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Spaceplane Carrier Help, Please.


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Here's the story.

I decided to build an SSTO that can carry a three-person spaceplane into LKO and leave it before landing at the KSC. That way, the spaceplane itself will be ready for its interplanetary burn with all its liquid fuel tanks full and I won't have to drop any debris on the surface. For an aerodynamic design concept, I viewed Matt Lowne's air-launched spaceplane video.

  • Though I followed the video while building my plane, I made sure to add my own parts in my version (e.g. rapiers instead of airliner engines, reaction wheels, NERVs, more oxidizer for the rapiers, etc). 
  • I plan to get BOTH craft in orbit instead of sacrificing one for the other.

 

 

Below is the image of my prototype (spaceplane included). I do not plan to change the spaceplane, so it's the carrier I need help on.

rzd3ZCF.png

  • STAGING SEQUENCE:
    • 5: Carrier rapiers
    • 4: Carrier NERVs
    • 3: Detach spaceplane from carrier
      • Attached via docking port
    • 2: Spaceplane NERVs
      • Should be separate from carrier by then.
    • 1: Spaceplane rapiers
    • 0: Spaceplane rear parachutes
  • Spaceplane has ~4,952 m/s of delta-V with its oxidizer tanks (99%) depleted, so there's the reason why I want this to work so badly.
  • Craft filehttps://mega.nz/#!iSglSAKT!iyaMk3pM_rt1HMWtCXu4ROxektOhf-tsHE4Y5OqtBig

 

While the appeal of this design seems awesome, the test results are anything but. For instance:

  1. My aircraft keeps sliding on the runway, even at maximum friction control.
  2. I can't achieve LKO.
    1. I did once, but I could not repeat it. More details on what happened later.
      1. Honestly, I forgot if it was this prototype or another one. Either way, I was lucky.
    2. The other times, I failed to achieve an apoapsis above 70 km before I got below 39 km (or whatever altitude you can't bounce back up from).
    3. When I did reach Ap=70 km, I couldn't get my periapsis up in time to avoid getting recaptured in the atmosphere. 
  3. (IN THE ONE TEST RUN THAT I ACHIEVED LKO IN) My carrier plane lost control after re-entry, and I spun uncontrollably in the air. In fact, my tailfins snapped right off.
  4. I had a bit of trouble keeping this plane straight when the carrier's rapiers activated. More specifically, it wanted to pitch way up for a bit until its oxidizer supply was depleted.

 

I then tried Matt's Stratolauncher design concept, but that prototype turned out even worse. When the rapiers switched to rocket mode, I rapidly pitched downward. 

  • The picture below shows the prototype in question.
  • If I had to guess why, it's because of the three rapiers on the top being so close together

nRi76Tf.png

 

 

What should I do for my carrier to work? What kind of design is necessary to function both with and without the spaceplane? Any and all help is greatly appreciated. If you want to try it out for yourself, feel free to ask for the craft file (aside from the one I posted here)

  • One more time: the spaceplane shall not do anything until it is released from the carrier in stable LKO.

 

Spoiler

My first prototype involved putting my spaceplane ON TOP OF my relay carrier SSTO, but it didn't even break Mach 1 before slowing down and losing altitude. That idea was then permanently rejected.

 

Edited by Mars-Bound Hokie
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Well, lots of little issues.

First, to prevent runway veering/sliding -- you want lots of friction on your rear wheels only. Remove the friction on your front wheels. Drag at the front end causes instability which causes veering. Drag at the back end is helpful.

Rear stabilizers cause a lot more drag than canards, and they do not provide lift like canards do. Delete those stabilizers and use tailfins for canards on the carrier.

Each air intake adds drag, and you only need about 1 intake for every 4 engines. So you've got way too many air intakes.

Each elevon on the wing adds drag and mass. Maybe the ones way out at the tips are useful (because they provide the most torque), but get rid of all the others.

You want to minimize the mass (and therefore part count) of the stuff you are dragging into orbit. Can you replace a chunk of those wings with a single BigS wing? BigS wings hold fuel, so they can help you eliminate fuel tanks.

