Hardkore Heaven Episode 1: KSP Release Done Technical
This page is companion to the video located here: https://www.youtube.com/watch?v=3D9p7IaqQYo
Scott Manley: "We're getting low, better press the space button to deploy the parachute." CMBG Part 3
Done Technical: Right click on the parachute in the Vehicle Assembly Building and adjust the "Min Pressure" to control the initial deployment altitude above sea level.
|0.04||20 004||Default setting; not recommended|
|0.20||9 013||Orbital entries, may shred on steep entries|
|0.40||5 131||High hops from KSC; avoid the mountains|
|0.50||3 882||Altitude boost phase stage recovery|
Solid Motor Thrust Limiters
Altitude Boost Phase: It is efficient and safe to accelerate as quickly as possible off the launch pad to the maximum safe speed the vehicle can handle the dynamic pressure at, or the speed at which drag acceleration is 1g. The former is about 300m/s. Efficient because high speed increases the amount of altitude you gain per m/s of delta-v during the sustain phase. Safe because of the need to get wind over your fins quickly to keep pointed upward (a stipulation of nearly every amateur rocket safety code in real life.) Design the delta-v of your first stage to have between 300m/s and 500m/s. Once there, divide the total thrust of motors and engines operating in the first stage by the lift-off mass of the vehicle, giving the thrust induced acceleration. If it is between 25m/s^2 and 40m/s^2, you should be okay. If the delta-v of the first stage is more than about 400m/s, it might be a good idea to tune it down by right-clicking on the motor in the Vehicle Assembly Building and adjusting the Thrust Limiter. The value to adjust it to is given by multiplying the mass of the vehicle by 25 (the desired acceleration of the altitude boost phase) and dividing it by the amount of thrust delivered by the motors (remember to use the sea level thrust numbers.) This prevents parachute burnup accidents.
Altitude Sustain Phase: The most efficient acceleration to maintain during the altitude sustain phase is 18m/s^2 because it will keep you just below your terminal speed in vertical flight if you have an aerodynamically dirty enough vehicle to approach it (this was almost unavoidable prior to Release). It is also very unlikely that you will tear apart or burn up an aerodynamically clean vehicle at this acceleration level. If you have the ability to control the vehicle at low speeds, it is quite alright to lift off the pad with this acceleration level, or even much lower. Bear in mind that you will lose quite a bit of delta-v to gravity if lifting off at lower acceleration. The minimum acceleration for the altitude phase is 12m/s^2. Any lower than this, and you are likely to either stick to the pad with the ground glue bug, or drift off the zenith prior to getting enough speed for your fins to do any good.
There are three methods available for recovering stages for reuse in Kerbal Space Program. Here, they are organized in order of performance each method requires/allows the jettisoned stage to reach.
Physics Range Fallback: 0-400m/s. This method requires the stage to land back on the launch pad before the vehicle leaves its physics range of 22.5km (it used to be 2500m, which made this method nigh impossible.) Doing this requires the stage to return to the ground very quickly, asking for relatively late parachute deployment events. Set the Min Pressure to 0.5, the maximum value available, and the Altitude to 300m (this will not work with Mk16 parachutes, which are garbage anyway.
Formation Fallback: All delta-v. One might wonder why there is a need to detach the last propulsion system from the spacecraft after it has been depleted, since the spacecraft and stage will be landing in the same area. The answer is that long vehicles have a tendency to fall over and break when they land upright, and the things most likely to break are the parts you most want to keep (the ones that contain the science and the crew.) They can be recovered by whaleboys with lassos while still in the vertical position (out-of-character, this means hitting the Recovery button before the stage falls over), but if it still falls over, it'll be relatively cheap empty tanks near the top that will break instead of the expensive engine.
