Atlas Payload Performance
The atlas launch vehicle uses a stage and half design so the process to calculate its Δv capability is more complicated than for a standard launch vehicle like the Titan II. A stage and half rocket has two flight phases, the booster phase (before the booster skirt jettison) and the sustainer phase. The Δv budget depends on the payload mass AND the propellant burned by the sustainer phase while in standard rockets the Δv capability depends only on the payload mass. The Δv budget can also be calculated as a function of Thrust-to-weight ratio (abbreviated as TWR) at booster skirt jettison to better integrate with the skirt autojettison module currently used by the Atlas booster skirt.
First let's understand the information in this chart.
- On the horizontal axis we have the payload mass in tons (same unit as ksp). It includes everything that goes on top of the rocket (spacecraft, upper stage, fairing, decoupler and so on).
- On the vertical axis we have the TWR at booster skirt jettison in Gees (1 Gee = 9.8m/s²).
- The horizontal dotted lines show the TWR after skirt jettison, also in Gees. This is important to know because it affects the rocket's stability..
- The two solid lines are the flight envelope boundaries, the bottom one represents a scenario where the skirt is jettisoned at launch (full sustainer phase) while the top one represents a scenario without skirt jettison (full booster scenario).
The interaction between flight envelope limits and required TWR after skirt jettison allow us to classify the rocket configuration (payload mass and TWR at skirt jettison) into 5 categories:
- Late Skirt Separation: TWR too high due to long booster phase, this results in Δv losses as the booster phase Isp is smaller than the sustainer phase. Consider using a smaller rocket.
- Good to go and Ideal configuration: booster and sustainer phases are well balanced, a steeper launch trajectory will benefit from having a higher TWR at a Δv cost.
- Separation at High altitude: longer sustainer phase increasing the Δv budget due to its higher Isp while keeping the TWR after skirt jettison at an acceptable value. Atlas II benefits the most from this configuration due to its long tanks.
- Early skirt separation: booster phase too short, this will induce larger gravity losses and might degrade rocket stability. It can be used for launching a small payload to a high energy orbit as jettisoning the skirt at high altitude will mitigate its negative effects on the rocket stability.
- TWR too low after separation: booster phase extremely short, avoid it at all costs with MOAR boosters™ or using a larger rocket.
You can find a detailed manual describing how to read these maps using a graphic method in the following link.
It is recommended to read the Atlas performance manual before using the Δv maps. The manual also describes the mathematical procedures used to calculate the Δv budget. It is not necessary to understand the mathematics used to create the maps, but some users might want to know it. (It’s not rocket surgery)
the Δv maps all have quite big dimensions (9000 x 3000), so it is recommended to open them in a new tab.
The first Atlases were derived directly from ICBMs. Reduce the booster engines thrust to 90%
Atlas-B Construction: B T
Atlas-C Construction: B S C
Atlas-D Construction: B M T
LV-3A & LV3-B Construction: B M T
LV-3C Construction: B M M
LV3-D Construction: B M C(1.5m conical adapter)
SLV-3 Construction: B M T
SLV-3A Construction: B M S T
SLV-3B, C and D Construction: B M M
SLV-3E Construction: B M S C
Atlas E/F Construction: B M
Atlas I Construction: B M M S
Atlas II Atlas II is recommended for high energy orbits. Atlas II has a very narrow payload range due to its large amount of propellant (LFO). Construction: B M M M
Atlas IIAS Construction: B M M M + 4x Dioscuri-4A (Castor-IVA)
Atlas IIA2 Proposed variant with 2 SRBs only Construction: B M M M + 2x Dioscuri-4A (Castor-IVA)
Atlas III This one doesn't use a stage and half design, so KSP can calculate its delta V accurately.