Payload Performance
Launch Vehicle maximum payload capacity to 100x100 LKO in JNSQ, assuming approx 5k dV required. These numbers are rough estimates and intended to aid LV selection for a given payload. In some cases, higher payload performance may be achieved by adjusting thrust or fuel loads in the VAB.
Several rockets are designed to achieve high orbit or planetary departure and have very low TWR in the upper stage. While the 'true' payload to orbit should therefore include the mass of the upper stage, I prefer to know what I can actually put on it. A good example is Titan IVB, which has a better capacity to Low Orbit if you remove the upper stage.
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By Zorg (if you have any problems with this section please ping me when opening an issue)
This is a work in progress section that will aim to go into deeper detail regarding the payload performance of various BDB launch vehicles. It will also attempt to explain some of the issues that arise due to how a launch vehicle is optimised for a certain type of mission.
The performance figures are for the JNSQ planet pack which is 2.7x the size of stock Kerbin or roughly 1/4 the scale of the real world. Performance in rescaled 2.5x Stock System or 2.5x KSRSS will be similar but slightly different.
It should also be noted that the data below is for vehicles being flown realistically with limited ignitions, ullage requirements and no throttling where the real engines did not have the capability. If you do not play with such restrictions you may find have more flexibility to find more optimal results but this guide covers the most difficult scenario. The figures should be relevant and helpful regardless.
Most of the results were achieved with single burn to orbit trajectories driven by MechJev PVG guidance or hand flown in some cases. Launches are from KSC at 0 degrees inclination.
The performance figures should be taken as a rough guide. Depending on how you fly and other conditions, you may do better or worse.
Notionally the official Dv chart for JNSQ states 4900 m/s of Vacuum Dv to achieve low orbit. This is usually a little optimistic unless you can fly really well and have very high thrust stages. 5000 to 5200 m/s may be more typical depending on various factors. However, you may find that with some rockets, once you attach a payload that maxes out your capacity low orbit, you find yourself unable to make orbit despite the VAB showing your rocket had the required 5200 m/s or so.
Many Rockets in BDB have upper stages with rather low thrust since they are actually optimised to send payload to high orbits where the lighter weight and higher efficiency of such engines are beneficial. The longer burn times that result can cause problems when attempting to max out performance to low orbit instead. In order to avoid falling back into the atmosphere you may have to "Loft" meaning aiming for an Apoapsis higher than your target orbit and end up circularizing after you pass the apoapsis since the burn is so long. Mechjeb PVG can sometimes loft and sometimes fail to do so. Lofting while flying by hand can be a little tricky if trying to squeeze every bit of performance out of the vehicle as there is the danger you might still fall back into the atmosphere.
However such stages usually will perform extremely well when sending payloads further away. And the further you're sending it, the more the benefit of the stage is realised. Thus we need to look at upper stages and vehicle configurations in terms of both thrust and efficiency to see which is more suitable for the tast.
We shall look at the example of the Titan IV-B first to see a clear example the difference various types of upper stages make (or lack thereof).
- LKO - The launch vehicles absolute max capability to place a payload in a Low Kerbin Orbit of 100km x100km. Where lofting is needed this may end up more eccentric by the time you're done. 140x100km etc.
- GTO - A transfer orbit that puts the apoapsis at Geosynchronous altitude and periapsis at low orbit. 8,986.1 x 100km (JNSQ). This may be done as part of a continuous burn starting from launch or by first establishing a parking orbit and then doing the transfer burn. The latter is more typical in real life. The former would only be done when you have limited ignitions. The final circularisation will be done by the payload.
- GEO - These figure show the capability to insert into Geosynchronous orbit directly. Thus the launch vehicle will do both the GTO transfer and final circularisation.
