Low mass science SSTO help

[Tychonoir]

I'm trying to make a small unmanned science plane mainly for use on Laythe as part of a larger ship. The idea is that it undocks and leaves orbit, flies around doing science, then returns to orbit to dock with the main ship.

I've seen a number of small SSTOs, but they are usually just an exercise in minimalism, and can't actually do anything useful. For this mission, it needs to hold a bunch of science, have the endurance and control to fly around, land at various sites, and still get back to orbit, rendezvous, and then be able to dock. (I guess it doesn't have to be unmanned, but I'm finding them to be lighter.)
Additionally, I'm trying to keep clipping and exploits down to a minimum. Oh, also using stock KSP.

I've got a working design down to 9 tons, but now I'm stuck. I have a few candidates in the 5-6 ton range using various combinations of engines, but they can never quite seem to get to orbital velocity, let alone have some reserves.

Current design features:
9 tons
Rapier main engine - To get to orbit
2x Juno - For efficient cruising
RC-001S RGU - Probe core that also collects science
Service Bay:
     Various science
     Small Monopropellant - Docking, additional dv
     2x Retractable solar panels - So as not to run out of power in orbit
     2x Small batteries - Power reserves
Liquid fuel: 300 units
Lq/Ox fuel: 225 units (Lq unit equivalent)

So, any ideas for getting this thing down to, say, 5 tons? Less? Is it even possible?

[Razorforce7]

5 Tons? I don't think so but we will see.
I got two tips. The first one is tricky and it requires you to drag your wings around a bit and try getting the CoL exactly on the CoM.
Then drain all your tanks and see if it still does. If it doesn't redesign the tanks and wings and other parts so that it does with both full and drained tanks.
Then you can remove all control surfaces except vertical stabilizer (no rudder needed so use the smallest fin)
Then assuming your not the worst pilot you can get it to pitch, roll, yaw with the probe cores reaction wheel torque. No rcs or control surfaces needed!
But your Col - Com relationship must be very precise with both empty and total vessel weight and depending on the relationship of the other parts you want it slightly under or above the CoM.

The 2nd one is that you can remove all your rcs and have a seperate rcs docking module part of your motherships cargo.
You can release and pilot this from your mothership and claw/dock to your orbiter to navigate it to your motherships docking port. That way you wont need rcs on your vessel.

Which requires some level of construction and piloting. But it will help in minimizing weight and drag. But it might be a bit to crude for your liking.

 

Edited April 5, 2017 by Razorforce7

[AeroGav]

Most of the mass of your ship is in its engines and fuel.    I suppose you need to find a lighter propulsive package that's still good enough for Laythe.

The RAPIER weighs 2 tons, with half a tonne of Juno on top, then you add in the fuel for jet and close cycle modes.

Does this craft need to be able to make multiple trips to and from the surface?

If not, you could jettison the jet engine (and intakes?) on flameout.    This will reduce the mass to be boosted to orbit and docked.   For example, you could decouple the RAPIER, and do the close cycle burn with a Spark - plenty of power for such a small craft.   This will reduce the LF/O mass you need to have on the lander.

Alternatively, you could swap the RAPIER for something lighter like a Panther, then decouple that.    Air breathing top speed will be some 800 m/s lower, so you will  need more lf/o for the Spark stage.   If this ends up being more than the 800kg you saved by going with Panther, then i guess it's not worth it, but remember the Panther has a super efficient non afterburning mode, so you could skip the junos.

 

[Aegolius13]

You can save a bit of weight by losing the monoprop and RCS thrusters.  The latter will also help with drag.   

How to dock then?  Get your plane near the mothership (30-50m, maybe).  Have each craft target the other and point at target.   Then just use your engine to move the plane slowly forward.   Alternatively,  you could put RCS on the mothership if it's not huge.

You also mention monoprop for extra delta-v.  But you're way better off adding a little more fuel for your Rapier.  Thrusters have bad ISP, mono tanks are relatively heavy, and the thrusters add mass and drag. 

[bewing]

Since you have canards, I don't think you need to be as careful as Razorforce7 says about aligning CoM and CoL.

But yeah, lots of little things. Do try to get rid of the RCS and monoprop. It's not that hard to dock with just your main engine.
Since you have the junos for cruising, you may want to try ditching the Rapier for a Terrier and see if that works better.
If you take the stuff out of the service bay, mount it all radially, and ditch the service bay -- you may find that your drag goes down. Service bays have a surprising amount of drag.
I did a test before and found that when your antenna is mounted crosswise like that, it does have more drag than if you mount it lengthwise.
And if you can ditch those rear horizontal stabilizers -- they are quite heavy and draggy.
It doesn't look like you have any incidence on the wings. Just a couple degrees will make a fairly large difference in drag.

