CBase

Members
  • Content Count

    128
  • Joined

  • Last visited

Community Reputation

84 Excellent

About CBase

  • Rank
    Spacecraft Engineer

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  1. I do have Breaking Ground DLC for the robotic parts, just not Making History. It didn't appeal me somehow. Actually what is recovery friendly on Twin-Boar ? I would always consider a Main Sail engine with 2 Jumbo tanks and small control wings more recovery friendly as the performance across all altitudes is better. Personally I added fixed and recovered 1.25m tank and engines to scale performance until I unlocked bigger diameters.
  2. Unfortunately I do not have Making History to check this specific design. I did rebuild it closely with some SpaceY boosters and a weight at top, but at this small dV differences even small build changes make too much difference. But on the topic of this thread about optimal TWR it is a nice example that the answer is not purely to look at TWR and indeed the added hammer booster only barely help to achieve orbit: TWR ~2 is high enough so you are not loosing a lot of efficiency to gravity and going more shallow with the extra thrust is eaten by increased drag, therefore your solution to throttle the main stage early on makes sense. However it might be more sensible to look at your vacuum Isp during ascent beyond 15km altitude: The twin boar engine is sub optimal compared to a main sail or pure vacuum engines. The major dV proportion for gaining orbit is mostly at low pressure. So the second stage or SSTO stage should always have an engine with okay to good vacuum Isp. Then you add either a first stage or boosters to get high enough TWR at launch pad. For SSTO you add engines until you reach at least 1.3 TWR. A true two staged designed will propably aim for higher TWR at launchpad and lower TWR with the vacuum stage, like RizzoTheRat posted.
  3. Indeed comparing the profile from SAOCOM-1B with CRS-20 is interesting: First off: CRS-20 was heavier MaxQ is 5s later than for SAOCOM-1B and speed still around 100 km/h less, but for both at ~14.5 km altitude. However at MECO T+02:27 CRS-20 is only 2s behind and with 5860 km/h @ 67 km altitude compared to SAOCOM-1B MECO at T+02:25 with 5940 km/h @ 78.7 km altitude. So steeper climb seems mostly related to land return, as the first stage should not pick up too much horizontal speed. The lighter payload then just raises the altitude. I always thouhht that the dogleg is pretty early as the faster you go, the more dV is spent for a plane change.
  4. Unfortunately the commentator did not go into the pretty obivous differences in ascent profile: Normally SECO and Landing burn are very close together, but this time minutes apart. Actually even at Max-Q and MECO you could see that SAOCOM-1B was on a much steeper trajectory which requires longer second engine burn. MECO this time was at T+2:25 with ~ 5940 km/h @78km altitude, last starlink for comparison MECO was at T+2:41 with ~7880 km/h @71.5 km altitude. It would be nice to know how much of this velocity difference is due to boost back to CC and which is attributed to the dog leg.
  5. Interesting, could you share the craft and a rough description of the slower profile ? If it isn't a wardrope without a fairing that you try to put in space, I am curious to see it. Absolutely true, I am only flying SSTO rockets, so my thoughts on optimal TWR are focused on these. If you consider booster an option, "Moar Booster" for short duration is definitely improving performance until you hit ~ mach 1 when it is aerodynamic drag that is main factor. SSTO and other liquid fuel rockets do waste a lot of fuel in first seconds when vertically fighting gravity. But just like Elon Musk recently twittered: even loosing something like 40% payload pays off after 3 launches if you recycle.
  6. No, since higher TWR => more engines => more dry weight => more overall weight => less efficient And on very shallow profiles skin overheating is another issue. Actually I even modified MechJeb to better throttle down to avoid overheating for very aggressive profiles. But without shallow ascent profile the high TWR looses advantages as you simply coast earlier. On real scale earth escape velocity is a lot higher, ascent profile very different and therefore aerodynamic losses bigger, so real rockets do throttle down and do not benefit from a high TWR. But for Kerbin it hardly matters.
  7. Actually I don't think so. For the launch profile challegene I did quite some test if limiting TWR on high drag phases helps to get more efficient. It didn't, the gravitional loss was bigger than aerodynamic gain. My guess after this challegene is that dry weigth impact from number of engines is main factor. At least if you do SSTO for recovery. With expandable booster very high TWR for short duration might pay off.
  8. Actually the rule is about center of gravity vector change and less about prograde which would land you in different target orbit than planned. So yes the rule of thumb only applies to mostly circular orbits. At highly eccentric orbits - like the one in video after a very straight ascent - the craft spends considerable time near the apoapsis. Therefore gravity direction hardly changes and you can plan longer burns. I did this kind of LKO ascents with extreme non aerodynamic payloads, you can do it in a single symmetric burn. You do loose of course some efficiency when node marker and prograde are not aligned, but as a starter that is perfectly fine. His problem is really about starting 7s too late and following the node marker to fine tune orbit at wrong point in orbit.