- TMI - Trans Munar Injection. The capacity to send a payload from LKO to a hohmann transfer to the Mun (JNSQ).
| Launch Vehicle | LKO 100 X 100km | GTO 100 x 8968.1km | GEO 8968.1 x 8968.1km | TMI |
|---|---|---|---|---|
| Titan IV-B No Upper* | 18.1t | 8t | ||
| Titan IV-B Centaur** | 18.7t^ | 11.7t | 8t | |
| Titan IV-B IUS*** | 9.5t | |||
| IUS (from 100x100km start)**** | n/a | 3t | 1.4t |
*Here we see something interesting straight away. A Titan IV-B with no upper stage (beyond the LR91 powered second stage), achieves roughly 18 tons to low kerbin orbit (LKO). However, despite adding the seemingly highly capable Centaur T final stage, in practice you can barely achieve more performance when going to 100x100 LKO. The added mass of the Centaur means you get less performance from the stages below and when it comes time for the Centaur to perform, it is of such low thrust you have to loft (losing performance) and burn radially up as well to try to keep the apoapsis up and avoid falling into the atmo. Thus the real world Titan IV also delivered large payloads to low earth orbit with no upper stage as further low thrust stages were not worthwhile.
** So what good is the Centaur? As noted earlier in Launch Vehicle Optimisation, the further we try to take a high efficiency upper stage, the more performance you get out of it relatively speaking. A Titan 4B with Centaur can deliver 3.7 tons more to a Geosynchronous Transfer Orbit and if you use the Titan 4B Centaur for a direct Geosynchronous orbit, the payload is as much as Titan 4B (no upper stage) can just to a transfer orbit. Note that we have only included the Centaur to LKO figure here as a point of comparison. In most cases where this is really not worthwhile, this page will exclude the figure.
*** IUS consists of 2 solid stages, Orbus 21 and Orbus 6E. In this case we are showing the capability of the vehicle where the LR91 2nd stage burns to depletion during the initial launch establishing a highly eccentric parking orbit (something like 100x2000km) and then the Orbus 21 performs the final burn to put the payload into GTO. Since the Orbus 6E cannot circularise a payload of this size it is excluded from these calculations. You may consider its mass part of the payload as something that helps circularise partly
**** The former case would not be very typical IRL. Usually The IUS would be placed in a fairly low parking orbit and it would perform the transfer. IUS can send 3tons (including the Orbus 6E mass) from low orbit to GTO using the Orbus 21 stage. However keep in mind The Orbus 6E can only fully circularise 1.4 tons of final payload at GEO.
^ Lofted trajectory. Not circular 100km x 100km.
| Launch Vehicle | LKO 100 X 100km | GTO 100 x 8968.1km | GEO 8968.1 x 8968.1km | TMI |
|---|---|---|---|---|
| Titan CT3 (No Upper) | 13.4t | |||
| Titan CT3 (TOS) | 13.5t | 6.8t | 4t | |
| Titan 34D (no upper) | 13.1t | 4t | ||
| Titan 34D Transtage | 6.4t | 4t | ||
| Titan III-E Centaur | 12t | 7.2t | 4.9t | |
| Titan III 23D (no upper) | 11.8t | |||
| Titan III 23C Transtage | 5.8t | 3.4t |
| Launch Vehicle | LKO 100 X 100km | GTO 100 x 8968.1km | GEO 8968.1 x 8968.1km | TMI |
|---|---|---|---|---|
| Delta IV Heavy | 21.5t* | 11.6t** | ||
| Delta IV Heavy 2xUS *** | 24.7t | |||
| Delta IV Heavy Plus **** | 39.3t | 25.5t |
*Delta IV Heavy due to its low TWR upper stage had to loft to a 140km target in PVG guidance for the LKO attempt. During the ascent it went all the way to 170km plus before sinking eventually to a 140x100km orbit during its 15 minute burn.
**Getting into LKO initially for the GTO attempt is easier but even at 2.7x scale it is only a quarter of the real world scale and with an 11 minute GTO burn required, cosine losses make this difficult and complicated to do optimally. (cosine losses are due to having to burn away from your desired vector which in this case caused by the small size of the planet and the long burn required). Starting your GTO burn from a higher initial parking orbit could be helpful to make it easier to manage but this is less efficient as well
*** One of many proposed upgrades to Delta IV, the twin RL10B2 upper stage despite the added dry mass achieves a notable increase in payload to LKO due to double the thrust and it does so relatively comfortably with only a minor loft to 120km. Final orbit 120x100km
**** Just for fun, this one consists of nearly all the upgrades proposed for Delta IV heavy but never implemented. Including 6x GEM 60 boosters, twin RL10B2 engine upper stage, regenerative RS68K main engines and propellant crossfeed. 120km slightly lofted trajectory for both LKO and GTO.