capi3101

Okay...still having issues with the Death Trap. This one was one that RCS Build Aid was telling me, though: And you can even see it there - the CoT is not inline with either of the CoMs. I can get rid of most of the torque by putting the wing lower on the fuselage, but doing that will put the CoL below the CoMs, and tilting the wing dihedral to put the CoL back where I want it puts the torque right back where it was. FAR gives me reds with the current design on at 10/1 on Xu at 15/2 on Zw, but in both cases the sims stabilize. FAR green-lights the design at the other benchmarks, including 30/4. What happens is that I get up to about 35k/Mach 3.8 or so, the jet flames out and so I put on the rockets. The torque kicks in and I pitch up and out of control. First time that happened I got to do the emergency landing I talked about last night. Tonight when it happened and I attempted another emergency landing I wasn't paying attention to my airspeed like I should've been - it dropped down to 45 m/s (when the plane's takeoff speed is 120), it nosed down and I didn't have enough time to get the speed/nose back up at that point... On the plus side, the spoiler/flap combination as an airbraking system works great. The problems are with the main wings - I need a way to lower their location on the main fuselage while raising the CoL so the plane will stay stable. I'd thought perhaps about making a "gull-wing delta", bending the wing dihedral at about the midpoint of the span (or thereabouts) while lowering the position of the wing in relation to the fuselage. Would that work, or does anybody have a better suggestion?

Yeah, I'm not really sure what happened with that fin - I was shooting for 20% of the wing area at a 2:1 ratio and I think I must've miscalculated something. On the plus side, the plane's yaw control was awesome.

Can't offer much that these guys haven't already; I would suggest installing a mod called RCS Build Aid, though, especially if you're planning to do a lot of future spaceplane flights. It'll show you how much and in what direction your fuel load is shifting in flight, which is going to be an issue until 1.0 is released. Just as useful as KER, IMHO. Since your plane is barrel-rolling with SAS off, it could be that the CoM is shifting upwards, above the CoL, as the tanks drain. Just a thought - these guys would tell you if there's any validity to it or not (I'm a FAR rookie, and not a pilot IRL...).

I see. Wish I'd seen this last night prior to the test flight of the re-designed Death Trap: (Yeah, I'd forgotten about the RCS Cylinders I was using for ballast before I took off - I should take those out and rebalance the fuel some more. Two more structural intakes on the underside that y'all can't see in that shot). Decided not to change the engine set-up despite the dragginess. Engine flamed out at 32k and I was only doing about Mach 3.5 at the time, and I started losing speed when I throttled down to let the engine catch. So I kicked the rockets on. You might suspect what the result was... I wish I'd paid closer attention to this prior to take-off; FAR was giving me Xw warnings from 15k/Mach 2 and up though the simulations were showing stabilizing tendencies, so I did nothing about it. On the plus side, I did get to practice an emergency landing, which I'm happy to say was a full success in its own right. Q maxed out around 13 kPa during the descent; started S-turns at that point and those really helped. Touched down about 80 m/s SAS-off and hit the brakes; got her stopped without anything breaking off. I guess putting the gear out on the underside of the wings was a really good idea...

If you want to do it by hand, try this route: 1) Since you know your rocket's thrust, set the TWR at 1.2 (generally considered the lowest acceptable value) and solve the TWR equation for mass (R=T/mg therefor m=T/Rg, where T = your booster thrust, R = 1.2 and g = 9.82). Compare the resultant mass with the known mass of your rocket. The difference between the two is the maximum payload based on TWR. 2) Now, since you know both the mass of your rocket and its mass when the tanks are dry, add that same payload mass to both the wet and dry figures and run the Rocket Equation to solve for delta-V. The result will be how much delta-V you'd get with that payload mass. 3) If the delta-V with the payload mass is less than 4500, the limiting factor on your booster is delta-V. If it is 4500 or greater, TWR is your limiting factor. 4) In the event that your determine that delta-V is your limiting factor, you're going to have to run the Rocket Equation backwards, solving for payload mass. Let me solve the equation for you there: delta-V = Isp * Go * ln(M/Md) = 4500, where Go = 9.82 m/s^2 and Isp is the specific impulse of your engine cluster. Set Delta-V equal to 4500. 4500 / (Isp * Go) = ln(M/Md) = x (divide the 4500 delta-V by the Isp and Go, and call the result "x") M/Md = ex = x' (find the inverse natural logarithm of x and call it x') M = x'Md (Solve for wet mass) M + P = x' (Md+P) (since the payload mass is part of both M and Md, pull it out as a seperate term, called P) Now, here's where we might have an issue: 1 meter and 2.5 meters fuel tanks (from the little FL-T100s all the way up to the Jumbo 64 orange tankls) have a wet to dry mass ratio of 9:1. For 3.5 meter tanks, the ratio is 8.2:1. Either way, you can say that the wet mass equals whatever ratio you'ure using times the dry mass. I'll assume you're using 2.5 meter tanks, and therefore I can say that M=9Md. So I plug that in: 9Md + P = x'(Md+P) And now you solve the equation for P: 9Md + P = x'Md + x'P 9Md - x'Md = x'P - P (9-x')Md = (x'-1)P P = (9-x')Md/(x'-1) You then just plug in the numbers; the result should give you the payload mass that will get you the 4500 m/s of delta-V. Now, if you're dealing with a multi-stage rocket, you'll have to go through this process for each stage in turn. And if you're using both size-3 and size-2 tanks in a single stage and find yourself in a situation where delta-V is the limiting factor, you'll need to figure out the wet-to-dry ratio of that stage yourself (and then put that in place of the "9" in the final equation if needed. Or you could save yourself the trouble of manual calculations and download NRAP as Alshain has suggested. Best of luck in whatever route you choose to take.

