Reusable rocket

[0x7be 2]

Hi, everybody.

Playing career, want to save some money. Spaceplanes are something beyond my understanding, so I've decided to make a reusable classic rocket SSTO. My simple rocket is 10 jumbos sitting upon 4 mainsails. Enough to throw full a jubmo to LKO without staging. For reentry I've tipped my rocket with heatshields and installed chutes and legs so rocket lands in engines-up position. Now, problem is that during the reentry aerodynamic forced invariably flip rocket around and it reenters sideways or engines-first. Never made it to the lower atmosphere in one piece.

Are there any tips for making reentrable rocket? Example designs are very appreciated. 

 

P.S. Playing with heat modifier == 2.0

 

 

[Cavscout74 1,022]

If you're flipping, you have too much drag up front and/or too much mass in the back.   I think with most of your fuel expended during launch, the majority of your mass is concentrated in the engines, so the rocket will try to flip tail first

My own rocket SSTO design reenters engines first.  I put a few airbrakes or (later versions) grid fins around the top to keep it draggier than the bottom, and a few chutes to slow down before a low power burn to drop my speed to a few m/s just before landing.  Which I realize is the opposite of what you're trying, but it works pretty well.   You could try putting airbrakes around the tail of your rocket & deploy them for reentry - watch the overheating - which should keep the nose first.  Also transferring all fuel to the forward-most tanks should help.

Edited December 14, 2018 by Cavscout74

[Pthigrivi 1,270]

Sure! Its a great way to go. I personally recommend re-entry wings, allowing you to slow down gradually in the upper atmosphere and glide to an altitude at which you can pop the chutes. Keep in mind landing something like this is still tricky. I've found circularizing at 80k, then bringing my PE down to 55k over my target landing site works well. You'll want to start facing radial out until about 55k, then transition to 45 degrees. Once you're through the plasma you can try to trim out to a gentle glide. You can also try keeping a fuel tank at the front and rear of your rocket and pinning them so you can move fuel up and down to control your COM. 

Results may vary, good luck!

[0x7be 2]

39 minutes ago, Cavscout74 said:

If you're flipping, you have too much drag up front and/or too much mass in the back.   I think with most of your fuel expended during launch, the majority of your mass is concentrated in the engines, so the rocket will try to flip tail first

My own rocket SSTO design reenters engines first.  I put a few airbrakes or (later versions) grid fins around the top to keep it draggier than the bottom, and a few chutes to slow down before a low power burn to drop my speed to a few m/s just before landing.  Which I realize is the opposite of what you're trying, but it works pretty well.   You could try putting airbrakes around the tail of your rocket & deploy them for reentry - watch the overheating - which should keep the nose first.  Also transferring all fuel to the forward-most tanks should help.

I've tried airbrakes. They are first to explode due to overheating  To my surprise they have very weak heat tolerance

40 minutes ago, Pthigrivi said:

Sure! Its a great way to go. I personally recommend re-entry wings, allowing you to slow down gradually in the upper atmosphere and glide to an altitude at which you can pop the chutes. Keep in mind landing something like this is still tricky. I've found circularizing at 80k, then bringing my PE down to 55k over my target landing site works well. You'll want to start facing radial out until about 55k, then transition to 45 degrees. Once you're through the plasma you can try to trim out to a gentle glide. You can also try keeping a fuel tank at the front and rear of your rocket and pinning them so you can move fuel up and down to control your COM. 

Results may vary, good luck!
 

Hmm. I've found that shallow reentry tends to deplete heatshields without really slowing down craft. Don't know why - in theory if you don't slow down much, you can't get too hot because it is kinetic energy is converted to heat. With my capsules reentry I usualy aim, to 25-30 km periapsis. Will try reentry wings.

[Cavscout74 1,022]

31 minutes ago, 0x7be said:

They are first to explode due to overheating  To my surprise they have very weak heat tolerance

Yeah, they are really more for aircraft than spacecraft.  You can toggle brakes on & off to help prevent overheating.  Or add a MM patch to raise the max temp of the brakes, there is one posted on the forums here somewhere I think. 

