The Rocket & Payload Selections sheet is the main sheet of CEPE. It allows users to select rocket components, orbit/trajectory parameters and then to derive the payload for the corresponding case.

The description of the content of each Rocket & Payload Selections sheet's cell is on the left side of the cell while the units are on the right side. The cells of the Rocket & Payload Selections sheet are color-coded. The green cells are the parameters that can be changed by the users. The yellow, grey and dark/light orange cells are calculated values and the equations of those cells should not be changed unless you know what you are doing. The grey cells are periods of time. For the stages they are burn times obtained with the equation: (ISP x propellant mass)/ Thrust. The orange cells are delta V or speed calculations which are described in attached notes to the cells. The red cell U25 is the surplus or deficit delta V resulting from parameters entered in green cells including the dark green cells N6 and N7 which will ultimately contain the sought estimated gross payload. Cell N3 shows the net payload calculated using the margin entered in N4.

2The information on the Rocket & Payload Selections sheet is regrouped by rocket stages (SRBs in box B24 to B31, core stage(s) in box E24 to E31, upper stage in box K24 to K31). The data from the SRBs and core stage(s) are then regrouped/separated into a first stage called "SRBs + core stage initial" (box B40 to B46) and a second stage called "Core stage left over" (box E33 to E38). For calculation purpose, the first stage will be fully expended before the ignition of the second stage. From these two modified stages and the upper stage, CEPE derives 3 virtual stages which parameters are in boxes Q40 to Q46, Q33 to Q38 and Q26 to Q31.

The rocket engine layout shown in cell T1 is a 3 digit number indicating the number of SRBs (hundreds), the number of main engines (tens) and number of upper stage engines (units). The rocket delta V value in cell T2 shows the total delta V produced by all the engines of your rocket. This value must overcome air drag losses (T3) and gravity losses (T4), which are individually shown in dark orange spreadsheet cells, and still be big enough to reach the desired orbit/trajectory. The graphic in the upper right corner, which is there for information, shows of the rocket speed versus time since take off.


3First you must select your rocket components in boxes B4 to B9, E4 to E22, K4 to K22 as well as in cells H16 and N16. For example, an Atlas V 542 rocket should have 4 in cell B4, 1 in cell E4, 1 in cell E16, 1 in cell K4, 2 in cell K16 and 5 in cell H16. It should be noted that the boxes F16 to F22 and L16 to L22 show for information the standard type/number of engines for selected core/upper stages. If you have the mass (kg) of the LAS, fairing, interstage and/or payload adapter they need to be entered respectively in cells H4, H19, H30 and/or N19. If you don't have those masses, CEPE will use calculated generic masses (cells H18, H29 and N18).

To simulate the Space Shuttle you must enter 2 in cell B7 for the SRBs and 1 in cell K15 for the orbiter. CEPE will then automatically enter 1 in cell E15 to account for the ET. Although the SSMEs are fitted on the orbiter (which is considered an upper stage by CEPE) they must be entered as part of the core stage data (3 in cell E18). For the upper stage engines, 2 OMEs must be entered in cell K18.

The next step is to enter the propellant % filled for the core stage and upper stage in cells E1 and K1. Unless you want a stage to be partially filled (This could be the case to test a Direct J-110 rocket for example), enter 99 in cell E1 and 100 in cell K1. The 1 % loss in the main core stage is there to account for main engine(s) start up expended propellant before the rocket leaves the launch pad.

Next, you must enter the initial and final inclinations (degrees) in cells T48 and T20. For flights to the ISS, these cells should contain 51.6 degrees. The launch site latitude (degrees) must be entered in cell N46 to permit calculation of the launch site level compared to the equator radius (earth is not a perfect sphere) and earth rotation induced speed at the launch site. For KSC, cell N46 should contain 28.5 degrees. Your selected perigee and apogee values (km) must be entered in cells Q22 and Q21. However, if you are doing a flight to EML1, EML2, SEL2, moon or Mars, you can enter 296 km in both Q21 and Q22 as this would be a normal parking orbit before TEML1I, TEML2I, TSEL2I, TLI or TMI burn. Then enter 1 in cell T12, T13, T14, T15 or T16 as required. This would give you a slow/propellant economical trajectory ideal for cargo flights. But, if you are doing a manned flight, consider entering 1 in cell T11 to reduce the transit time for the astronauts. The table below shows the nominal delta V requirements assumed by CEPE depending of the destination. These values are slightly modified by CEPE to account for LEO altitude (including orbit decay) at time of TxI burn.

