Predicting how my spaceplane will fly

[quasarrgames]

It gets tiring spending hours test-launching my spaceplanes to see if they get to orbit. So, is there any way i can tell if a spaceplane i make will be able to get to orbit? Like wether it will have enough thrust to do a 25 degree climb, or wether the rocket stage will have enough delta v and thrust to make it to orbit? I have Kerbal engineer, if it helps.

I understand that this is a difficult question, so if you can answer it, i will give you rep.

[SAI Peregrinus]

Learning to use FAR's stability analysis window is pretty much necessary for this. If you're using stock aero you should just get used to trial and error testing, and getting a feel for what works with your piloting style. With the FAR tools you can actually see if something will work with your expected flight profile.

[ajburges]

There are 4 important stages of a typical hybrid engine SSTO ascent: climb to a low pressure altitude, accelerate past mach 3 on aerobic engines, climb past the majority of the atmosphere, assume a gravity turn to orbit.

The three limiting factors of you flight envelope in order are

> ability to climb past 7 km (10 preferred) subsonic

> ability to reach supersonic

> having 1.6+ dV and .8 TWR for anaerobic engines at the end of aerobic profile.

The first limit is a factor of lift and TWR. The second is a factor of thrust to drag. The final one is the application of the rocket equation and having enough extra LF for the earlier parts (and landing if you want powered landings).

Advice about tonnes per jet are a good guideline for you aerobic TWR. I think current advice is 13t a jet at takeoff. If you have problems climbing, you have too little wing; if you have trouble going supersonic you have too much drag (which could be because of too much wing!). I am recalibrating what my desired aerobic TWR is so I can't help there. I prefer using less aerobic engines than most and instead supplementing them with anaerobic engines to break the sound barrier.

Edit: derp! Are we talking stock or FAR?

Edited July 17, 2015 by ajburges

[O-Doc]

It gets tiring spending hours test-launching my spaceplanes to see if they get to orbit.

Problem identified.

[Xannari Ferrows]

Kerbal engineer gives Delta-V = Invaluable.

CoM aligned, and in front of CoL = Immediate stability.

CoM changing upon resource drain = Potential disaster. Make sure it moves as little as possible.

CoT inline with CoM = No thrust torque.

Just the basics. I might get into more things later.

[cantab]

KER can help you explore TWR at various speeds and altitudes. That depends just on the engines and the plane mass of course, so you should soon get a feel for where the different engines work well.

Stock has no real SPH tools to study aircraft stability. FAR does, and if you're using FAR you can check the handling of your aircraft at various speeds and altitudes. It won't really tell you your performance though.

[Val]

I have a few design guides that I use, for pure Rapier SSTOs that only go to LKO.

For each Rapier you need:

• LF tankage: 2x Mk1 Fuselage or 1x Long Mk2
  
• LFO tankage: 1x FL-T800 or 1x Long Mk2 or other equevalents, that add up to the same amount.
  
• Wing Area: 3 (2-2.5 main wing, rest for horizontal stabilizer) + Control surfaces + Vertical Stabilizer.
  
• Intakes: 1x Shock cone (lowest drag)
  

This will allow you to lift 4.5 t to orbit comfortably. 5 t just barely.

The 4.5-5 t is the rest of the craft and cargo:

• Cockpit
  
• RCS+Monoprop
  
• Cargo bay with cargo.
  
• Landing gear
  
• Batteries
  
• Power Generation
  
• Reaction Wheel
  

Other things of note:

• The only recommended surface attachments are, Wing parts (incl. Airbrakes) and Retractable Wheels. Fuel tanks are OK, if used to mount an engine. RCS and Radiators may be needed. Everything else goes in a cargo or service bay.
  
• As little control surface as is necessary. On many of my single Rapier designs i only have 2 control surfaces. Combined Roll and Pitch in a V-tail or Elevons.
  
• I always mount wings with a little upward angle, so I can have positive lift, even with the craft at zero degrees pitch, at high enough speed.
  
  • I do it mostly to minimize drag around and right after Mach 1.
    
  • Because it's better to have only the wings at an angle to the airflow, than the whole craft.
    