 

[Tychonoir]

I've never tried to control a plane with just reaction wheel, but it doesn't sound like it'll fly very well, what with aerodynamic forces and all. It does have to land and stuff, but maybe I'll be surprised. It should be noted that at sub-mach, it currently has more control than it needs, but when getting up to orbit, I find that the extra control is needed or I can maneuver at all.

I'll give it a try, but even if I ditch the control surfaces, I'll have still have to replace them with wings to maintain the lifting area, no? So I don't really see any mass savings there.

Yeah, the mono prop is pretty heavy at .3 tons, but switching to the orbiter is a bit of a pain. Unless there's other savings, it's not really worth ditching on its own. Of course, if it becomes the difference in getting a min-space plane or not, then sure.

Here's a little more analysis, and possible weight savings:

Engines:
So far, trying to go lighter seems to point to ditching the rapier, since it's 2 tons by itself. The panther would be nice at 1.2 tons, but I just can't get the speed or altitude with it before needing rockets, and I don't see having the fuel to fire them early. When I add more fuel, then I need to add more lift, etc. and I end up back in the 9 ton range after all is said and done.

The Juno's are half a ton together, but ditching them seriously gimps any endurance to do actual work. With a light enough plane I might be able to go down to just one.
The panther is efficient dry mode, but again, lacks the power to get close to orbit even with afterburner. Maybe if the plane gets light enough.
Whiplash has the power, but doesn't have the ISP for cruising around, similar to the rapier and almost as heavy.

As for drag, the RCS ports do get some drag, but I've tried to be fairly minimal with them, and the single ports are less draggy than the 4-ways. The atmosphere analyzer isn't as bad as it looks for drag, but 1) There's no where else it will fit, and 2) It looks kinda cool there. There's a docking port jr. on the underside that probably causes some drag, but removing it requires a claw on the orbiter, and those things can be a bit buggy in my experience.

EDIT:
Oh, a bunch of replies while I was typing.
I'll try a version with no rcs and minimal drag and see where that gets me.

Edited April 5, 2017 by Tychonoir

[Razorforce7]

It will work! You will indeed have less lift without control surfaces but, seeing your image I don't think you need that. But you have to try that youself, I haven't got your craft.

I made something real quik that looks a bit off. It's more to show the concept.

It only has the cockpit reaction wheels, nothing else. Your probe plane would probably need 1 of the smaller reaction wheels though. Which equals about a single "Elevon 4" in weight.
It sure doesn't turn quikly. The 270 degree circle that I took to land back at the grass is the turn rate.

Because the Com and Col are perfectly aligned you will have better control once out of the atmosphere, not less.

My assumption is that if you collect science and fly to places that you do this in a straight line.
So you wont need to turn much anyway.
 

 

[Spricigo]

1 hour ago, bewing said:

Service bays have a surprising amount of drag.

Just checking: Are you sure its the service bay and not the stuff inside clipping out?

[Tychonoir]

Well, I tried a bunch of stuff ditching rcs, wing incidence, docking port, and got no better than my previous attempts.
I hadn't thought to try the Terrier, but got nowhere with it. The problem is that the Junos can't get me high or fast enough to make much use of it.

The closest I got with the Panther was with 4 Twitches and a total of 5.5 tons. It should be noted that 2 Sparks just doesn't cut it. Additionally, the Twitches afford a bit more control with their greater gimbal range.

As far as lifting surfaces, the 9 ton version is about at its limit.
One thing I've noticed is that I've seen a lot of people complaining about controlling their plane designs, eventually concluding that they just suck at it, or flying via keyboard is hard, or both. Well it turns out, many control issues actually come down to not having enough lift, despite the issues not really looking like they are lift related. Yet they are.

Reducing the current lift of the 9 ton craft, without reducing mass, begins to make it quite a challenge to fly in a reasonable manner, even though the challenge appears not to stem from lift problems. It's deceptive like that.

I'm wondering about the service bays too. I've done some drag tests, and they appeared to help. Though I wasn't specifically controlling for drag from the service bay, just items inside or not.

[Foxster]

Just a bit of lateral thinking...

Does it have to be an SSTO? Cos, if not, here's an idea that weighs about 6.5t...

You fly around doing some exploring and a bit of science. Your scientist is there to reset the experiments. 

Then when you are ready for home you make sure all the science is in the ESU and launch the mini rocket to orbit, where you rendezvous with your manned mother-ship, EVA a Kerbal over, collect the science, return to Kerbin and Robert's your mother's brother.