Destination: EML1 EML2 SEL2 moon Mars
Unmanned Flight 3070 m/s 3120 m/s 3198 m/s 3080 m/s 3700 m/s
Manned Flight 3100 m/s 3180 m/s 3236 m/s 3150 m/s 4300 m/s

The Destination data box Q14 to Q19 shows EDS information post LEO insertion. The "Initial T/W ratio" value (cell Q18) is also used to calculate the gravity losses during TxI burn (cell T19). Cell U22 show the same type of gravity losses (caused by weak T/W ratio burns) during earth orbit burn to reach the selected apogee. Based on the destination selected in box T11 to T16, cells U18 and U17 will show the expected transit time and delta V required at destination for orbit insertion. These two cells are only there for information as it is expected that the burn will be accomplished by the payload and not the EDS. Also for information, the calculated C3 value of the selected trajectory is shown in cell T17.

For EML1, EML2, SEL2, moon or Mars destionation, CEPE allows you to add extra payload in LEO (cell Q6). This cell can be used for 1.5 and 2-launch missions requiring docking in LEO of a payload to the EDS stack. The cell Q7 will then show the propellant mass required in LEO to insert the added payload to the selected trajectory. The cell Q8 shows this propellant equivalent mass after TxI burn. The Q8 value is added to cell Q29 (end mass+CEV prop) in order to reduce appropriately the EDS propellant available (cell Q30) to propel the payload launched from earth (cells N6/N7). Cell Q9 allows to enter loss of altitude while in LEO to simulate orbit decay over time. This value is used to increase the delta V requirement shown in cell T18.

The final step is to find the estimated payload in cells N6 and/or N7 that will leave the smallest surplus delta V in cell U25. If after guessing a payload value, the value in cell U25 is negative, you need to reduce your payload entered in cells N6 and/or N7. If the value in cell U25 is positive, enter higher payload values until the value in cell U25 becomes negative. There is no point to go below 100 kg increments as this is already well beyond the accuracy of CEPE. So, for example if you obtain a small surplus delta V with a payload of 12,700 kg and a negative delta V value at 12,800 kg, the CEPE estimated payload is 12,700 kg.


The downloaded Rocket & Payload Selections sheet shows data for LEO flight of the original CaLV (LV27.3) with 5 SSME engines as described in the NASA ESAS final rpt Nov 05 p. 431. According to CEPE, a 147,200 kg gross payload (cell N6) leaves a surplus delta V of 1.97 m/s (cell U25). This is the maximum payload that can be entered in cell N6 which leaves a surplus delta V.

1Let say we want to know what would be the payload of the same rocket if it was equipped with 6 SSME engines instead of 5. First you have to enter 6 in cell E22 to account for the extra engine. This increases the surplus delta V in cell U25 to 110.13 m/s. If you then increase the gross payload in cell N6 to 152,000 kg, the surplus delta V is reduced to 9.78 m/s. Now enter 153,000 kg in cell N6 and you will see the surplus delta V become negative at -10.80 m/s. If you reduce the payload to 152,500 kg, the delta V deficit shrinks to -0.52 m/s. Finally, entering a payload of 152,400 kg, leaves a small delta V surplus (1.54 m/s). Therefore, according to CEPE, adding a sixth SSME engine on the original ESAS CaLV rocket would increase its payload capability to LEO by 5,200 kg (152,400-147,200).


Instead of entering your guessed crew/cargo and/or propellant values in cells N6 and N7, it is now possible to use macros (if enabled) to calculate the payload values for you. Ctrl-c can be used to calculate the crew/cargo gross payload in cell N6. Ctrl-p can be used to calculate the the propellant gross payload in cell N7. Ctrl-z is also available to zero both cells N6 and N7.