(Sorry, but I didn't download and examine your craft file.)

Do you have a probe core somewhere on that carrier? It can't fly without one. You have a lot of reaction wheels and they eat a lot of power. Are you sure you have enough batteries?

 

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*bleep* Did I forget to click on "Submit Reply"? Again? *bleep*

So here are my 0.05€, again in addition to what @bewing wrote. *grrr*

6 hours ago, Mars-Bound Hokie said:

My carrier plane lost control after re-entry, and I spun uncontrollably in the air. In fact, my tailfins snapped right off.

I guess the carrier plane became aerodynamically unstable once you detached the spaceplane and burnt most of your fuel.
The Nervs at the back are pretty heavy, and I don't see much in the front of the craft to balance them.

6 hours ago, Mars-Bound Hokie said:

I had a bit of trouble keeping this plane straight when the carrier's rapiers activated. More specifically, it wanted to pitch way up for a bit until its oxidizer supply was depleted.

Probably the center of thrust of the RAPIERs is not inline with the center of mass - at that stage of the flight.

Checking the center of thrust is tedious but comparatively easy: set the thrust limiter of all other engines to zero, then the thrust indicator in the editor will show you the center of thrust of the RAPIERs. And don't forget to reset the thrust limiter of the other engines again. (Like I have a tendency to do... :cool:)

Checking the center of mass at a certain stage of the flight is harder - if it moves significantly while fuel is burned. So my approach to this is to distribute the fuel in a way so that the CoM doesn't move (much) when fuel is burnt.

6 hours ago, Mars-Bound Hokie said:

I can't achieve LKO.

Short answer: you don't have enough TWR. Longer answer: achieving LKO on mostly Nervs is hard. But why do you have the Nervs on the carrier plane in the first place? I understand that it is only meant to get to LKO and then get back to land on Kerbin. For that I would use only the RAPIERs, and increase the amount of oxidizer (and Lf if needed) to get enough closed-cycle dV.

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To address everyone's responses (in order of posting)

 

7 hours ago, bewing said:

you want lots of friction on your rear wheels only. Remove the friction on your front wheels. Drag at the front end causes instability which causes veering.

Got it.

 

7 hours ago, bewing said:

Rear stabilizers cause a lot more drag than canards, and they do not provide lift like canards do. Delete those stabilizers and use tailfins for canards on the carrier.

Where would I put these canards? Wouldn't adding them and/or removing my rear stabilizers throw off my CoL?

 

8 hours ago, bewing said:

Each air intake adds drag, and you only need about 1 intake for every 4 engines. So you've got way too many air intakes.

But where would I put them? With the engines all spread out like that, how will the air from the intakes reach them? Sure, I could trade the engine fuel tanks for pre-coolers, but that also means I will have 40 Lf; I will be trading (a lot of) fuel for intake air.

 

7 hours ago, bewing said:

Can you replace a chunk of those wings with a single BigS wing? BigS wings hold fuel, so they can help you eliminate fuel tanks.

You mean this? How would I place them so that they can hold the spaceplane AND the necessary engines (which is another issue)?

  • The FAT-455 wing is way longer, and it can hold 600 units of fuel (the Big-S Delta wing can hold 300)

 

8 hours ago, bewing said:

Do you have a probe core somewhere on that carrier? It can't fly without one. You have a lot of reaction wheels and they eat a lot of power. Are you sure you have enough batteries?

Probodobodyne OKTO2 is literally the first piece I started with when making the carrier (merging the spaceplane (with docking ports attached) later). After assembly was complete, I made the spaceplane cockpit the root part.

I also made sure to include A LOT of batteries (one behind each rapier).

  • I should probably add an RTG on the carrier too, but I don't want it to blow up during re-entry.
  • I didn't want to add solar panels to the carrier, or else I might trigger something on the spaceplane that I don't want to. 
    • If I get rid of the NERVs, I could set "0" as the panel action group. It also toggles the surface harvester on the spaceplane, but that won't work unless the drills are deployed (which requires the "4" button).