  • I aim for neutral lift around 375 m/s, at sea-level. At 9 km this gives neutral lift around 600-800 m/s.
    

Once I've built my SSTO, I evaluate it's performance using quick tests, that I've found to work for me. It takes a maximum of 4 minutes to test the basic viability of a new SSTO. That is not to say, that an SSTO which fails my tests, can't get to orbit. But it will probably take much longer than it needs to and probably uses more fuel than needs.

Which brings me to the ascent profile. I don't use the standard, slow climb to 10 km. I fly pretty much level right off the runway and don't start climbing until I hit 450 m/s. I've found this to be the most efficient ascent for my designs. Especially the heavier ones, many of which could not climb past 5 km with the standard ascent.

I baked the tests into this album.

Javascript is disabled. View full album

Craft file

In general I prefer combined Rapier/Nuke SSTOs. I find them to be much more efficient than pure Rapier. They use a very similar ascent profile. There's a bunch in the SSTL album in my signature. Mostly made from Mk1 parts, but there's a few made from Mk2 and Mk3 parts, also.

Here's a proper Rapier/Nuke SSTO: G+ post (with craft files). Although that one is a bit overpowered. It doesn't need a sea-level acceleration. It can accelerate through a shallow climb.

Edited July 19, 2015 by Val
Added Vertical Stabilizer and Airbrakes

[CybSeth]

I did an analysis on several space plane designs and some of my own. There are some ratios i've calculated out that i use as a rule of thumb.

total_mass * 35-60% = total_fuel_mass

>> Does not including dry mass of fuel tanks - keep ratio of LH to O2 at about 2:1 or 3:1

total_mass * 10-25% = combined_engine_mass

>> You need about 1 rapier per 16-20 tons of total mass - 20 tons per rapier makes for very shallow ascent profiles, 18 is a good spot, 16 tons per rapier usually means you can practically just aim at 20 degrees the whole way and you'll get to orbit

total_mass * 6% = fuel_tank_dry_mass

>> fuel_mass / 8 = fuel_tank_dry_mass (about the average weight of dry tanks)

total_mass * 4% = lift_body_mass

>> total_mass * 40% = lift_body_needed (I aim for about 40-45% lift units, if you have lots of TWR you can probably get away with less - e.g. 20t craft should have about 10 lift units total)

>> lift_body_needed / 10 = lift_body_mass (average weight of most lift bodies is 1/10 the lift)

total_mass * 10-45% = other_parts_mass

>> "other" variable fixed part mass costs like:

>> cockpit, landing gear, airbrakes, tail fins, air intakes, batteries, solar panels, science equipment, radiators, filled ore tanks, etc.

How I apply it:

I always start with 50% fuel ratio, 20% for engines, 10% dry tanks/wings, and other stuff to 20%. That's what I expect for a reasonable flight profile (climb at about 20, level off at 10k, gain alt slowly, activate o2 around 25k, aim for AP of 75k'ish, circularize).

Step 1) I usually start with the number and types of engines first (say 2 rapier and a nuke - 7t of engines)

Step 2) With a 20% engine ratio, this means my total mass should be around 7t engine / 20% = 35t

Step 3) From there I figure about 50% fuel as a baseline expectation (17t fuel -- 3:1 ratio so that's about 12.75t lh and 4.25t o2)

Step 4) I presume 10% of total mass goes toward wings/dry tanks

Step 5) remainder goes toward other stuff, but i try and restrict this to 20% if at all possible (so 7t limit of cockpit and other stuff).

I'll start building with these goals in mind, putting together fuel tanks, then engines, then wings (balancing CoL), then put on other accessories. As I build it, and it comes in UNDER 35t total, I know I can expect the flight profile to be more relaxed. If it comes in OVER 35t then i can expect the flight profile to be more finesse and shallow ascent. These rules work even for big birds - e.g. a 180t bird i'm working on now.

If you want to have an even more lazy flight profile - go for more engines and more fuel and reduce your 'other' mass:

55% fuel, 25% engines, 10% dry tanks/wings, 10% other - That will very likely do a 30 degree fire-and-forget style ascent profile.