As a bonus you get to leave the scientist on Laythe and the aircraft as a base (i.e. you abandon him to his fate. Its what he would have wanted). 

Edited April 5, 2017 by Foxster

[Aegolius13]

Regarding reaction wheels,  keep in mind that in thinner atmosphere (such as Laythe),  reaction wheels work better and control surfaces work worse.  

I've not done a pure reaction wheel-based plane on Laythe, but I have used them as the only yaw and roll control, and that worked fine.   For small planes like yours,  small elovons, in conjunction with the reaction wheels,  have sufficed for pitch control. 

[Tychonoir]

11 minutes ago, Foxster said:

Just a bit of lateral thinking...

Does it have to be an SSTO? Cos, if not, here's an idea that weighs about 6.5t...

That's an interesting idea, I'll have to explore that.

Also, I just did some quick service bay tests. For rockets of equal mass, the service bay doesn't seem to add extra drag, and it clearly seems better to put stuff in the service bay, rather than just nix it all together and hang it off the side. The exception is when that .1 tons of the service bay makes more of a difference than the accumulated drag effects.

In my case, while .1 can make a difference on a 9 ton craft, it is far outweighed by the potential drag when going 1500 m/s in the upper atmosphere.

[Foxster]

...plus a service bay can make a handy air brake when open. 

[Tychonoir]

Ok, so using the "Science Torpedo" method, I've got a 2.5 ton disposable UAV. Naturally, this isn't a SSTO anymore, but this might be a better option.

Atmospheric entry is a problem for the the various science items and the small intake, so the heat shield became necessary, adding .326 tons with the connector.

After flying around for a bit, the whole back end and undercarriage with the Juno, wings, gear, and science get detached and the remaining rocket will get back to orbit. (Atmosphere analyzer on underside)

@Foxster
Maybe you have your reasons for 6 Oscars, but I found that bringing an ESU to LKO from ground only requires 4 with dv to spare.

[Foxster]

1 hour ago, Tychonoir said:

@Foxster
Maybe you have your reasons for 6 Oscars, but I found that bringing an ESU to LKO from ground only requires 4 with dv to spare.

I spent about 15 minutes knocking it together, I didn't mean it was the optimal solution, just a working proof of concept.

You have tried re-entry with your craft? Just that the heat shield will only protect stuff in a cylinder behind it, so won't be doing anything for the wings etc. But if they are high temp items then I guess that might not be a problem. 

Edited April 6, 2017 by Foxster

[Spricigo]

3 hours ago, Tychonoir said:

Atmospheric entry is a problem for the the various science items and the small intake, so the heat shield became necessary...

 

I'm not so sure, just a few days ago I made a spaceplane with low-tech parts. I was expecting to see things melting in the atmosphere but the highest temperature registered during reentry was 926K in the LY-05 Landing Gear. Notice my craft has (wing lift)/mass > 1,  with high AoA it slow down so quickly that there is no time to building up heat.

 

 

[Tychonoir]

Yes, I tested entry. It's only the delicate parts that need the shield. Now I did shift things around a little, so I should retest that the strut and decoupler are safe since they peak out, and have slightly less heat resistance than the wings.

Yes, with the original craft, I can use high AoA for both slowing and to protect the atmosphere analyzer, however with the smaller craft, there's really no safe angle possible, hence the shield.

[Foxster]

I've had a bit more playing with a little aircraft on Laythe and I've decided its not worth the trouble. Its really hard to land because of the thinner air and even then there are few biomes to make it worth the bother, especially without a scientist to reset the experiments. 

I think its probably worth taking along an aircraft only for sightseeing. Other than that, a science rover and a rocket return is more practical. 

[Tychonoir]

I've tested on Laythe, and landing wasn't an issue. I'm betting you don't have enough wing area.

[Spricigo]

1 hour ago, Tychonoir said:

Yes, with the original craft, I can use high AoA for both slowing and to protect the atmosphere analyzer, however with the smaller craft, there's really no safe angle possible, hence the shield.

I will make mine your words:

1 minute ago, Tychonoir said:

I'm betting you don't have enough wing area.

I cant test it now. Maybe will take a look later.

[AeroGav]

I had a go at this, decided to call my entry the "Peacock" because of the long tail feathers.

Only got the weight down to 8 tons.   Jet propulsion is by a single Panther,  which then gets jettisoned and it flies to orbit with a single Spark.

