Light but bulky

[RizzoTheRat]

Does anyone else struggle with light but bulky payloads?

I've built a long range probe to go and scan other plants (with ScanSat and RemoteTech), but its a right pain to launch.

If I stick it on the nose of a rocket it's a massive amount of drag (without or without a faring) way ahead of the CoM.  If I strap a pair of them to the sides of a rocket it's way too draggy and seems no mater where I mount them the CoM still drops too far back.  It fits nicely in 1.5 Mk2 cargo bays so I'm thinking I might have to figure out SSTO spaceplanes as I can't come up a decent vertical launch design with that length of cargo bay

[Starhawk]

This was my solution.

However, if you've never built spaceplanes before there's somewhat of a learning curve.

Happy landings!

[RizzoTheRat]

Yeah, I've not built any spaceplanes since version 0.something and I'm finding it a bit tricky.  Currently thinking about a vertical launch glide land system like a shuttle but with 2 of them sharing a central tank so no asymmetric thrust issues.

ETA...except you can attach stuff to the belly of a M2 part, but you can, but you can't attach a Mk2 part to something else via it's belly.  AAARGHG.

Edited January 31, 2016 by RizzoTheRat

[Starhawk]

One alternative is to use a conventional rocket lifter and overpower the drag with a high TWR.  That'll cost more, of course.

OTOH the vertical launch glide land system should work just fine as well.

My understanding is that there is a bug in the fairings at the moment which places the centre of drag in the wrong place.  I expect it will be fixed in 1.1.

Happy landings!

[sal_vager]

This might work.

Though you really need more mass than this on the top of the rocket so it acts as an arrow or a dart with the CoM nearer the front.

Another possibility would be to drag the payload to orbit.

(Launcher for example only)

[RizzoTheRat]

I'm liking Sal's second one, but I can't help thinking a pair of of orange tanks and skippers seems and overkill for a <2 tonne payload.

 

Experimenting with a rough copy of Starhawks SSTO design, my previous attempt lacked the speed despite having more engines so might be a flight profile problem rather than a design problem,  If I've nicked a design that I know should work I can experiment with profiles until I get it right.

 

ETA:  Since when did craft start drawing fuel from all tanks even if they're not directly linked?

Edited January 31, 2016 by RizzoTheRat

[sal_vager]

Aero engines draw evenly but they will stop if there is a decoupler in the way, this is to help keep the CoM of planes from moving too much.

[RizzoTheRat]

That makes life a lot easier than before.

I need to learn a bit more about flying them though, I just managed to accelerate Starhawks to about 1500m/s at 24 km whereupon the cockpit overheated and blew up.  

[Starhawk]

Heating's the thing in 1.0.5 that is most tricky with the ascent profile.  That cockpit can only take so much heating.  It sounds like you're close.  You probably just need to climb a little quicker/earlier.

Happy landings!

 

[FancyMouse]

3 hours ago, RizzoTheRat said:

If I stick it on the nose of a rocket it's a massive amount of drag (without or without a faring) way ahead of the CoM.

With a fairing it shouldn't - just install the stock bug fix for the stupid fairing bug. My rocket with 1.25m fairing works totally fine with this kind of payload with the bugfix, and I don't see significant drag because of fairing.

[ForScience6686]

Am I the only one who doesn't experience issues with the fairing?  I wrap all payloads in fairing and never have issues if I stay within prograde.  But I build my payloads for space so nice sharp edges.  Maybe a high  Twr counters these issues.

[RizzoTheRat]

Well after spending ages faffing about at 23-25km and 1200-1500m/s I gave up trying to get quicker and higher, switched to rockets, and made it to orbit with loads of fuel left, so I assume your design is intended to carry a bit more payload.  Now to see if I can land it...

[Snark]

Yeah, the M700 scanner is notoriously inconvenient to launch.  My own solution is generally to put a fairing around it, and then either make sure that the launching ship has plenty of steerable fins placed far enough back to keep the thing stable, or else do a deliberately inefficient launch in which I keep the speed down and delay my gravity turn until I've climbed enough that aero instability's not so much of an issue.

Being inefficient doesn't matter so much, because when I launch an M700, it's usually on a small probe that doesn't weigh much, so I'm not paying that much of a penalty if I'm burning 50% more fuel than a "good" launch would.  My typical M700 survey satellite looks a lot like yours.

