Launch Simulator

Working with flight data

Two different datasets come off the launch pages, and both are free to download with no account. The simulated flight is a second-by-second reconstruction of an ascent, exported from Data Mode as a telemetry table, a full data file or a ground track. The launch record is the real published data about a launch: the vehicle, its price and capacity, the individual booster's flight history, and where the launch sits in the running totals. This page shows how to open both in common software and what you can do with them, including for classroom and research projects.

The simulated flight

A reconstruction, not measured vehicle telemetry. Exported from Data Mode on the launch tracker or the simulator.

What each file contains

Two dynamic pressures, and why they differ

Dynamic pressure, q, is the aerodynamic load on a vehicle: q = ½ρv², where ρ is air density and v is speed. Its peak, Max-Q, is the most structurally demanding moment of an ascent. The export gives it two ways.

dynamic_pressure_pa uses a textbook atmosphere: density falling exponentially from 1.225 kg/m³ at sea level with an 8.5 km scale height, and the vehicle's ground speed standing in for its airspeed. It is available for every flight and is what most back-of-envelope calculations use.

dynamic_pressure_measured_pa uses the atmosphere actually forecast over that pad at that hour, and it corrects both halves of the formula:

• ρ comes from the real sounding. Density at each pressure level is computed from the ideal gas law using that level's own pressure and forecast temperature, then interpolated in log space between levels because density falls close to exponentially with height. On a real day this runs several percent away from the textbook value, and the gap is often widest at 9 to 13 km, which is exactly where Max-Q happens.
• v becomes airspeed, not ground speed. Dynamic pressure is defined against the air, so the wind at each altitude is subtracted from the vehicle's velocity as a vector. A headwind raises q and a tailwind lowers it, and neither shows up at all if you use ground speed. This is usually the larger of the two corrections: jet-stream winds of 40 to 50 m/s at 11 km are ordinary, against a vehicle doing 400 to 600 m/s there.

Across a sample of real launches the two figures differed by roughly -6% to +15%, in both directions depending on which way the vehicle was flying relative to the wind. Neither column is more "correct" in the abstract: the model column is reproducible and vehicle-independent, and the measured column is specific to the conditions on the day. Having both in the same file is the point.

Three things to know before using the measured column. Flight altitude is treated as height above the pad and offset by the pad's own elevation before the profile is read, which matters at a site like Jiuquan that sits about a kilometre up. The column is left blank above the top of the forecast profile (around 24 km) rather than extrapolated, so it stops where the data stops. And the ascent it is applied to is still a reconstruction, not measured vehicle telemetry, so it is a better answer to "what would this flight have felt" rather than a record of what it did feel.

The launch record

This one is not simulated. It is what the launch databases actually publish about a real launch, passed through unchanged apart from four figures we compute and label. Download it from the Launch record panel on the launch tracker after opening a launch.

Reading the columns

Two things are worth knowing before you compute anything with these.

An empty cell means the source has no value. Coverage is uneven and varies a lot by operator: across a typical feed, vehicle height and diameter are present on about 95% of launches, mass and thrust on about 90%, published price on about 70%, and booster serial numbers on about 40% (essentially all the ones that land and fly again). Nothing is interpolated, averaged from a similar vehicle, or defaulted to zero to fill a gap, so an empty cell is a real absence and should be treated as one rather than as a zero.

Four columns are computed by ISSC, not published by anyone. They are listed in the # header of every file:

• cost_per_kg_leo_usd_kg is launch_cost_usd / capacity_leo_kg. This is a best case: it assumes a completely full rocket, which almost no real mission is, and the price is a published list price rather than what any particular customer paid.
• vehicle_liftoff_twr is vehicle_thrust_kn * 1000 / (vehicle_launch_mass_t * 1000 * 9.80665), the thrust-to-weight ratio at the moment of lift-off. It has to be above 1 or the rocket does not leave the pad.
• vehicle_success_rate and agency_success_rate are successes divided by total attempts. Watch the denominator: a new vehicle at 3 for 3 is not more reliable than one at 400 for 410.

Any of these four is left empty rather than published when the inputs produce a physically impossible answer, which does happen: bad rows exist upstream, and a plausible-looking wrong number is worse than a missing one.

Opening the data in different software

Pick the tool you already use. Nothing here needs an ISSC account or any paid software.

import pandas as pd
import matplotlib.pyplot as plt

df = pd.read_csv("falcon-9-ascent-telemetry.csv", comment="#")
plt.plot(df["time_s"], df["altitude_km"])
plt.xlabel("Time (s)"); plt.ylabel("Altitude (km)")
plt.show()

# Max dynamic pressure (Max-Q) and when it happens
row = df.loc[df["dynamic_pressure_kpa"].idxmax()]
print("Max-Q:", round(row["dynamic_pressure_kpa"], 1), "kPa at T+", int(row["time_s"]), "s")

import json

data = json.load(open("falcon-9-flight-science.json"))
print(data["vehicle"]["name"], "->", data["orbitSummary"]["periodMin"], "min orbit")
print("Provenance:", data["provenance"]["source"])

Straight from the API

If you would rather not click a button, the same data is available as JSON over plain HTTP. No key, no account, no rate limit beyond ordinary politeness. Everything is same-origin on issc.space and CORS is not needed for server-side use.

https://issc.space/api/launches
https://issc.space/api/launches?window=previous

https://issc.space/api/launch-weather?id=<launch id>

https://issc.space/api/launch-orbit?id=<launch id>

What you can do with it

A real-ish flight is a rich dataset. A few things it is good for: