VTOL/SSTO aircraft problem - need suggestions and tips/tricks

[IncompetentSpacer]

Do anyone have tips and trick for building a VTOL that is also SSTO capable for at least LKO? I have tried below an idea which relays on Rotation Servo M-12 and KAL-1000 (Breaking Ground DLC) to rotate engines.

With 6 rapiers (two more than the design below needs to reach space in a normal setup) I can (carefully!!!) lift off vertically.

The problem I have is that once I turn engines horizontally and start flying once I reach 200 ms (and even before that) the engines start flailing around and disintegrate the aircraft in ensuing vibrations. I have experimented with rigid attachment and autostruct settings but did not solve the problems just change how they manifest themselves. The culprit is the servo component that seems to flex with increase in speed (see the bottom picture for an example).

Any ideas or alternative solutions on how to design an VTOL? Or solve the rigidity issues with servo components?

 

Image below shows engines pointing inwards more and more as speed increases.

[AHHans]

3 hours ago, IncompetentSpacer said:

The culprit is the servo component that seems to flex with increase in speed (see the bottom picture for an example).

When you set the servo to "locked" then autostruts can reach across that servo. So you can try setting autostruts that will reach to a part on the other side of the servo on the engine nacelles and locking the servo once it is in horizontal flight.

P.S. You don't need that many air intakes. A single shock cone intake can feed at least two rapiers. So if you didn't add the adjustable ramp intakes for aesthetic reasons then feel free to remove them.

Edited November 26, 2020 by AHHans

[Entropian]

One small thing I noticed: those FAT-type control surfaces are going to melt upon atmospheric entry.  I would recommend replacing them with the elevon-type ones, as they have a much higher heat resistance.

[IncompetentSpacer]

3 hours ago, AHHans said:

When you set the servo to "locked" then autostruts can reach across that servo. So you can try setting autostruts that will reach to a part on the other side of the servo on the engine nacelles and locking the servo once it is in horizontal flight.

P.S. You don't need that many air intakes. A single shock cone intake can feed at least two rapiers. So if you didn't add the adjustable ramp intakes for aesthetic reasons then feel free to remove them.

I get the message that it cannot lock the servers while they are moving . Tried to lock them several ways but no good.

 

1 hour ago, Entropian said:

One small thing I noticed: those FAT-type control surfaces are going to melt upon atmospheric entry.  I would recommend replacing them with the elevon-type ones, as they have a much higher heat resistance.

Not really. I have tested the same setup dozens of times on Kerbin and two times on Laythe. Of course my modus operandus is for this vehicle to go to LKO or in case of Laythe to do drop from parking orbit into the atmosphere. I am not trying to do atmospheric breaking with this thing or tested it on EVE so maybe that is why i never straggled with heat issues.

[AHHans]

4 minutes ago, IncompetentSpacer said:

I get the message that it cannot lock the servos while they are moving . Tried to lock them several ways but no good.

.oO(That bug again.) What I do is to put "activate lock" (not "toggle lock"!) on an action group, and then spam that key when I want to set the locks. Usually that sets the locks after hitting the key a few times. If that doesn't work, they the parts may actually be moving. Try reducing the throttle - or setting it all the way to zero -  to reduce the mechanical load on the connection, spam the "set locks" button, and throttle up again. If that also doesn't work, then I'm out of good ideas.

[IncompetentSpacer]

Did as you suggested and....

Edit - I removed the engines and saw that unbalanced landing gear was making the plane jump up and down. Fixed that and it solve the problem when aircraft was landed. In air - no good.

Edited November 26, 2020 by IncompetentSpacer

[king of nowhere]

you nailed the problem. robotic parts are not so sturdy that they can take a rocket pushing agains them and stand still.

there are two ways to counteract that:

1) use more resistant robotics

2) use more robotics

for 1), as far as i know, bigger is more resistant. and yes, it is awfully inefficient; you generally don't have much spare mass on an ssto, now i'm telling you to put several tons worth of hinges or servos. yep. vtol looks cool, but it's not terribly practical.

for 2), try using multiple servos and attaching one engine to each. perhaps they will be able to resist the push from one single engine.

I have a rover that does vtol from tylo, and i used 8 terriers attached to 8 hinges; this way, every hinge is not too stressed. i tried using more powerful engines, and the hinges broke apart.

there is also option 3, instead of angling your rockets, you may be better off adding different rockets. they may be cheaper than all those servos. I would suggest a vector, it has a high thrust for its mass and it doesn't take up too much space. you may be able to fit it decently inside a cargo bay.

or perhaps you could use a propeller; it may be lighter than a rocket, but it will screw up aerodinamics once you fly, unless you can put it inside a cargo bay. or you can try a mixed system.

[Entropian]

2 hours ago, IncompetentSpacer said:

Not really. I have tested the same setup dozens of times on Kerbin and two times on Laythe. Of course my modus operandus is for this vehicle to go to LKO or in case of Laythe to do drop from parking orbit into the atmosphere. I am not trying to do atmospheric breaking with this thing or tested it on EVE so maybe that is why i never straggled with heat issues.

Ah, thanks for the explanation.

[IncompetentSpacer]

18 hours ago, king of nowhere said:

you nailed the problem. robotic parts are not so sturdy that they can take a rocket pushing agains them and stand still.

there are two ways to counteract that:

1) use more resistant robotics

2) use more robotics

for 1), as far as i know, bigger is more resistant. and yes, it is awfully inefficient; you generally don't have much spare mass on an ssto, now i'm telling you to put several tons worth of hinges or servos. yep. vtol looks cool, but it's not terribly practical.

for 2), try using multiple servos and attaching one engine to each. perhaps they will be able to resist the push from one single engine.

I have a rover that does vtol from tylo, and i used 8 terriers attached to 8 hinges; this way, every hinge is not too stressed. i tried using more powerful engines, and the hinges broke apart.

there is also option 3, instead of angling your rockets, you may be better off adding different rockets. they may be cheaper than all those servos. I would suggest a vector, it has a high thrust for its mass and it doesn't take up too much space. you may be able to fit it decently inside a cargo bay.

or perhaps you could use a propeller; it may be lighter than a rocket, but it will screw up aerodinamics once you fly, unless you can put it inside a cargo bay. or you can try a mixed system.

Well I tested number 1 option and changed M-12  servo to larger and heavier m-25 servo. It did work, the speed of 1400+ ms was reached with only slight flexing but no breaking. However by this point the mass has increased to a point where VTOL is almost impossible due to extra weight.

I actually think that contrary to real life rotating engines seem more costly and less efficient than having separate engines for hoovering (solution which is simplest but the one I wanted to avoid because it entails having extra weight which is dead weight outside VTOL operation) for my VTOL design.

In pure SSTO mode the design in first post can be reduced to 42 tons - with M25 servo, extra engines to keep it hoovering, it is reaching 57,25 tons (in SSTO pure mode, the design needs only three rapiers I discovered). 

So for option 3, except me going for airspikes instead of Vector (4 tons vs 2X airspikes for 2 tons and 1,4 TWR for Kerbin at sea level which also give me better air dynamics since they can be fully hidden inside MK2 cargo hold), I can get away with design weighting 44 tons, although I loose lot on fuel efficiency by using rocket engines to hoover in atmosphere....

 

[Lt_Duckweed]

I know this answer is really late, but there is another option.  Currently they bend and flex because the thrust from the rapiers is not pointing through the attachment point of the servo.  If you use the shift key to increase offset range, you can offset the engines so that they are centered on the attachment point of the servo.  This should let you get away with a smaller, lighter servo while also having much less flex.