you what you need to". So, aiming to make full use of asparagus-staged external fuel tanks, leaving whatever batteries, solar and comms behind that might be considered excess, maybe even leaving the liquid engine and fuel tanks on the Mun and planning on mono and RCS for the trip home?

All the while, giving due consideration to crash survivability and redundancy. I created a save point and did 4-5 landings, some were complete failure but some were what you'd call a partial success, with bits broken off but that were only needed to get there. And that's with a basically uncontrolled crash at 30-40m/s into the Mun. It can right itself too. And the design 'kneels' on the landing struts, thus saving a ladder.

Crashed and tumbled at about 30m/s. Science Jr broken off rear, a other bits damaged but in theory still usable.

 

5th time lucky! No damage

 

Now, the challenge is to make the exposed parts the ones I don't want to return, hopefully break them off, or if not use a decoupler to drop them in one 'pack'!

[lemon cup]

What I like to do is divide the labor into 2 separate craft Apollo-style. The key is the science container!

One vessel is the crewed command and return vehicle, with a science container clipped all the way through the capsule (to shield it from reentry heat; some consider this cheating but wouldn’t that logically be an internal component in real life?)  . The other is the lander and that also has a science container somewhere on the ascent stage. 
 

Set up two action groups: each container gets an action group for “Collect All”. Pressing an action group will transfer all data on the entire vessel to the respective container (some experiments can’t be transferred, neither can duplicates).

The mission plays out like this. The crew lands on the surface with the lander vehicle and collects as much science as possible, storing all of it in the lander’s container. Then they return to the command module, board, and use the right action group to “Collect All” data into the capsule’s science container. Complete the mission by landing the crew safely on Kerbin in their capsule and recovering all that juicy science...!

[paul_c]

7 minutes ago, Wobbly Av8r said:

I'm not sure what your other limitations might be (cost? part availability?) but it really isn't so hard to include the Science Jr with your command pod for reentry - maybe put a Service Bay between the Science Jr and the heat shield if the low heat rating of the Science Jr is a problem, or something similarly more heat resistant. What's the issue of using another parachute or two?

That's food for thought. I've tended to put 2 parachutes on any lander anyway, for redundancy. Perhaps, one on the science module, one on the cabin. If the cabin one breaks off, I'm committed to bringing the science module home. If the science one breaks off, I have a choice of trying to slow down a descent (aerobraking, 1.5 or 2.5 orbits round, etc) or just transmitting and ditching (or maybe docking with a science lab on a space station, but that has its own challenges). If both chutes are still with me, the design could have 2 heatshields (extra weight) and I can recover the Kerbal and/or science independently. Their salaries are getting expensive now, I'm running out of them!

I am warming to the idea of having the science module and strategically using it, I like the fact that it mounts end-on-end but is slimmer than the fuselage, it means components can be mounted on it and are crash protected. Somewhat like I have mounted a bunch of essentials onto the hexagon in the screenshot above. I seem to be developing a Volvo Lander!

[king of nowhere]

i'm not exactly clear on why you want to send back your pilot and science jr in two pieces when you can send them in one piece.

that said, it is possible, because it's true that you can only control one object in the atmosphere, but since you are sending your two pieces on separate trajectories, they are going to hit the atmosphere at different times.

 

anyway, an unprotected science jr cannot reenter atmosphere without burning up. you don't necessarily need a thermal shield, just an engine or fuel tank to shield it from the worst of the impact can be enough. ultimately, to find out specifics you have to experiment.

[Wobbly Av8r]

FWIW, a Service Bay can have a lot of your external components included within and allow a more traditional configuration; while 'unique' configurations are indeed a hallmark of KSP designs, the functionality of a traditional design will allow for a higher rate of success. When push comes to shove, while you may think you are being spartan and minimalist in your design, you may instead realize you are making things more difficult to achieve...

Fair skies and following winds, sir - Good luck!

[paul_c]

I'll be doing a bunch of research/discovery/testing on what exactly all the parts do - or rather continuing, since I am still discovering new things as the game career goes on and so far I've been adding bits without 100% knowing what they do. Or....not adding parts then wondering why it doesn't work! The one that still gets me is the little 909 engine doesn't have an alternator, so any trip of more than a few minutes once undocked requires a decent solar setup. And solar is somewhat "breakable" on an uncontrolled landing!

Also need to start looking at electrical power, transmission date rate/power draw, etc etc more methodically.

2 hours ago, Wobbly Av8r said:

................... the game dynamic which only allows you to focus on one object that is operating below 70,000m at a time.................

