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Space Nerd

Advice on building a space plane?

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I'm trying to build a dream chaser analog,so I built a small space plane, drained half of the fuel, and used set orbit to bring it to lko.

Then I burned retrograde to lower my pe to 30 km, and did a prograde reentry.

At first everything went well, only some minor heating problems, but nothing exploded.

Then it slowed down too slowly (over 600m/s at 7km), I tried using bigger wings, but it didn't help.

Can anyone help me?

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Is your reentry precisely locked on prograde all the way down? And what's wrong with being at 600m/s at 7km altitude? That's a perfectly nice speed for that altitude for a spaceplane.

 

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Try using airbrakes, drogues and chutes to slow you down. Furthermore for ssto reentries i recommend you not facing prograde but having a "reentry" attitude. Hope it will fix your problem 

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Posted (edited)

I slightly point it up when reentering.

I afraid it's going to crash(it's sort of nose diving).

Edited by Space Nerd

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Now I'm building a rocket to get this into orbit, my design keeps failing, any advice on that?

ps.I can't add my screenshot

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More than likely, your wings are adding asymetrical lift to your rocket. Try using an X-style wing configuration, and add large tail fins to the rocket to bring your center of lift behind your center of mass.

You may also want to have the whole thing take off as a spaceplane. It may actually be easier to make a small SSTO since the aerodynamics will be more consistent.

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Well, I put it inside a 5m faring, it's the rocket lifter that's keep failing.

 

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On 8/14/2019 at 1:42 AM, Space Nerd said:

I slightly point it up when reentering.

I afraid it's going to crash(it's sort of nose diving).

So, point it up more, then.  If you don't want it to nose-dive, then just pitch up, right?

Is the problem that you can't point it up?  i.e. you're holding down the S key to try to pitch up, and it just won't pitch up enough?  If that's the case, then it likely means you've designed your craft without enough pitch authority on its control surfaces, which in turn indicates a design issue (with their size, placement, and settings).  Could you post a screenshot of your craft while it's re-entering?

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I fixed the reentry problem, now I'm trying to build a working launcher for it.

10 hours ago, Space Nerd said:

I put it inside a 5m faring, it's the rocket lifter that's keep failing.

 

 

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Can you help explain "how" your rocket is failing? Is it just sitting on the pad? Does it flip over during flight? Something else?

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Posted (edited)

When doing a gravity turn, the faring base just  'snips' off.

Edited by Space Nerd

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If you have a lot of weight above the fairing base and the fairing base itself is small, you can try making that section stronger by adding some struts or using a larger fairing base. 

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10 hours ago, Space Nerd said:

When doing a gravity turn, the faring base just  'snips' off.

How sharp is your turn?  Are you doing a degree or two pitchover at the start, or shifting 90 degrees in one go?

What size decoupler are you using?  If it's a tiny one, it might be getting overwhelmed.

How is the clearance between the payload and the fairing?  If something collides into the fairing, bad stuff can happen, though I wouldn't expect the fairing base to fall off.

You might want to try autostruts.  E.g., autostrut the fairing base to the heaviest part (which will probably be something in the booster),  and autostrut something in the booster to the root part (which should be in the plane).

A screenshot would also help diagnose the issue.

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https://kerbalx.com/direstorm/Dream-Katcher

You know, landing Dream Chaser on Kerbin is kinda a boring thing to do. Try this one that I made specially for you, which you can definitely use to land on the Mun or on Duna (not both on the same trip, sadly). The real question is, can you land this on Duna and then get back to Kerbin?

If I did my math right (which I might not have) and you're a great pilot (which you might not be) then you -should- be able to get back to Kerbin. I think. Pretty sure. Good luck!

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Well... My dream chaser is for getting kerbals to my space station, so your craft is a bit overpowered for me.

Also, what's the problem with my rocket?

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12 hours ago, Space Nerd said:

Well... My dream chaser is for getting kerbals to my space station, so your craft is a bit overpowered for me.