 

3 hours ago, AHHans said:

But why do you have the Nervs on the carrier plane in the first place?

When I tried the carrier WITHOUT the spaceplane (and NERVs), I ran out of oxidizer before I could complete my orbit. I added the NERVs so that I could have enough delta-V to achieve a high enough orbital altitude for my spaceplane.

  • Though it has a maximum thrust of 120 kN on NERVs alone, I don't want its periapsis to get below 70 km during the ejection burn (I learned about that possibility the hard way).

 

3 hours ago, AHHans said:

For that I would use only the RAPIERs, and increase the amount of oxidizer (and Lf if needed) to get enough closed-cycle dV.

At the same time, I need to watch my CoM. Not only will I lose the spaceplane before my re-entry burn, but I would have used up a good chunk of my oxidizer and Lf and Ox by then. Where would I put the Lf and Lf+Ox tanks?

  • I'm considering using the Mk. 2 tanks for this, since those parts include a Mk. 2 bicoupler (that I can put two rapiers on).

 

Interesting points you two brought up. After I take care of my college stuff IRL, it's back to the SPH for me.

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Take a look at my Jool-5 mission. full album The interplanetary ship isn't a spaceplane, but you can still look at the carrier SSTO for ideas. It has engine pods consisting of 4 Rapiers and 1 shock cone apiece, and uses 12 pods. As far as functionality is concerned, the game doesn't actually care if the intakes are inline with the engines. I could have stuck them all on one pod if I wanted.

For fuel, I have a mix of LF-only and LF/Ox tanks, with the LF/Ox tanks mostly near the CoM and the LF tanks balanced fore and aft. The engines are also near the CoM. This helps minimize CoM shifts as fuel is burned.

2UPZVph.png

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6 hours ago, Mars-Bound Hokie said:

Where would I put these canards? Wouldn't adding them and/or removing my rear stabilizers throw off my CoL?

Put them on the nacelles of your carrier, approximately inline with your spaceplane's canards. Your stabilizers shouldn't be adding any lift, so removing them shouldn't change your CoL.

Quote

But where would I put them? With the engines all spread out like that, how will the air from the intakes reach them? Sure, I could trade the engine fuel tanks for pre-coolers, but that also means I will have 40 Lf; I will be trading (a lot of) fuel for intake air.

Air is shared all over the surface of your craft. So you don't need a path from the intake to the engine, unless you are trying for realism. Part of the point is that I don't think you need that many rapiers, either. Each of your engines is mounted on a short nacelle -- so you make the nacelle longer, stick a 1.25 bicoupler at the back end, and attach 2 rapiers. Or, you use the fact that rapiers are stackable and you stack maybe three of them and use the move tool just enough to make sure their exhaust plumes don't get mutually obstructed. (Unless you are a fanatic about clipping.) Additionally, since you aren't using all/any of those rapiers for the "rocket phase" of your ascent -- whiplashes give you more oomph for a lot less mass. You gotta either use the non-airbreathing function of the rapiers, or you should ditch them for better engines (whiplashes).

Quote

You mean this? How would I place them so that they can hold the spaceplane AND the necessary engines (which is another issue)?

It's almost never a good idea to mount engines directly on wings. It would almost certainly be better to attach your rapier nacelles directly to the sides of your main NERV nacelles.

Quote
  • The FAT-455 wing is way longer, and it can hold 600 units of fuel (the Big-S Delta wing can hold 300)

But the FAT455 wings have no heat resistance, and will almost always burn off of any spaceplane unless you treat them like they are made out of feathers or something else that's flammable.

Quote

When I tried the carrier WITHOUT the spaceplane (and NERVs), I ran out of oxidizer before I could complete my orbit. I added the NERVs so that I could have enough delta-V to achieve a high enough orbital altitude for my spaceplane.