One of my ambitions is to do a "powered return" of a 1st rocket stage/booster or something like that, similar to how SpaceX do it so well. Of course, with the above restriction and there being no auto-pilot (in the standard game) it means I'd need to either set up the onwards stage(s) to do their job on their own (eg reach orbit on a constant throttle and heading setting) or sacrifice it for the purposes of the ambition.

All the bits are there in KSP to do it, so I bet someone has already, but it would be fun to nail it.

Edited November 14, 2020 by paul_c

[jimmymcgoochie]

The only reason to actually bring back a science jr is because you're desperate for the funds, otherwise stick a pair of science lunchboxes experiment return boxes on the sides of your capsule and bring a scientist with you: the two boxes and your crew pod can hold three separate copies of each experiment and the scientist can EVA to both grab the results and store them, and reset the experiment. Bung a probe core on top of the capsule so it can still use SAS and you'll have a much easier time of it.

I always favour a one-stage Mun land/return ship because docking is a bit of a faff and the delta-V requirements aren't that high for the Terrier to manage it all. I usually go for an overbuilt second stage that manages everything from completing the Kerbin orbit insertion to doing most of the Mun landing burn before running out of fuel, which requires a larger launch stage but means the lander can be lighter or carry a heavier payload without running out of fuel. 

[Spricigo]

4 hours ago, paul_c said:

Could I simply attach a science storage box to my final stage, do a transfer before take off off of Mun, which would save weight and thus improve dV, t

hen I can leave that 0.8ton behind me (I'll need a decoupler)

Yep. If you get the science into a science storage or crew capsule (or just transmit it) then you can discard the instruments.

5 hours ago, paul_c said:

It seems the trend in KSP is to return a 'last stage' all-in or almost all in; but in real life, when they went to the moon they even used separate descent and ascent engines, but the ascent module of the LEM was really very small (but then had to dock)

It goes without saying that KSP, being a game, is quite different from real life. 

 

 

[Popestar]

One issue you may have, apart from the Kerbal universe swallowing the science module when you are too far away from it, is direction of travel.  With no command module or pilot attached to it, you can't be certain as to which way it is pointing, which means the heat shield may not even be at the right angle to protect it.  Or even on the right side.  And without a command module or pilot, you can't turn it to the direction you want. 

[paul_c]

I thought aerodynamics (or a Kerbin Space Program tweak for gameplay) tended to flip it ablator-side down, and it had a little stability in that position? With something light and not too thin/long, it should be okay? 

In any case, with the science storage box I can incorporate that into the return stage and leave the Science Jr and all other instruments on the Mun. Even the expensive ones.

[Popestar]

2 hours ago, paul_c said:

I thought aerodynamics (or a Kerbin Space Program tweak for gameplay) tended to flip it ablator-side down, and it had a little stability in that position? With something light and not too thin/long, it should be okay? 

In any case, with the science storage box I can incorporate that into the return stage and leave the Science Jr and all other instruments on the Mun. Even the expensive ones.

The weight of the heat shield may flip it so the shield is pointing to Kerbin...but that may not mean anything if you aren't falling straight down.

[paul_c]

I'll fire up sandbox mode and do a test tomorrow, I'm quite interested if my theory is right or not now! Also I'm not 100% sure if there's a smaller parachute available in the game, I remember a smaller heatshield I think. I am in career mode now and been quite conservative with my trading in of science points for the availability of more goodies (I need to work out how to use the stuff I have so far!)

I believe its how a lot of the science data in the early days of space was recovered back to earth (little boxes which were de-orbited and dropped) and weather balloons do it too.

[lemon cup]

26 minutes ago, Popestar said:

The weight of the heat shield may flip it so the shield is pointing to Kerbin...but that may not mean anything if you aren't falling straight down.

Drag forces and momentum come into play during reentry, not so much the gravity of Kerbin. If the heat shield is large enough, center of mass is close enough to the heatshield, and there is not too much drag-inducing parts on the top of the craft, then the heatshield should have no problem staying oriented correctly.  

[paul_c]

I've done plenty of flights with a mk1 crew capsule and 2-pax cabin coming down in front of the heatshield, the cabin alone is very stable (I suspect they made it so, otherwise the game would be somewhat inaccessible until you understood the navball and controlling retro position thru descent, which can be hairy). In my in-game career any new pilot with no stars usually does an orbital sightseeing thing to gain their first star and the prograde/retrograde buttons.

I tried 2x2pax cabins, ie 5 all in, but personally I couldn't control it well enough. It might be possible if I understood the physics a bit more.