Glad you were able to work things out. Since you're still relatively new to the realm of spaceplanes in general, I'll point you to '>my general advice for rookies. Keptin's guide is a must for all beginners, DocMoriarty's guide is good for specifics if you're planning on sticking with the pre-1.0 stock aerodynamic model for the time being. And should you decide to switch to a different aerodynamic model, there's always a chronology of my shenanigans in that area. I would second StainX's suggestion of the RCS Build Aid mod and I would go so far as to call it an "essential" mod; if you can build your plane such that you don't have to worry about the CoM shifting in flight, you'll meet with a great deal more success. That mod gives you the tools you need to do that accurately - it tells you where your CoM will be when the tanks are dry, i.e. in which direction the CoM will shift and how far it will go. Have you tried landing on Kerbin yet?

Er...went for it. A while ago.

Really? I'd have thought you wouldn't need that much thrust at that point in the ascent...is that choice more for the rocket's Isp? Or is it more a matter of getting a decent amount of thrust out of it while the throttle is set low (i.e. giving you a good amount of "crossover" thrust between the jets and the rocket)?

This morning I'm starting to give some thought over where I need to head next. Obviously de-orbiting and landing are things I need to practice; I kept last night's plane in orbit so I can start doing that this evening. I'm also beginning to think about how I'd lift a Jumbo 64 or S3-14400 to orbit via plane (I've done a J64 in stock before - never a 14400), and eventually I'll need to get back over to my career game, which has been on hold since I started playing around with FAR. One thing I know there - I haven't unlocked RAPIERs yet. I guess that last bit begs a question - one of my very first FAR plane was the Flaming Death Trap 7: Y'all warned me off of the engine arrangement I had going there (the Turbojet/24-77 combo), due to the drag the 24-77s produced. I used to use that arrangement in stock because it reduced the overall engine footprint I needed for my planes. But let's say in the future I had a design that only needed a single Turbojet (and let's say this is in my career game so I don't yet have RAPIERs). What would be a good rocket engine arrangement for such a plane? Maybe a pair of flanking 48-7Ss? EDIT: I really should revisit this design. So many things to be fixed with it...

Alright - followed your guys's suggestions for a redesign this evening and came up with this: Missed the suggestion about structural intakes at the base of the fins but pretty much did everything else y'all suggested. That is a large cargo bay BTW, and the CoM is right in the center of it. Added the canards to improve the pitch authority and bring the CoL closer to the CoM (first few flights of the redesign went into the drink with the elevators set to full deflection) and then later added the third engine when I realized just how fast the takeoff speed was on this thing - neighborhood of 140 m/s (which seems high to me but also is an indication that I shouldn't be affected by drag from the wings so much at altitude, right?). Nose lifted just fine once the canards were added, but the plane didn't want to take off until it got going better. Probably should've paid more attention to the wing geometry - the fuselage is longer than what I started with but I pretty much left the wings/fins there from the first design. I am going to have to calculate how much payload this thing should be capable of carrying to orbit - I think I've got a reasonable general purpose transporter pretty far along at this point, though. Still need to try to land it.

Had a couple of you ask about the monoprop tank - this design started out as a tweak of SalehRam's design from the day before yesterday, and it had an inline monoprop tank. I didn't see any harm in leaving it in since some of the advice with that particular design was "make the fuselage longer and draw the CoM forward". In my experience, monoprop makes for reasonably good ballast if nothing else. I did move it well forward from where it was originally; in retrospect I wonder if it would've been better to put it in between the cockpit and the cargo bay. Design actually incorporated a third engine originally - I took it off and replaced it with the drag chute once I started flying it. TWR was over 1 with the extra engine and the design was getting some pretty high Q at low altitude. Forgot to drain that back fuel tank after I removed the engine though; from my final fuel stats I definitely didn't need it. I suppose the design has a good fuel reserve as a result though. I was pretty sure I'd overSASed it. On the other hand, RCS Build Aid was telling me I had 10 kN of torque from the main engines, so I knew I needed at least one. Plus I figured the extra mass would draw the CoM forward a bit. I think that if I were to double up the PB-NUK and move it over to the sides of the cargo bay there'd be enough space for a small payload. Another reason to move that monoprop tank in between the cockpit and the cargo bay...get the cargo bay closer to the CoM. Of course, that would make the SAS less effective, wouldn't it? Got both NavUtilities and TAC Fuel Balancer installed - can't really see that from my UI on account of low graphics settings. Will have to think about those chutes...

