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Ncog Nito

Oxidizer usage in atmosphere

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Good evening to you all.  Hopefully I have a simple question here.  I have built an early tech plane, and, in order to make it long range I added a Terrier rocket engine to it so I could get it off the ground with all the extra fuel it's carrying for the Juno engines that are on it.  I set the thrust limiter on the Terrier to 20, just enough to get the plane off the ground, and then I shut it down once I have enough speed to keep the plane in the air.  My question is, does the oxidizer burn at the same rate no matter what you have the thrust set at?  When I do activate the Terrier in flight it seems that the oxidizer is still burning at the same rate it would if the Terrier was at full power.  If it does, is there any way to limit the oxidizer so I can maximize the effectiveness of the Terrier?

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By the way, @Ncog Nito, to address the specific things you mentioned in your original post:

  • Rocket engines use oxidizer, and only oxidizer, to burn their fuel.  They don't (can't) use atmospheric oxygen at all.  When you run them in atmosphere, they use the exact same amount of oxidizer (relative to liquid fuel) as they do if you're running in vacuum.
  • When you are running in an oxygen atmosphere (like Kerbin's), jet engines win over rocket engines, by a lot, for airplane-style flight.  It's simply no contest.  They're hugely more fuel-efficient.  So there's basically no reason to put a rocket engine on your airplane, unless you need it for getting up to altitudes where the jet engines don't work.
  • Note that the Terrier is an especially bad idea for taking off with.  Not all rocket engines are the same.  Some of them work well in atmosphere (and are therefore useful for taking off from the launchpad), but the Terrier isn't.  It's a "vacuum engine" and has horrible efficiency at sea level-- its thrust drops to practically nothing, while still gobbling plenty of fuel.

So if your goal is to build a long-range plane that cruises at conventional airplane altitudes... don't use rocket engines.  At all.  The only reason to put rockets on your plane is for higher altitudes (e.g. getting to orbit, or suborbital hops).

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At sea level the terrier is heavier,   less efficient and less powerful than the Juno. 

It's just terrible for what you want and limiting the thrust make it even worse. 

If you limit the thrust you use less fuel. 

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As Spricigo said, the terrier is a space-engine.

 

If you are just trying to get off the ground and have enough thrust once airborne, just strap on a couple SRB's on the wingtips (with radial decouplers ofc), and make your plane a RATO (Rocket assisted takeoff). Once you are in the air, you can ditch them, maybe add some parachutes for recovery.

 

They are cheaper, provide more thrust, and don't weigh your plane down once in the air.

Edited by Ketatrypt

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Actually you may tell us what tech us available and what is the plane's purpose. 

Thus we can present some design proposals and a few pointers to better utilize it. 

Alternatively a image of your plane so we can help improve it

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Well the planes purpose is to complete contracts around Kerbin that are lower than say, 19000 meters.  The highest tech item I have on it are the small retractable landing gears.

Ok...I went to Imgur.com, uploaded my screenshot, clicked the "insert other media" box, copied the BBCcode into the URL line there, but I can't seem to get the image up.  What am I doing wrong?

Edited by Ncog Nito
Trying to insert screenshot

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24 minutes ago, Ncog Nito said:

Ok...I went to Imgur.com, uploaded my screenshot, clicked the "insert other media" box, copied the BBCcode into the URL line there, but I can't seem to get the image up.  What am I doing wrong?

The URL line needs an actual URL. The bbcode is for when you are inserting it into the message part of the screen.

 

 

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2 minutes ago, bewing said:

The URL line needs an actual URL. The bbcode is for when you are inserting it into the message part of the screen.

 

 

OK...I'm stupid, where do I get that?

DXlBfOI.png

Ok...here is the bbcode for the screenshot...how do I insert a URL?

Sorry...I am trying this again...bare with me...

DXlBfOI.png

Ok...got it figured out...thanks on the screenshot @bewing, now about my plane...lol

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I don't know why you'd need a rocket engine to take off, you can build a juno powered plane that circumnavigates kerbin that can take off under its own power.   Most of what you are lifting here is rocket fuel for the rocket engine.     The only reason to build something like this is if you're trying to make a spaceplane, suborbital plane or are using the rocket to boost you to altitudes over 17km.

The limits of what you can take off with tends to be that when your plane is too heavy, the gear breaks before you get enough lift to take off.   Having a rocket engine won't help with that, you'll just reach the speed where the landing gear comes apart quicker.

However, a properly designed and flown aircraft does not need anywhere like that much fuel.   If you're going long distance, remember to fly high and keep speed below 240m/s,  AoA below 5 degrees.    However there is also the issue of human patience...   are you really going to spend 2 hours flying to the opposite side of kerbin for one measly contract?

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I suspect the problem isn't thrust, but an aerodynamics issue where pulling up while traveling down the runway doesn't pitch the nose up. Put your rear wheels just behind the center of gravity, so your rear control surfaces have some lever arm length to work with. Alternately, use canards as control surfaces.

8 hours ago, AeroGav said:

However there is also the issue of human patience...   are you really going to spend 2 hours flying to the opposite side of kerbin for one measly contract?

Allow me to again sing the praises of Atmosphere Autopilot. Build a sturdy plane, set a course, kick up to 4x warp and walk away.

Edited by Jarin

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1 hour ago, Jarin said:

 Put your rear wheels just behind the center of gravity, so your rear control surfaces have some lever arm length to work with. Alternately, use canards as control surfaces.

Just notice that If you keep the wheels too far from the CoG it may be still difficult to pitch up. Consider the torque  relative to the wheels, you want the [aerodynamic arm length]x[aerodynamic force]>[weight's arm length]x[weight].

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3 hours ago, Spricigo said:

Just notice that If you keep the wheels too far from the CoG it may be still difficult to pitch up. Consider the torque  relative to the wheels, you want the [aerodynamic arm length]x[aerodynamic force]>[weight's arm length]x[weight].

Yeah, this. ^ Basically what I was saying, but math. 

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6 minutes ago, Jarin said:

Yeah, this. ^ Basically what I was saying, but math. 

Maybe I was a bit blunt and not clear, sorry.

My comentarry was more in response to the suggestion of of canards. For planes with scarce lift and not much of it provided by canards itsellf can still have a hard time toking off.

On the other hand, if this is not a concern, too much plane behind the landing lags can result in tailstrikes. As always therre is compromisses to take.

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5 hours ago, Spricigo said:

For planes with scarce lift and not much of it provided by canards itsellf can still have a hard time toking off.

That's true, but I don't think it would be an issue here. Any canard the OP could use on a plane that size would be providing a notable percentage of lift.

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Thanks everyone for your input.  I was able to design a plane to do what I was trying to achieve with only the Juno engines, just took a little more refinement than I originally anticipated.  Your input helped a lot.

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If you use rocket engines in atmosphere, then Terier is definitely not a good choice. Terier is a vacuum engine, and its performance at sea level is lousy.

Spark is a tiny-sized engine that’s very effective in the atmosphere. It’s thrust at sea level (16.2 kN) is greater the the one of Terier (14.8 kN). At the same time, it consumes 3 times less LfO at sea level, and it is 5 times lighter!

Twitch and Spider radial engines are also good enough.
If I understand correctly and you’re using only 20% of Terier’s thrust (3 kN), then two Spiders are a possible variant.

Edited by Teilnehmer

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