[Wobbly Av8r]

So the reason the heat shields help reentry stability is due to two properties; 1) They are relatively heavy in comparison to the pods they are attached to, which shifts the CoM close(r) to the heat shield, and 2) They are slightly convex which causes them to be aerodynamically self-correcting, within reasonable limits, including the CoM location.

When you look at the image, the normal flow and CoM of a command pod with convex heat shield is displayed. In B the pod starts to tip and the more flattened in-to-the-airflow side creates greater drag while the side more tilted produces less drag creating a rotating moment at the CoM that corrects the tipping. However, if you make the vehicle long as in C and thus move the CoM far enough from the heat shield, because the CoM will easily exceed the  point of the 'corrective' force of the heat shield, they will actually cause the vehicle to rotate around the CoM more and create instability. With situation C, the only hope is to keep the CoM within the angle limits that allow the corrective action of the heat shield to maintain stability - and as the vehicle gets taller, the angle gets smaller.

Edited November 15, 2020 by Wobbly Av8r

[paul_c]

Aaaaah cool, I vaguely remember something similar from my boating days. Its something to do with the distance between the centre of gravity and the centre of buoyancy. When a ship rolls, it has a righting force due to the buoyancy 'pushing it back upright'. There is a metacentre and a metacentric height. And it linked to how "tender" or how "stiff" it feels.

Obviously this is aerodynamics stuff, not boating, I'll need to research a bit more!

[Spricigo]

4 hours ago, paul_c said:

Obviously this is aerodynamics stuff, not boating, I'll need to research a bit more!

It's fluid dynamicseither way. 

[paul_c]

Update:

I tried a "test" scenario. I built up a basic design rocket which I knew would make it into at least low-Kerbin orbit; and configured a command capsule + heatshield then Science Jr + heatshield, and added a few modules to it (goo, temp, pressure) and a chute.

I figured, with the dV I had spare - around 600m/s - I could take it out at least part-way to the Mun, to simulate a much faster-than-from-LKO re-entry. 

Test 1: at Apoapsis, I did a small re-entry burn to lower the Periapsis to ~35,000m. The capsule went 2 1/2 orbits round ( I didn't monitor it closely though, I was looking at the debris/other parts) and made it back safely. The Science Jr and instruments burnt up, the only remaining part was the chute itself which went 2 1/2 orbits and landed safely - it deployed at 5000m and came down at 2.5m/s. The rocket stage survived the first encounter with the atmosphere and went round again, then survived the re-entry but was destroyed upon impacting the surface at ~280m/s, unsurprisingly.

Test 2: As above but I went for 65km entry. Interestingly, I noticed the decoupling (at apoapsis) itself lowered the periapsis by 5k - something to be aware of! The science module 'bounced off the top of' the atmosphere, and went for another orbit...and another...and another. In all, 12 orbits were made, each time a little lower on the Ap (but not really the Pe) until last time round, its speed was low enough to drag the Ap below 70km and it made a safe re-entry - science recovered too. the capsule...is still orbiting.......

Possible additional tests

* Put some RCS onto the command module, so it doesn't make multiple long orbits round.
* Work out if the extra fuel needed to carry the Science Jr (plus other instruments) 'home' from Mun, offsets the extra fuel/size (in all components) in having to carry the Science Storage Box in addition to everything else. I suspect its very much worth using the Storage Box and leaving the instruments behind on Mun.

The good thing is, in my KSP Career I'd not observed the outer space around Kerbin so I gained +32.7 science points from my tests!

[Spricigo]

1 hour ago, paul_c said:

I tried a "test" scenario

That's a good way to learn. However, it's also fine to get what others already find when they faced the same situation.

Pe35km is often too low, Pe65km is often too high. Pe45km tend to be just fine.

Also, it may be a good idea to test thing on a sandbox save. 

[paul_c]

I suspect there is a relationship between approach speed (which would be higher from an eccentric orbit ie distant place/Ap) and ideal Pe. We are fortunate, KSP is quite forgiving (on Kerbin at least....don't know about the other planets yet) with it and there's a wide envelope which will "get you home" - and the Time Warp, to make 2+ orbits okay. The Apollo crew didn't have that luxury, they had limited power and CO2 filters etc etc....but that's the difference between real life and the game! Also, I dare say an orbit in outer space is somewhat predictable due to the vacuum, while an atmospheric re-entry on a real planet needs to deal with variations in the upper (and lower) atmosphere weather and many other considerations, like not landing over populated areas etc.

Its all fun!

Anyway, just a quickie, I've now landed my contraption on Minmus but I can only transfer certain and not other experiments - I can't transfer the Science Jr and Seismometer results (but have transmitted them). I know there's some experiments which it won't store, are these some of them?