[Hodo 436]

I have a couple of completely reusable rockets, that work in FAR... 

The first one is WAY easier to fly and land.. 

I found coming in engines first is fine.. you engines have a MUCH higher temp tolerance than the rest of the craft.  And when you put air brakes on the other end to keep the nose pointing away so you can create as much drag as possible to slow down.   

Once I am below 7.5km I should be going subsonic and can deploy my landing chutes.  

Edited December 15, 2018 by Hodo

[Pthigrivi 1,270]

On 12/14/2018 at 6:38 PM, 0x7be said:

I've tried airbrakes. They are first to explode due to overheating  To my surprise they have very weak heat tolerance

Hmm. I've found that shallow reentry tends to deplete heatshields without really slowing down craft. Don't know why - in theory if you don't slow down much, you can't get too hot because it is kinetic energy is converted to heat. With my capsules reentry I usualy aim, to 25-30 km periapsis. Will try reentry wings.

Ahh the divine mystery of flux. The eternal question of reentry is: to suffer intense heat for a short time or mild heat for a long time? That's the difference between the apollo capsule and the space shuttle. You'll find that a shallow reentry--so long as you can control it--is survivable by most parts with a surface max temp. above 2000k. Also keep in mind that heat shields have exceedingly high max surface temperatures even when their ablative surface is expended. I often use them completely depleted for interplanetary aero-capture. 

Edited December 16, 2018 by Pthigrivi

[swjr-swis 2,193]

On 12/14/2018 at 11:45 PM, 0x7be said:

My simple rocket is 10 jumbos sitting upon 4 mainsails. Enough to throw full a jubmo to LKO without staging. For reentry I've tipped my rocket with heatshields and installed chutes and legs so rocket lands in engines-up position. Now, problem is that during the reentry aerodynamic forced invariably flip rocket around and it reenters sideways or engines-first.

As others have mentioned, the problem is drag, or rather, because of the specific way you want your rocket to point/land, a lack of it. Once the fuel is expended, the engines at the bottom are by far the heaviest part of the rocket, and the rest of the rocket is, as rockets tend to be, rather sleek, so there's no drag to counter the engines from 'wanting' to get to the ground before the rest of the rocket.

Airbrakes at the top would be one way to counter this, but as you've noticed they are rather sensitive to reentry heating. Two methods to protect airbrakes from too much heat: 1) pumping the airbrakes - when they start heating up too much, close them for a bit so they cool down, then reopen, etc. 2) limit their authority slider (advanced tweakables in the game settings) so they don't stick out too much outside the reentry wake.

 

An easier alternative: place service bays between the mainsail engines and the bottom tanks, and add toggling them to the brakes action group. That way when you engage the brakes, the service bays open and provide a bunch of drag at the engine end, which should help keep the rocket pointing the way you want.

[0x7be 2]

Reporting on the progress (or lack of )

I've rethought the reentry process for my vehicle - now coming engines first. With my difficulty settings Mainsails do actually oveheat upon reentry no matter how I reenter. Tried shallow and steep curves - all the same. My solution to the problem: I've reduced maximum payload for this rocket to save more delta-v. This way I have some fuel to retrofire during reentry. Once I detect overheating, I kick the thrust and slow down a bit. This way can survive slowing down without loosing parts of my rocket.

Last problem to solve - this way I end up lanidng without fuel. Dry mass of my rocket to too big for chutes to slow it down to safe speed. Touch down on speeds ~10 m/s is fatal even for Kerbal-built machinery

I've put aside this project for some time - got tired of explosions. Next time I will try to find some landing profile that will save fuel for last-moment-before-touchdown deceleration.