  9. Thank you for that video ! Actually if you look close at the start of the burn (around 0:33s in video) you see that estimated burn duration is 2 minutes however only 53s are left until apoapsis. Yes this is pretty close, but hey this is rocket science ! And at 2:38 in the video you reached an 80x58 km (sub-)orbit, which is actually pretty good . Just after it you messed it a little up by trying to correct right now by following the manoveur target. If you are around 10 m/s dV left, but already 1 minute behinde the node time, corrections on the original nodes (as displayed by the moving blue cross) are not the best choice. Remember fixing periapsis is best done at next apoapsis. So accept the node as executed once it is that close and rather create an new node to correct your orbit. This might even mean that you coast quarter or half around Kerbin. But hey did you watch the Crew Demo return on this weekend ? They streamed 11 hours while they were constantly doing small corrections on multiple orbits. This is traveling in space. Luckily Kerbal space has some time warp to skip long coasts Oh and a minor hint for your next ascent: 1400 m/s circulization burn indicates you did ascend mostly straight up. If you turn little more sideways early in ascent you burn more horizontal and need less time to circulize. Which makes it easier to time. But really only do very small corrections, you will see that drag and changed ascent curve change big even for small turns early.
  10. Maneuver nodes show where you end, if you apply all forces instantaneous. This means you would need a incredible powerful engine to burn only fractions of a second. This is of course not possible. Normal burn durations are several seconds up to 1-2 minutes. If you need a longer burn for interplanetary travel you might want to split it into 2 or 3 nodes. As a rule of thumb you should never plan to burn longer than 1/16th of your orbital period. And it is important to properly time your burn. If you start when the count down is at 0s, which means you reached the node position, in average all of your forces are behind the planned position. But you want them in average to be at the node. So if you have a burn duration of 60s as example, you want to start 30s before the node and stop 30s after the node. This way your average actual forces align with the planned node and you should end in approximately right orbit.
  11. I just posted some pre releases of current state for MechJeb and Trajectories. I only tested it with KSP 1.9, but should work. Please note: These are private dev releases with some unreviewed changes ! Don't bother the official Trajectories team or bug report if you have any issues. https://github.com/cbase2/KSPTrajectories/releases/tag/2.2.2.0_MJ https://github.com/cbase2/MechJeb2/releases/tag/2.9.0.0_Traj
  12. This is my typical 1st stage with deployed wings for reentry: My additions to trajectories for this: include deploy angle for control surface calculations fix orientation bug with up / down being wrong on retrograde entries, which affects tilted wings changed reentry altitudes and sliders to allow better control between -10° AoA (low altitude) and -60° AoA (reentry) Kind of. I did add some Trajectories API code and a major rewrite of MechJeb autoland to follow and adjust Trajectories AoA with the same goal. With a drag parachute at final approach the system is slow and stable enough to hit always the runway and often even launchpad with less than 10m difference. Without any parachute difference is still around 100m, so no pure SpaceX like landings. The rocket is simply to fast, Trajectory predictions not exact enough and since it should work a vast variety of rockets fine tuning control code for one design does not pay off.
  13. The size of Kerbin and therefore required skill level to reach orbit is the fact I really admire most about KSP 1. And the decision to publish the API and embrace plugins was probably key to it's long running success. Without it would have been a great game, but never received the community and long term motivation of all players here.
  14. I would guess that load and revert do set up the universe differently, and since any choice of origin is arbitrary, does it matter ? Compare rootPart.rb.worldCenterOfMass and vessel.mainBody.position with vessel getWorldPos3D: If all change, simply the origin was choosen different.
  15. Actually you should plan as well for differences in surface altitude. Predicting the true landing position for the suicide burn is way harder than the speed timing. But without perfect position you do not have perfect altitude... So as Laie wanted to express: Add some safety margin for things that you do can not know exactly when starting the burn. Kerbin has atmosphere, the Mun not. The difference is huge even without parachutes, even from 1000m down. No single equation will get you landed on both. Unfortunately the equations are not that easy even for the Mun as the problem is 2d: You need to kill horizontal speed and gain vertical speed, both are connected by the vehicle angle which constantly changes. However you are right, basically it is a rocket equation reverse, so iterative solutions or known solvers might help. I did recently touch this topic by MechJebs Landing scripts, but only changed final descent, which kicks in once your are falling mostly straight.