SSTO Assistance

[Choctofliatrio2.0]

So I built my first working SSTO recently... sorta. I call it "Old Semi-Reliable." It can make it into orbit sometimes, if I fly it right, and with major help from monopropellant.

Spoiler

My flight path is using the jet engines, flying at about 15-20 degrees until the engines become worthless around 20km. Then, use the aerospike to get out of the atmosphere. The main problem is that the oxidizer gets eaten up really fast. If I try and fly lower and gain more speed, the plane burns up. So I am asking for assistance. Any suggestions to improve the design (without radical changes), or changes to the flight plan would be infinitely appreciated  

[Exothermos]

Dancing that line between burning up at low altitude vs. being too slow at rocket switch-over is the name of the game.  In the case of this design, my guess is that you have way too much thrust below 12k, and no where near enough between 15k and 25k where most of the speed is gained.  

Try switching to 2 rapiers and dropping the spike entirely.  

Change your wing angle so that the angle of attack on your main wing is 5 degrees or so.  This will allow you to keep your nose flying more-or-less directly into the path of the wind, while still gaining altitude.  

Fly at 10 degrees the entire time below 30k.  

Those things should do it.  I bet if you just changed the wing angle and flew at that profile you would still make it into orbit better with the design the way it is.

[Frank_G]

Like Exothermos said, a low Thrust-to-Weight-Ratio is good for an SSTO. While rockets need a TWR of 1.x, a SSTO spaceplane can successfully reach orbit with a TWR below 0.5. It only takes its time to get there.

A key point in your ascent profile is breaking the sound barrier. This can be quite a challenge, if you fly a low TWR craft. Once you pass Mach 1 and reach altitudes above 10.000 m, the R.A.P.I.E.R engine really wakes up. If you cant pass 330 m/s while going up, it is no crime to take a little dive to build up speed.

The design you have there, already looks capable. Try to exchange the two jet engines and your rocket engine for a single R.A.P.I.E.R. I dont want to fight you, Exothermos, but a single engine should be enough for such a small craft. The extra loss of mass will increase fuel efficiency for sure.

[selfish_meme]

Such a small plane could be converted to liquid fuel only, replace the spike with a nuke, ditch all bifuel tanks and replace with liquid. You must pass 20km at 1300ms+ and then head for the horizon, you can even run the nuke and jets at the same time for a speed boost.

[Rune]

First of all, yes the RAPIER is awesome. It's awesome because it has the highest cutoff speed of all airbreathers, and that really makes things easy for the rocket part of the ascent. But...

There is always a "but". It is perfectly reasonable to make a SSTO with any airbreathing engine, even Junos. The trick is to understand that the crappier your airbreathers are, the more your SSTO will look like a rocket SSTO, that is, something which has 3,5km/s of dV on LFO (therefore, is mostly fuel by weight), and high TWR to waste littel to gravity losses (>1 at least). In this case, you are using turbojets, which are almost as good as RAPIERs, so your SSTO only has to be a bit more "rockety" to make it to orbit. Increase the fraction of your weight that is LFO mix, increase rocket TWR if you need to (unlikely, really, turbojet cutoff speed is already high enough that gravity losses are low form there), and you can probably just go with the airbreathing TWR you have now. As Frank and Exo say, TWR when flying like a plane is much less of an issue, anything that allows you to go over Mach 1 at ~10,000 is something you can work with. However, the minute you turn into a rocket (stop using air for ridiculous effective Isp), TWR becomes important again, and it's all about that, and of course your fuel fraction. Never forget the rocket equation!

 

Rune. TL,DR: add moar fuel.