 

 

Edited December 18, 2018 by 0x7be

[Ozymandias_the_Goat 424]

On 12/14/2018 at 6:38 PM, 0x7be said:

I've tried airbrakes. They are first to explode due to overheating  To my surprise they have very weak heat tolerance

I had similar problems with air brakes (I do all my launches with a two stage fully reusable rocket). What I have found to be essential in shaving off the bit of speed that separates exploding from not exploding is air brakes with a 30-35% authority limiter (you can toggle this by right clicking on the breaks). The air brakes should not explode with this much exposure.

1 hour ago, 0x7be said:

Reporting on the progress (or lack of )

I've rethought the reentry process for my vehicle - now coming engines first. With my difficulty settings Mainsails do actually oveheat upon reentry no matter how I reenter. Tried shallow and steep curves - all the same. My solution to the problem: I've reduced maximum payload for this rocket to save more delta-v. This way I have some fuel to retrofire during reentry. Once I detect overheating, I kick the thrust and slow down a bit. This way can survive slowing down without loosing parts of my rocket.

Last problem to solve - this way I end up lanidng without fuel. Dry mass of my rocket to too big for chutes to slow it down to safe speed. Touch down on speeds ~10 m/s is fatal even for Kerbal-built machinery

I've put aside this project for some time - got tired of explosions. Next time I will try to find some landing profile that will save fuel for last-moment-before-touchdown deceleration.

Yes, sometimes it is necessary to fire engines during descent, this should only be an emergency method for when the overheat bar is nearing 90% full. For landing, you should not need more that 1-2% of your original fuel. With all this is mind, you should design a rocket that can get to orbit with 5% or more of its fuel. Otherwise, consider adding a small second stage. All of these things come from my own personal experience with reusable rockets, and as you fly your rocket more and more. You can develop a landing routine that will deliver results consistently on your own. 

Edited December 18, 2018 by Ozymandias_the_Goat

[ZL647 34]

You're playing with heat modifier two, you need to do a powered reentry, spam heat shields, or change that back to normal.

[Snark 10,624]

Moving to Gameplay Questions.

[bewing 2,261]

Saving a little fuel for touchdown really is important for landing heavy stuff with engines attached -- parachutes can only do so much. So I think it would be wise for you to look into other ways of surviving heat than just burning your engines during reentry.

During ascent, you want low drag. During descent, you want high drag. So  you want your aero drag to change before descent. The best way I've found to do that is with aero control surfaces and their "deploy" function. If, for example, you have two sets of two control surfaces and you set them to deploy in opposite directions -- that creates a tremendous amount of drag, without creating a lot of roll torque. There are other parts that also change their drag when deployed -- wheels, legs, cargo bays, and service bays were mentioned above. Airbrakes are not your only choice and I do not think they are your best choice, either.

As said above, putting some little wings near your empty CoM is another good method. During launch they create very little drag, but during descent you can point your craft in a "bad" direction to create a lot more drag. With the wings near the CoM, it doesn't take much control authority to hold your craft in that draggy orientation for a long time.

Another method is to deliberately tumble your craft. This distributes the heat some, and puts your craft into a draggy orientation for 50% of the time. (Which is a lot better than nothing.)

 

Edited December 18, 2018 by bewing

[0x7be 2]

Okay, I've managed to land this thing down in one piece near KSC. Hooray! :) Made only minor design changes, mostly did it by refining landing procedure.

But I've noticed a weird thing. I have a probe core covered by 2.5m heatshield. During reentry the probe tends to dangerously overheat (and sometimes to explode due to extreme heat) even with heatshield fully intact and full of ablator. Don't know what to do with it. I've thought that right behind heatshlied probe should be safe...

[bewing 2,261]

Heatshields probably don't work quite the way you think. In order to operate at all, they have to get really hot. And then some of that heat gets conducted to the nearby parts. So heatshields protect against extreme heat, but they basically do nothing against moderate heat. Additionally, a heatshield takes a little bit of time to warm up and get operational, shall we say. So for just returning from orbit around Kerbin, a service bay is probably a better deal for you. You don't really need a heatshield until you are returning from the Mun, and you don't need any ablative on the heatshield until you are going interplanetary.