One thing:  you would make your life easier if you can redesign your satellite a bit.  Your design is making life more difficult than it has to be:  you've got your M700 perched on top of that stack of octagonal struts, which is greatly magnifying your problem by sticking the M700 way out in front.  Get rid of those struts, and perch the M700 directly on top of your probe core.  It will make things quite a bit easier.  Then find somewhere else to put those ScanSat scanners.

You may also want to consider ditching those Gigantors in favor of 6-8 of the smaller folding solar panels; the Gigantors are pretty heavy, their power-to-weight ratio is considerably worse than that of the smaller panels.

 

5 hours ago, RizzoTheRat said:

...except you can attach stuff to the belly of a M2 part, but you can, but you can't attach a Mk2 part to something else via it's belly.  AAARGHG.

Actually, you can, though it's hacky and inconvenient enough that it's generally not worth bothering with (you have to play games with re-rooting and subassemblies).  Put a BZ-52 radial connector on the Mk2 belly, use the root tool to make it the root part, save the whole thing as a subassembly.  Then open up the craft that you want to attach it to, load the subassembly, and you can attach it anywhere there's a node, via the BZ-52.

 

[RizzoTheRat]

Yeah I was being a bit lazy with the satellite design,  stuck the big panels on as I want sure how far from kerbal a few smaller ones would get me. On the plus side I'm now learning lots about SSTO space planes because of it :-D 

[Plusck]

2 hours ago, ForScience6686 said:

Am I the only one who doesn't experience issues with the fairing?  I wrap all payloads in fairing and never have issues if I stay within prograde.  But I build my payloads for space so nice sharp edges.  Maybe a high  Twr counters these issues.

Mixed experience here. I've recently sent a few probes up which look very like the OP's one, in a fairing, and they were fine as long as I was really careful about keeping to prograde.

[ForScience6686]

5 hours ago, Plusck said:

Mixed experience here. I've recently sent a few probes up which look very like the OP's one, in a fairing, and they were fine as long as I was really careful about keeping to prograde.

Does that rule not apply to all rockets though?. Keep prograde or lose efficiency, and at the extreme, control.

[Snark]

1 hour ago, ForScience6686 said:

Does that rule not apply to all rockets though?. Keep prograde or lose efficiency, and at the extreme, control.

True for efficiency, false (or, at least, not universally true) for control.

An aerodynamically stable rocket will stay under control no matter how you perturb its orientation-- it wants to be prograde and will fight to go back to prograde as you disturb it from that orientation.

An aerodynamically unstable rocket wants to be retrograde.  However, the torque pushing it away from prograde is proportional to the deviation from prograde, so even a moderately unstable rocket can stay controllable if you're really careful about sticking to prograde.

So the case of "you lose control if you don't stick to prograde" applies to unstable rockets.  Essentially, what Plusck was saying was that he had a rocket that was somewhat unstable, but not too unstable, i.e. flyable as long as it sticks close to prograde.

 

[Plusck]

9 hours ago, ForScience6686 said:

Does that rule not apply to all rockets though?. Keep prograde or lose efficiency, and at the extreme, control.

7 hours ago, Snark said:

True for efficiency, false (or, at least, not universally true) for control.

An aerodynamically stable rocket will stay under control no matter how you perturb its orientation-- it wants to be prograde and will fight to go back to prograde as you disturb it from that orientation.

An aerodynamically unstable rocket wants to be retrograde.  However, the torque pushing it away from prograde is proportional to the deviation from prograde, so even a moderately unstable rocket can stay controllable if you're really careful about sticking to prograde.

So the case of "you lose control if you don't stick to prograde" applies to unstable rockets.  Essentially, what Plusck was saying was that he had a rocket that was somewhat unstable, but not too unstable, i.e. flyable as long as it sticks close to prograde.

 

 

Precisely that - the way fairings work at the moment give a lift vector that is very far forward. If your fairing is any significant size compared to your rocket (which it would be in the OP example), that means that you either have to add huge fins at the back to ensure aerodynamic stability, or you are very, very careful to keep to within a degree or two of prograde (literally a degree or two - that 5° angle corresponding to the edge of the prograde circle is generally far too far off when at the 16-25 km stage).

So yes, it's true for all rockets that you need to keep prograde or lose efficiency. The issue with fairings is simply that they make designing a stable rocket virtually impossible. What should be a self-correcting gravity turn tends to become a tightly-controlled, HECS-assisted, fine-control-tapping, sweaty-palmed prograde follow.

Edited February 1, 2016 by Plusck

[Wemb]

21 hours ago, RizzoTheRat said:

Does anyone else struggle with light but bulky payloads?