 

Liquid Fuel 3.37T - 674 Units

Oxidizer 880 KG -  176 Units

Monoprop 60 KG (tank not filled all the way up) - 16 Units (tank holds 80 max)

It has linear rcs thrusters for translating back / up / down / left / right -  the u/d/l/r ones are hidden in the service bay, the reversing one is stuck on the nose.    It has a reaction wheel for pointing, an antenna, an RTG and an inline clamp o tron.

The "tail feathers"  are there to offset the rear CG caused by the presence of the panther.  When it goes, so do they, and the handling is the same coupled or uncoupled.  

Although only a ton lighter than the OP's first ship, it does hold quite a bit more LF.  Coupled with the low drag airframe and higher ISP of Panther vs Juno, it should pack a serious amount of atmospheric flight time.

In fact during the test flight I ran full afterburner at mach 2.8, and circumnavigated Laythe with fuel to spare.

The video shows my only flight to orbit in the thing.  It was a bit close for comfort, but i didn't help myself by carrying all that unburned jet fuel with me.  Should have waited till more jet fuel burned off before going for orbit.  I was planning to use the RCS thruster to circularise, but lost comms before AP,  disabling RCS controls.  Fortunately we just barely had enough oxidizer.   Maybe add more Oscar tanks, or attach the rear landing gears to the jet engine, that way they aren't carried to orbit.

Other positives - it flies stable on prograde hold.  Stall speed, depending on the amount of Jet fuel aboard, is between 30-40m/s on Kerbin. 

[Tychonoir]

As a general rule, if the plane doesn't take off on Kerbin between 50-70 m/s, it's probably wanting wing area. Or if it has a tendency to dip it's nose after take-off (even with SAS)

There are other reasons that could lead to these these problems, but those are usually the first warning signs for me saying I need more lift.

If you mean to imply that the smaller craft has re-entry issues due to wing area, then no, that's not really the case. Both craft are extremely easy to fly and land.
There's no safe angle mainly because of the small intake in front. On the underside is the atmosphere analyzer, and on the rear and slightly topside are other science instruments. But the small intake is the hardest to safeguard. Even if I enter tail first, the aerodynamic forces will eventually flip me forward.

Now, it might be worthwhile to experiment with airbrakes, seeing as the heat shield is a little unwieldy. However, it does have the benefit of not having to touch the controls or worry at all, and being extremely forgiving on entry speed and angle.

@AeroGav

Does it get to orbit on Kerbin?
I ask because I'm usually doing my initial testing there, and if a single spark/panther is viable on Laythe, I'm wondering if some of my other failed tries would have worked.

Edited April 6, 2017 by Tychonoir

[AeroGav]

17 minutes ago, Tychonoir said:

Does it get to orbit on Kerbin?
I ask because I'm usually doing my initial testing there, and if a single spark/panther is viable on Laythe, I'm wondering if some of my other failed tries would have worked.

Laythe has lower orbital velocity.  1800 m/s vs 2200,  and the atmosphere ends abruptly at 50km.  It's gravity is only 80% of Kerbin's and sea level pressure is also just 80% of Kerbin's, so landing speeds will be the same, but the middle atmosphere is much thicker, enabling jet engines to work much higher.   Notice that the Panther is giving me level flight at 30km !  On Kerbin i'd be lucky to see 20km.

So yes, way way easier on Laythe than on Kerbin.

BTW,  I don't normally do Laythe missions this way.  I am unused to building really small planes,  and my rockets flip out if mount a drogue chute asymmetrically,  goodness knows how people manage to fly rockets with airplanes hanging off one side.   So I do it the other way round - very large space plane with small rocket carried inside for tylo.

You should see how underpowered my Jool 5 motherplane was at Laythe - having dumped it's Whiplash engines on Kerbin - yet it still makes orbit with over 3k dV left.

 

[bewing]

3 hours ago, Foxster said:

 

I've had a bit more playing with a little aircraft on Laythe and I've decided its not worth the trouble. Its really hard to land because of the thinner air and even then there are few biomes to make it worth the bother, especially without a scientist to reset the experiments. 

 

I find the opposite, myself. I always land in the water and then boat to shore -- and landing in the water really isn't that hard for most of my planes. All you have to do is touch one wheel to the ground to be landed, so if you land close to shore you can get several biomes worth of experiments pretty fast.

[fourfa]

Agreed - I find Laythe much safer if you can tolerate a water landing.  Roll around on shore to find a suitable takeoff strip - or engineering for water takeoff is pretty interesting too. There has been some discussion lately about hydrofoils - I've been playing with a Laythe SSTO seaplane whose main control surfaces double as hydrofoils to lift the fuselage out of the water drag, at which point takeoff is a cinch.