You can always lose the drop tanks and add a crew cabin and a cargo bay. The dreamchaser is supposed to deliver 7 crew to ISS and a bunch of supplies. That puts you firmly into Kerbin SOI-only territory and might put Mun landings out of your reach. So your ship would be able to deliver supplies to an orbital mun station, as well as an LKO station.

12 hours ago, Space Nerd said:

Also, what's the problem with my rocket?

Well, you've got what, two tons of plane wrapped under like five tons of fairing? That should give you a hint that things are out of whack. The big issue are the big, swoopy wings, which are perfect for taking off from the runway and traveling long distances, but less good when mounted on the nose of a rocket. The radius of the wings squared gives you a giant pancake nose, which you've already found tends to get ripped off at the slightest provocation. And they're kind of overkill for something whose job is basically to fall out of the sky.

Regarding your mission profile, your ship seats one astronaut and has zero docking ports, which means you're going to have a hard time delivering crew and supplies which is the whole point of the ship.

Things you did right: your ship has airbrakes and parachutes, which are perfect for a controlled descent whether your craft has wheels or not (it should have wheels too, but I can't tell). Your swoopy wings mean your center of lift is behind your center of mass even when your fuel tanks are empty, which means you'll have a controlled descent all the way down. You have SAS, a battery, a solar panel, and monoprop fuel (I assume that means you have RCS thrusters and they're balanced around COM, since you look like you know what you're doing in space). That means that if you do have a docking port that I can't see (maybe radially on the belly?) you'll have the easiest time possible docking. Well done. 

Things you could do to fix: Notice on my ship I have tiny, tiny wings around COM, and then big, angled tail fins to provide lots of drag and control. My ship fits under almost the tiniest fairing possible, although you could make the fins angle inwards instead of outwards and save a bit of space, but that's not how the actual Dreamcatcher does it. Add a mk1 crew cabin, a docking port and a cargo mod and you'll be able to accomplish your crew-and-resupply missions, and you should still have tons of dV for LKO

One advantange the mk2 form factor offers is actually that I've got fuel tanks on both ends of the craft, which means COM basically doesn't move around too much when the tanks are empty vs full. This compares favorably to having your CoL behind the engines, which does work but means you have more wing than you need.

Anyway, it was actually faster for me to just build a craft to show you than it was for me to type all that and you to read it. Hope that helps!

 

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I did a quick replication of your ship and managed to fly it into orbit without having the fairing break (and I didn't use any struts). Interestingly, I ended up grabbing the 3.75m fairing and ran with that. I didn't go back to see if there's something different internal to the 3.75m vs. 5m fairings, so there's a possible issue there. It may also be something with the adapter you're using.

 

Another possible cause of the problem is thrust to weight ratio. The big reaction wheels especially can sometimes bend or crush under the weight of a ship on top of them. This usually comes up with rockets much bigger than yours (i.e. there is a lot of weight on top of the reaction wheels), but could still be a factor if your takeoff TWR is particularly high. This could be popping up when you do the gravity turn since the reaction wheels and/or fairing are already under stress from the TWR, then it gets some bend during the gravity turn. I have my doubts here because I also tried flipping the rocket sideways and it stayed together, but it can still be an issue.

I also noticed (not unexpected) that the rocket wants to flip over. There's so much surface area on the fairing that turning too much off of prograde while at semi-high speeds causes the rocket to really want to flip over. I'm not sure if that's also part of the issue you're running into.

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Yep, it flips when I detached the first stage.

Also, I tried, but it won't fit in a 3,75m faring.

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Posted (edited)

Maybe consider removing that second stage engine and adding just a bit more fuel (to keep the dV about the same). That might also enable you to remove many of the struts, which will slightly improve performance. Additionally, removing an engine could reduce the overall price of the rocket (if you're dealing with funds at all).

If it's still a bit flip happy, you could also try putting some fins near the bottom to help improve aerodynamic stability and control (in a manner visually similar to the Saturn V).

 

Spoiler

Y6wdioD.jpg

 

Edited by Claw

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