I've built a long range probe to go and scan other plants (with ScanSat and RemoteTech), but its a right pain to launch.

If I stick it on the nose of a rocket it's a massive amount of drag (without or without a faring) way ahead of the CoM.  If I strap a pair of them to the sides of a rocket it's way too draggy and seems no mater where I mount them the CoM still drops too far back.  It fits nicely in 1.5 Mk2 cargo bays so I'm thinking I might have to figure out SSTO spaceplanes as I can't come up a decent vertical launch design with that length of cargo bay

My survey sats are more or less the same, and I don't have a huge amount of problem launching them - they do produce a huge amount of drag, but I've not installed the fairings fix as yet.  Having said that, taking them up on a reasonably sedate launch seems to go well - plenty of control authority on the back end will do the trick.

 

The light show as one of these things goes through the upper atmosphere is pretty terrific, but there's no damage to it, 

 

You may want to strut the joints between the scanner and the probe core, and again between the spark (?) engine and whatever launch vehicle is under it - the size of these could make a wobble a big problem. 

Wemb

[RizzoTheRat]

I think the general gist to this is I need to get better at launches then, I never manage to get the initial turn spot on to be able to just chase prograde the rest of the way up, meaning I have to do adjustments, which, as I'm using keyboard not an analogue stick. means I can easily hit 10+ AoA and the whole thing flips if its too light and draggy on the top.

However I've leaned a chunk about spaceplanes getting this thing in to orbit.  A design similar to Starhawk's made it to orbit on the second attempt with loads of fuel to spare despite a fair bit of faffing with speed/altitude before I switched to rockets, and while I missed to runway I did manage to put it down in one piece on the grass next to the KSC.  I'll fit some airbrakes to the next one though as I nearly ended up in the sea

[Warzouz]

Hmmm, I always go for big stuff : why launching ONE probe when you can launch 2 with a 2 time bigger rocket ?

For my Laythe/Tylo explore mission I sent 4 * 1.25m landing probes on a single mission. 4 probes on a Rockamax size rocket with one fairing which detached before entering Jool SOI (so all probes would arrive at different time.

For my Eve probe lander mission, I even sent a "probe carrier" with 8 rockamax sized probes on it. That was big, but quite light as the probes weren't really powered except for deorbiting around Eve.

When I launch a single 1.25 probe, I usually use a 1.25 fairing and 1.25 rocket. The head of the rocket is a bit bigger, like many IRL rockets.

[Snark]

5 hours ago, RizzoTheRat said:

I think the general gist to this is I need to get better at launches then, I never manage to get the initial turn spot on to be able to just chase prograde the rest of the way up, meaning I have to do adjustments, which, as I'm using keyboard not an analogue stick. means I can easily hit 10+ AoA and the whole thing flips if its too light and draggy on the top.

One thing to bear in mind about gravity turns:  Torquing your ship pitch is not the only way to "tune" the turn as you ascend.  If you're climbing too steeply, but have pitched over enough that you're not climbing mostly straight up, then you can back off the throttle a bit to let your gravity turn tighten up some and lower your climb angle.  Or, if you're climbing not steeply enough and aren't already at full throttle, then throttling up can help pull it out of the hole.

This technique is limited (you can't use it in all cases-- it only helps you at certain combinations of speed + angle), and it may cost a bit of fuel efficiency (throttling down = increased gravity loss), but at least it has the benefit ot being very safe:  it doesn't require you to deviate from prograde at all.  If you have a ship where you're not concerned about efficiency so much, and the primary problem is just getting the darn thing to orbit without flipping, it can come in handy.

[RizzoTheRat]

3 hours ago, Warzouz said:

Hmmm, I always go for big stuff : why launching ONE probe when you can launch 2 with a 2 time bigger rocket ?

Which is another annoyance with the M700 scanner, it really needs to go on the end of a satellite, making stacking them tricky.  For my LKO Komsats (RemoteTech) I launched all 4 in a stack.  Before stock farings I used to use the Procedural farings which had the option to stack a faring base on another faring, very useful for this kind of launch.

[Vegetal]

You could always make your final stage out of mk2 parts if planes aren't your forte.

[Warzouz]

3 hours ago, RizzoTheRat said:

Which is another annoyance with the M700 scanner, it really needs to go on the end of a satellite, making stacking them tricky.  For my LKO Komsats (RemoteTech) I launched all 4 in a stack.  Before stock farings I used to use the Procedural farings which had the option to stack a faring base on another faring, very useful for this kind of launch.

On my space station, I also plug satellites radially. It works fine