Downrange Recovery: 2100-5000m/s. This method counts on the stage being recovered to reach high enough altitude and speed that will not return to an altitude of 25km (23026m to be precise) before you can leave the payload and return to fly it back to the surface. The lower limit is incumbent upon reaching a speed of at least 1300m/s surface frame and having a high enough thrust upper stage and a high enough trajectory to get the upper vehicle out of the atmosphere and have enough time before it reaches its apoapsis to land the booster. The upper limit is about when the stage will escape the Hill Sphere of Kerbin into Kerbol orbit (an unlucky encounter with the Mun is negligibly less, and the difference puts it on such a long orbit that the stage will probably be obsolete long before it can land and be recovered.) I will have my own details in later episodes of Hardkore Heaven, but until then, you can tide yourselves over with this really old video by Scott Manley: Raven by Edgar Allen Probe
An unsuccessful attempt to roll a Mk1 Command Pod out of the Launch Pad biome.
A Mk1 Command Pod launched by an RT-5 profiled for altitude sustain phase and recovered using a Mk16 Parachute. Reached 5682m altitude and 286.4m/s speed. Loaded mass 2.10t in 3 parts and delta-v 650.2m/s in vacuum. Flown once by Bob Kerman; the flight was perfect.
A two-stage, three section vehicle divided by TR-18A decouplers unlocked in Engineering 101. The first stage was a full-thrust RT-5 for the altitude boost phase, and a profiled RT-10 (unlocked in Basic Rocketry) for the altitude sustain phase. Mass was 5.20t in 6 parts, with a total of 1569.1m/s of vacuum delta-v (229.2m/s in the first stage and 1339.9m/s) in the second stage. It was flown once by Jebediah Kerman in a perfect flight which reached 592.4m/s and 23489m altitude. The immediate separation and inversion of the Command Pod was deliberate and normal to improve crew comfort by avoiding -X acceleration and improve performance and safety by ditching the low-beta empty RT-10. This flight was the first to reach high altitude and the first to land outside the Launch Pad biome.
This seven part vehicle was the first to feature dedicated science instruments and was flown by Bob Kerman for this reason. It had a loaded mass of 4.25t, vacuum delta-v 1197.2m/s. The flight recovered 38 science points and featured Hardkore Heaven's first serious anomaly: the booster overtook the spacecraft because the spacecraft decoupler and parachute were on the same staging command. The error was not corrected as the vehicle only flew once.
This two stage vehicle is 16.09t and 28 parts, making it one of the most complex and highest performing vehicles to date. The first stage was a BACC solid booster providing 579.3m/s of vacuum delta-v, followed by an LV-T45 engine fed by six FL-T200 bipropellant tanks providing 2857.7m/s of vacuum delta-v for a total of 3437.0m/s. The spacecraft separated from the formation-upright recovered second stage, and was the same type as that flown on Hopper 3. Valentina Kerman flew this vehicle above 70km with over 2 tonnes of propellant remaining. At the time, it was thought that the danger of the spacecraft colliding with the second stage after separation was small, but later events were to disprove this.
The second version was the suborbital tourism vehicle at 17.08t and 19.9m long, getting close to the limit of the Level 1 Launch Pad. With the heavier spacecraft, the first stage provides 540.0m/s of vacuum delta-v followed by the second stage of 2624.5m/s of vacuum delta-v for a total of 3164.5m/s. The pilot rides in a standard command pod while the passenger rides in a new Mk1 Cockpit on top, selected less for its ruggedness and more for its transparent canopy to allow our valued passengers to view their surroundings during the brief flight. Pilot Jebediah Kerman, along with FAQ#Why_do_some_Kerbals_laugh_during_the_entire_mission.2C_while_others_panic_easily.3F unperturbable passenger Joedorf Kerman flew the maiden voyage. Both were briefly perturbed by the booster smashing its decoupler against Jebbers' command pod.
The main reason for Hopper 5 was to reduce operating costs by replacing the BACC first stage with an RT-10. The second reason was to increase safety by providing an off-loaded RT-5 to separate the spacecraft from the second stage (Sepratron I has not yet been unlocked.) This final stage provides 40.3m/s of delta-v, while the ascent system is reduced to 2711.3m/s (274.9m/s in the smaller RT-10 first stage and 2436.4m/s in the unchanged second stage.) The first flight was a nerve-wracking abort because I forgot the decoupler between the first and second stages. During this abort, the separation motor prevented a collision between the second stage and spacecraft, an event more likely to be fatal with so much propellant left in the second stage. The second flight with pilot Valentina and passenger Ralssa, was a complete success ended with a typical memory-out crash of KSP.