Just wanted to let y'all know that I have been paying attention... Kinda wish I hadn't reverted the flight - I could use the landing practice. It's become reflexive...

It's the Caps Lock key.

Yeah - I was thinking about suggesting moving the mono tank up between the cockpit and the drone core, but I didn't know if that would've moved the docking port into an impractical position or not (I never got a good look at the underside of the plane so I don't know if the original designer put an extra set of structural intakes down there or not; if he hadn't, I wouldn't see any issue with setting the port up to open up from the ventral side instead of the dorsal). Small fin crossed my mind when I ran the 30k/M4 and got the red/unstable Lß figure. Say I was working on that same design; how much more wing does it need? I'm assuming more chord, not necessarily more span. Maybe some strakes up in front of the intakes? EDIT: Whoops - Wanderfound posted while I was typing...yeah, I wasn't sure suggesting canards to the guy was a good idea, especially after I'd cloned the craft and saw that it already had pitch-up tendencies.

Cool - I'll try those out those tweaks on the BWOT when I get a chance. I'd like to digress from my designs for today - yesterday, another forum member posted this FAR plane design: I went ahead and cloned it and tried flying it around a bit last night. Only real change I made was adding a bit of dihedral to the wings. Now, from what y'all have told me - and this is what I told them - that design has A) too many engines and insufficient pitch authority. My general advice was to cut the number of engines to two and add a set of canards up front (I suggested AV-R8s high up on the fuselage; high up so they wouldn't be in-line with the main wing). Was that good advice or not? I've read that canards are de-stabilizing surfaces... My experience with the plane was that it had really lousy pitch authority, a high takeoff speed (150 m/s before it even wanted to start nosing up), a tendency to tear itself apart on takeoff due to excessive Q, and that nasty un-controllable pitch up there at 20k/Mach 3. FAR also predicted the design would have Lß problems at 30k/Mach 4 - not enough fin/rudder, but I figured the design wouldn't make it past the Mw instability at 20k/Mach 3, so I didn't bother trying to fix it... What's involved in designing a good tail-less delta, in general?

Went ahead and built a clone of your original craft this evening. There's some pretty hellacious Q right there at take-off; more than once I pulled up too hard and ripped the plane apart from the dynamic forces...and the plane has a very high takeoff speed, around 150 m/s. Level flight AoA for takeoff was 12 degrees. That's assuming I built the clone correctly. I did get it off the ground a few times - I just had to nurse it a bit to keep the Q survivable. Stability analyses showed a number in the red - Mw - at 20k / Mach 3, and the simulation studies showed the system to be unstable at that point. Mw in the red means pitch-up behavior, and it's been established the plane has poor pitch authority. That's why you were going into the stall.

What others have said is true. There's also the fact that even if asteroids in KSP had gravity, it'd be so negligible that it wouldn't be worth simulating. Take the case of a class E asteroid; they max out at a mass of about 3800 tonnes (3.8*10^6 kg) and are about 30 meters across. Surface gravity can be derived from Newton's Law of Gravitation, which states g = GM/r^2, where G is the gravitational constant of the universe, M is the mass and r is the radius (we'll treat the rock as a sphere even though it's procedurally generated and never perfectly spherical). 30 meters across means a radius of roughly 15 meters. Given those values, the surface gravity - and mind you, this is for the largest rock that KSP generates - is: g = GM/r^2 = (6.67*10^-11 * 3.8*10^6) / 15^2 = .000001126 Which is about 1/43,500 the surface gravity of Gilly, if I've done the math right.

Yeah - my guess is the CoM shift. I'm also wondering if I really should've suggested canards; as a rule they're destabilizing surfaces. Might try a smaller part there - like an AV-R8 winglet, perhaps - something that won't throw the CoL quite so far forward.

I can't see your craft at the moment; I think imgur is having a bad hair day, because it's not just you... Don't know if it's a "normal thing" or not. What is the FAR stability analyses telling you? Check these benchmark values: 0k/Mach 0.35, 5k/Mach 0.85, 10k/Mach 1, 15k/Mach 2, 20k/Mach 3, 30k/Mach 4. Any red numbers pop up there? If so, you'll need to do simulation studies to determine if it's something that needs to be fixed or if it's just going to be annoying. Might also check to see what the level-flight AoAs are doing at each of those benchmark levels. Get RCS Build Aid too - it's possible that your fuel load is making your CoM shift aft of your CoL at about the point you go into your stall. Wouldn't be able to say one way or another without being able to see the plane...

