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Thorn_Ike

SSTO Ascent profile urgently needed

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Hi,

This is the Longneck. Bainbridge Aerospace hasn't decided an official name for it yet, but we'll figure it out:

am6tmmD.png

As you can see, Longneck has a good 11k dV, however, when Longneck is passing through the atmosphere, we only have 1k dV to circularize, and 300-400, and if we're lucky 500 m/s dV to deorbit.

I am pretty sure I'm using the wrong ascent profile. I use the entire runway, accelerate to 500m/s below 1000m, and pitch up 20 degrees. 

Help me?

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You have a decent amount of wing and a streamlined MK1 design. While you are in airbreathing mode, you are burning very little fuel -- so you are free to make a slower, more efficient ascent. The steeper you go, the less "good" you are getting out of your lifting surfaces. Lifting surfaces save you fuel, if you let them work. A vertical climb doesn't use your lifting surfaces at all. The more horizontal you fly, the more they can do for you.

There are two tricks that I can see you aren't using though.

The most important is putting some incidence on your wings. The front edge of your wing should be tipped up 3 to 5 degrees. This keeps the rest of your plane more horizontal, which reduces drag a lot.

The second is your choice of cockpit. That MK1 cockpit is very heat sensitive. If you actually use your engines to their potential, your cockpit will blow up. Using an inline cockpit helps a lot -- and then putting anything (even just a nosecone) in front of it will make your plane a lot less likely to blow up.

So, the third thing is that when you get up to around 20km altitude, you want to mostly level out and let your engines really work for you. They can easily get you to 1500m/s before you have to switch to closed-cycle mode. And the longer you accelerate in airbreathing mode, the more fuel you will have left when you get to orbit.

 

 

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After lots and lots and lots of experimentation, here's the most efficient profile I've managed to find:

  1. Climb and accelerate to 10k, so that when you hit 10k, your speed is about 1000 m/s. Your climb angle depends on your craft and engine configuration. When using Rapiers, you might have to accelerate straight and level near sea level to get to around 400 m/s, at which point you'll be in the "ramjet feedback loop" with your TWR climbing nicely as you start ascending. 
  2. At 10k, level off to around 5 degrees climb. Hold it there -- alternate between regular and prograde SAS to keep your nose down. You will start accelerating rapidly. 
  3. At 15k, nose down even a bit more so you're only just above the horizon.
  4. At 22k or so, you should be going near the thermal limits of your craft, or around 1500 m/s, when your jets flame out. Switch to closed-cycle/your rocket engine, keep your nose just above prograde, climb to orbit, and circularise -- at this altitude air resistance won't cost you all that much anymore.

The only exceptions to this are significantly un-aerodynamic SSTOs, such as when you're lofting bulky cargo. In that case you will want a very high TWR -- so a few more Rapiers than you'd use otherwise --, and you'll want to blast your way up as quickly as possible, otherwise you'll lose too much to air resistance. So in this case just burn up at 40 degrees to 10k, then at 15-20 degrees until in space. 

Edited by Brikoleur
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I usually pitch up to 20 degrees straight after takeoff, aiming to get to 1100 or 1200 m/s by the time I get to 20km which is roughly where I have to change over to closed cycle mode.  Not sure why you would want to be doing 500m/s below 1000m as that is where you are going to encounter the most drag and thus have a higher fuel consumption rate.

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On 12/2/2018 at 5:36 PM, bewing said:

You have a decent amount of wing and a streamlined MK1 design. While you are in airbreathing mode, you are burning very little fuel -- so you are free to make a slower, more efficient ascent. The steeper you go, the less "good" you are getting out of your lifting surfaces. Lifting surfaces save you fuel, if you let them work. A vertical climb doesn't use your lifting surfaces at all. The more horizontal you fly, the more they can do for you.

There are two tricks that I can see you aren't using though.

The most important is putting some incidence on your wings. The front edge of your wing should be tipped up 3 to 5 degrees. This keeps the rest of your plane more horizontal, which reduces drag a lot.

The second is your choice of cockpit. That MK1 cockpit is very heat sensitive. If you actually use your engines to their potential, your cockpit will blow up. Using an inline cockpit helps a lot -- and then putting anything (even just a nosecone) in front of it will make your plane a lot less likely to blow up.

So, the third thing is that when you get up to around 20km altitude, you want to mostly level out and let your engines really work for you. They can easily get you to 1500m/s before you have to switch to closed-cycle mode. And the longer you accelerate in airbreathing mode, the more fuel you will have left when you get to orbit.

 

 

Good idea. Problem being, my PC's memory got corrupted, and I lost all my craft. :(

I'll try it with a Mun-capable SSTO when I get my PC back.

Edited by Thorn_Ike
Blame Pecan

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23 hours ago, Thorn_Ike said:

Good idea. Problem being, my PC's memory got corrupted, and I lost all my craft. :(

I'll try it with a Mun SSTO when I get my PC back.

"Mun SSTO" spaceplane?  You're funny.

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On 12/5/2018 at 7:58 AM, Pecan said:
On 12/4/2018 at 8:14 AM, Thorn_Ike said:

Good idea. Problem being, my PC's memory got corrupted, and I lost all my craft. :(

I'll try it with a Mun SSTO when I get my PC back.

"Mun SSTO" spaceplane?  You're funny

"Sometimes you gotta run, before you can walk." -- Tony Stark

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3 hours ago, Thorn_Ike said:

"Sometimes you gotta run, before you can walk." -- Tony Stark

You misunderstand - I have a particular bugbear about misuse of the abbreviation/expression 'SSTO'.  If you really mean you're going to try a Mun SSTO, that would be a vehicle that launches from Mun surface to Mun orbit in a single stage.  Ascent profile would be as for any other vacuum-body rocket launch.

(Feel free to ignore this or just reply 'shut up' if you wish.  I don't take my mission to be pedantic too seriously).

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2 hours ago, Pecan said:

You misunderstand - I have a particular bugbear about misuse of the abbreviation/expression 'SSTO'.  If you really mean you're going to try a Mun SSTO, that would be a vehicle that launches from Mun surface to Mun orbit in a single stage.  Ascent profile would be as for any other vacuum-body rocket launch.

(Feel free to ignore this or just reply 'shut up' if you wish.  I don't take my mission to be pedantic too seriously).

Got it, will edit that post.

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