Part of the point of spaceplanes is the flight path that you take. You have a lot of drag and a lot of wing. That's kind of a bad combination. With a lot of wing, generally you want a shallow ascent that stays in the atmosphere for at least 10 minutes. Climbing at no more than 5 degrees, with an additional 5 degree AoA at most. And since you're in the atmosphere a long time, you need to be super careful about minimizing drag and part count.

If you're just going to point the nose at the sky and light the rockets -- then NERVs are a bad choice. It takes them a very long time to get a spaceplane to 70km and orbital velocity. The maximum realistic mass for a spaceplane powered by 2 NERVs is about 50 tonnes, and yours is definitely too heavy -- and you again need a shallow ascent with NERVs to give them time to develop all that deltaV. If you want a steep ascent, then you need to use Ox powered rockets (like the rapiers), and you can probably get away with a lot less wing.

 

Quote

At the same time, I need to watch my CoM. Not only will I lose the spaceplane before my re-entry burn, but I would have used up a good chunk of my oxidizer and Lf and Ox by then. Where would I put the Lf and Lf+Ox tanks?

You put the Lf+Ox tanks on your nacelles, and make them longer.

 

 

Edited by bewing
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After looking at @sturmhauke's Joolia design, I decided to go big or go home.

LZBbydh.png

 

With twelve rapiers, it managed quite an immense acceleration. In fact, I had to ease up on the throttle a bit after 1400 m/s so I didn't blow up. The next picture shows me detaching the spaceplane at 160 km altitude.

FK44sst.png

 

HOWEVER, I lost control after re-entry. Sure, nothing came off, but I still spun around and couldn't fix it. To my surprise, the Mk. 2 tanks had been almost drained while the 2.5-m tanks (the Rockomax X200-16 and the C7 Brand Adapter) still retained most of their fuel. I blame the fuel ducts for throwing off my center of mass, but I installed them so that the rapiers could receive the Lf+Ox from the Mk. II tanks when it was time to go into rocket mode. 

 

How can I fix this problem? Worst-case scenario, I could deploy the chutes (added after this test run) on the carrier and just plop on the ground.

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1 hour ago, bewing said:

Pump all your fuel as far forward as you can, before you reenter, to get your CoM as far forward as you can.

Can I also set the fuel flow priorities to keep the CoM near the front? If so, what tanks should get what numbers (40 is default)

  • I tried that myself, but I ended up pitching up during the orbital ascent.
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18 minutes ago, Mars-Bound Hokie said:

Can I also set the fuel flow priorities to keep the CoM near the front? If so, what tanks should get what numbers (40 is default)

That might work, but it's a lot of trial and error. I use the RCS Build Aid mod during the design process. It shows you how the CoM moves over time so you can visualize where to place stuff, without having to manually set the tank levels.

The general rule is balance. You should have an equal tank volume fore and aft. Heavy dry mass parts should go close to the center, or be balanced with something across the CoM. The better your balance, the less your CoM shifts and the more predictable your performance.

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1 hour ago, Mars-Bound Hokie said:

Can I also set the fuel flow priorities to keep the CoM near the front?

Theoretically, yes -- but that can easily unbalance your plane and overcome the ability of your control surfaces to compensate. It's a lot easier just to pump a little fuel around when you need to.

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I DID IT!

 

G8BuBXa.png

  • Look at the cute spaceplane at 175 km orbit with 4,975 m/s of delta-V
  • Goodbye Minmus stops.

 

Besides the monopropellant tanks in the front, I also lowered the flow priority for the small Mk2 Lf tanks so that they'll keep the CoM near the front. It was a struggle to fly this monster to the KSC, but I managed... and voila.

 

sLz9wqg.png

 

One thing I would like fixed is the control point on the carrier once I detach the spaceplane. Since its cockpit is the designated root part, my screen focuses on that part until I switch to the carrier. However, when I do, it is being controlled from the underside docking port. 

  • I would like to fix that issue at the SPH, but it's not as important as actually flying this thing. If fixing it at the SPH isn't possible, a user could just hit "Control from here" on the probe core.
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