Welcome to CEPE. The CaLV/EELV Payload Estimator


Photo Credit : Ben Cooper / www.launchphotography.com/

With the current VSE initiative, some new rocket designs/modifications have been pursued by NASA (Ares I/Ares V) or proposed by individuals such as the Direct team. This has led to several postings on Internet forums about the merit/capability of various designs. For the non-experts, it is hard to assess the claims of these options with regard to payload capability. CEPE is a tool that should help you with that.

CEPE is for "armchair rocket scientists" who want to come up with their own rocket version or test proposed designs with their own variations such as adding an engine or more propellant in a rocket stage. It uses a spreadsheet which includes the stages characteristics of several existing/proposed designs. This allows the user to easily select rocket components and quickly figure out the resulting rocket payload. CEPE works on both MS Excel and Open Suite spreadsheet Calc available for free here

CEPE allows the use of existing SRBs, one or several (3 are required to simulate the Delta IV Heavy rocket) core stage(s) with selected engine(s), mass or size of an interstage adapter, an upper stage with selected engine(s), mass or size of payload fairing, mass or size of payload adapter, mass of a LAS and obviously a payload mass. This can be crew/cargo mass or useful propellant left in the rocket last stage. It should be noted that CEPE can also do Space Shuttle payload estimations. Propellant offloads for the core and upper stages are also possible. Finally, CEPE allows to add extra payload in LEO (such as the docking of the CEV for example) and orbit decay while waiting in LEO for EDS burn. From selected data, CEPE creates a virtual three stages rocket built as follows:

Rocket type SRB(s) + core stage + upper stage used No SRB No core stage No upper stage/EDS
First Virtual Stage SRB + core stage (take off to SRB burnout) core stage (take off to 70 seconds of flight) SRB alone (take off to SRB burnout) SRB + core stage (take off to SRB burnout)
Second Virtual Stage core stage (from SRB burnout to core stage burnout core stage (71 seconds of flight to core stage burnout) upper stage (from SRB burnout to one third of propellant used in upper stage) core stage (from SRB burnout to one third of core stage remaining propellant used)
Third Virtual Stage upper stage (from core stage burnout to upper stage ECO) upper stage (from core stage burnout to upper stage ECO) upper stage (from one third propellant used in upper stage to upper stage ECO) core stage (from one third of core stage remaining propellant used to MECO)
Examples Ares V Atlas V 401 Ares I Direct J-130

For sake of simplicity, the payload fairing is jettisoned on completion of the virtual 2nd stage burn. The LAS and interstage adapter are also jettisoned on completion of the virtual 2nd stage burn unless the rocket has no core stage such as the Ares I rocket. In these cases, the LAS and interstage adapter are jettisoned on completion of the virtual 1st stage burn. The LAS will also be jettisoned on completion of the virtual 1st stage burn if there is no upper stage such as the J-130 rocket. The payload adapter is kept until completion of the virtual 3rd stage burn.

Obviously, CEPE uses the rocket equation to calculate delta V of each virtual stage of the rocket. Vacuum ISP/thrust values are used for the 2nd and 3rd virtual stages. For the first virtual stage, the ISP/thrust values are derived using a combination of sea level and vacuum values. In addition, for rockets using SRBs and a core stage the ISP value is proportionally calculated (SRBs thrust vs core stage thrust) using the SRBs' ISP and core stage's ISP values. CEPE takes into account gravity losses (including those caused by weak T/W ratio after reaching LEO) and air drag losses using empirical equations. The launch site latitude, initial/final orbit inclinations, perigee, apogee and trajectory away from LEO (if used) are also used to calculate the final delta V required.

CEPE has three sheets with color-coded fields: Rocket & Payload Selections, Rocket Components and Payloads Comparisons. But the Rocket & Payload Selections is the only sheet you need to operate if you limit yourself in using pre-defined rocket components. It is easy to use and it takes less than a minute to figure out a valid estimated payload. Payload estimations can be obtained manually by entering directly your guessed crew/cargo mass and usefull propellant left-over or automatically using macros (Thanks Lee Jay for your provided macro codes!).

CEPE is only an estimator tool as it does not use some more complex parameters such as maxium G allowed, engines throttling up/down or flight to orbit profile. However, I find that the CEPE estimated payloads are within 10 % of the published payloads most of the time. So download the spreadsheet here and use it to test your favorite rocket and try to optimize it by adding more SRBs/engines, reducing propellant loaded or changing orbit parameters. If you have any question or idea to improve the tool, contact me at plafor@rogers.com . Good luck with your rocket estimating!