There isn't a true "bottom line" for TWR once you're in space, other than "is the craft capable of making the maneuver you need it to make". For a ship with a TWR as low as the one you've got, I'd suggest using the "periapsis kick technique" to raise your apoapsis in stages (basically, you keep your periapsis low - say between 70-72 km around Kerbin and make a couple of burns (say three to four minutes each) to gradually stair-step your apoapsis until it's where you want it. If you're trying to go interplanetary with a craft like that, you'll want to do your kicks in advance of your transfer window, of course. Gotta ask - how many engines do you have and how much mass are they pushing?

FAR eh? Well, dammit. If it'd been a stock aero craft I could've told you what was wrong with it - I'd have pointed you to DocMoriarty's KSP Space Plane Construction and Operation Guide (which would've confirmed everyone else's diagnosis of too much engine and not enough wing). As it is, I'm still learning the rules for FAR planes my own self. From what I've learned so far, you've definitely got more engine than you need; in general a TWR of between 0.55 and 0.75 at takeoff is plenty. Your pitch authority is also minimal; I might suggest a set of canards for this design, placed high on the cockpit. Take that advice with a grain of salt as I'm still learning; generally the pitching surfaces will have less effectiveness if they're placed in-line with the main wing. The rear wheels could stand to be moved forward a tad; finding the exact right spot for those things can be a pain (even in stock aero). Start with that and see where it gets you. Myself, I think I'll clone your design and see what I can learn from it... EDIT: Have you thought about installing RCS Build Aid? I'd highly recommend it for any spaceplane designer; it'll let you know how much (and where) your CoM will shift in flight. Also tells you how much torque you'll get from your engine placement and (as you might expect) greatly assists with the placement of RCS blocks. One other thing I've learned - and this in the stock game - is that when it comes to spaceplanes the little single-spurt RCS ports tend to work a lot better than the general RCS blocks. Just a suggestion there.

Right - so stick the Engine Nacelles back on in front of the engines and then re-balance the torque. Got it. Running a fair number of mods on a box with only 4 GB of memory, 2.9GHz Dual Core and an on-board card whose equivalent was high-tech about eight years ago. I'd upgrade my box (I want to be able to play Star Citizen when it comes out) but there's this annoying thing called "life" that refuses to stop happening.

I think tonight I'll ask you guys about how to build a proper rudder... Up until now I had been building a rudder/fin out of a moving wing, but y'all have been recommending building a T-tail or at least putting the tailplane up on the fin for a while now, and so I decided to give building a fin with a dedicated rudder a shot. This was my first try. Doesn't look right. Changes to the Bigger Waste of Time were otherwise reasonably successful; FAR greenlit the redesign at 30k/Mach 4 and the torque was down to a little over 1 kNm. Damn thing failed to reach orbit because (of all damn things) the engines overheated and exploded when it switched to rocket mode (missed it because the FAR flight data was covering that part of the screen). I was able to maintain control long enough to get the plane over land (1.2 degree grade grassland) and get the horizontal speed down to 80 m/s and vertical speed to -5 m/s; still lost the plane on landing. I usually equate engine overheat to having the engines trying to handle too much mass or there being too few heat-dissipating parts. Which one do y'all think it was?

Okay...so that set of calculations I made earlier? Yeah - forget it; you've got a reasonable booster just the way it is: Hell, I even modded the damn thing a bit for FAR - extra weight for the fins nosecones, see - and it still worked well.

Alright - new design: Let me guess what y'all's advice is going to be before you make it - 1) Fill in the gaps between the external tankage and the fuselage with wing connectors, both fore and aft. 2) Move the CoM forward. 3) Move the CoL a shade closer to the CoM. 4) Fix the funkiness with the entire tailplane assembly. Maybe increase the sweep of the tailplane or loose it entirely. 5) Reduce the wingspan and length of the tail because reasons. 6) It doesn't look enough like a plane because reasons. Make it look more like a plane. The MiG-21 doesn't count. This plane made it up to about 45k/1900 m/s in rocket mode before I lost control of it; plane broke up due to aerodynamic failure as I was attempting to recover from the resultant spin. I'm guessing that was the point at which the control surfaces could no longer control the inherent engine torque RCS Build Aid was telling me about. Ultimate hope for this design is eight tonnes in the cargo bay or thereabouts; this flight I was empty so it had a little more oomph to it. So - was I close? Yes, I was being facetious on #5 and #6; call that a bit of frustration creeping in...