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I was looking online for a profile depicting the Dragon re-entry similar to those showing the launch and first stage re-entry (e.g. https://www.elonx.net/wp-content/uploads/profile_Inspiration4_Infographic_EN.png) but could not find anything.

Ideally that chart would contain time, velocity and altitude information for the different phases of the re-entry.

Is something like that online? (If this has been asked and answered already feel free to flag as dup, but I didn't see anything.)

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  • $\begingroup$ @Giovanni Thanks for the correction. $\endgroup$ Sep 20, 2021 at 10:00
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    $\begingroup$ @Peter-ReinstateMonic - "Keep from potential enemies" might not mean "protect the capsules from attack"; it might mean "keep secure data that might be useful for devising ICBM re-entry vehicles." Lots of rocket-related stuff is ITAR-protected and can't be made public. $\endgroup$
    – antlersoft
    Sep 20, 2021 at 14:00
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    $\begingroup$ @antlersoft Ah, ok, I see. Seems <strike>not</strike> to be rocket science to find that out if you are able to start an icbm with a nuke but yeah. All the pesky differentiation and integration, angles and stuff, it's really not everybody's cup of tea. $\endgroup$ Sep 20, 2021 at 14:04
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    $\begingroup$ @NgPh There are precise reentry charts of Apollo and the Space Shuttle. $\endgroup$
    – user43968
    Sep 21, 2021 at 12:32
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    $\begingroup$ I think I found the similar question space.stackexchange.com/q/46177/40257 $\endgroup$ Oct 22, 2021 at 21:47

1 Answer 1

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In lieu of authoritative data there is always homebrew simulation.

The necessary details about Crew Dragon & its entry, short of aerodynamics, are relatively easy to find:

Parameter Value Justification
Final Orbit 418 km x 22 km curve fit of altitude callouts from Demo-2 splashdown livestream
Cross-Sectional Area 12.6 $m^2$ 4 m diameter circular cross-section via SpaceX
Entry Mass 9,615 kg item 4 of Top 10 Things to Know for NASA’s SpaceX Demo-2 Return
Orbital Inclination 51.6° inclination of ISS

Critical aerodynamic parameters are the drag coefficient, $C_d$, and the lift to drag ratio, $L/D$. While in general a function of Mach number, these values are (pretty much) invariant in the hypersonic regime (Mach >= 5) and are therefore treated as constant in this simulation.

I was able to infer a drag coefficient from the abstract of A. A. Gonzales et al., "Mars Sample Return using commercial capabilities: Mission architecture overview" and its associated presentation focusing on EDL.

From abstract:

Total entry masses between 7 and 10 mt were considered

Plot from presentation:

Red dragon Mars entry plot

Where $\beta$ is the ballistic coefficient $\beta=\frac{m}{C_D S}$:

Area, S ($m^2$) Mass (kg) $\beta$ ($kg/m^2$) $C_D$
12.6 7,000 450 1.24
12.6 10,000 650 1.22

Thus $C_D=1.23$ (and $\beta=622$ $kg/m^2$).

The $L/D$ is dependent on the axial displacement of the center of mass of the vehicle (which determines the trim angle of attack) and is configurable prior to entry/flight (I believe Apollo experimented with this in test flights; see $L/D$ differences in flight data from AS-202 & Apollo 4).

The value used in the plot, 0.27, is definitely plausible; however, it is higher than other low-Earth orbit crewed entry capsules (Soyuz: 0.26, Dragon 1: 0.18, Gemini: ~0.15, Mercury: 0, ballistic). I used $L/D$ as a fudge factor to make the maximum inertial loading (g-force) about 4.2 g's as described by Demo-2 astronaut Bob Behnken. Here is a plot of the (significant) variation of some entry stats based on differing $L/D$ values, with 0.13 being the selected value:

variation of lift to drag ratio

With all of that out of the way, here is a selection of figures from the simulation (please comment what others you wish to see):

Entry Setup:

entry setup geometry

Note I have defined entry interface at 120 km (extremes of my atmospheric model), in reality this is sometimes a dynamically sensed condition (g-detect) so the duration may seem a little long (click to enlarge):

trajectory description

3D Earth view

References:

  • A. A. Gonzales et al., "Mars Sample Return using commercial capabilities: Mission architecture overview," 2014 IEEE Aerospace Conference, 2014, pp. 1-15, doi: 10.1109/AERO.2014.6836421.
  • L. G. Lemke et al."Mars Sample Return Using Commercial Capabilities: Propulsive Entry, Descent and Landing," 2014 IEEE Aerospace Conference, 2014 (retrieved from NTRS id: 20140013203)
  • Xu Guowu et al., Effect of Recession on the Re-entry Capsule Aerodynamic Characteristic, Procedia Engineering, Volume 99, 2015, Pages 377-383, ISSN 1877-7058, https://doi.org/10.1016/j.proeng.2014.12.550.
  • Trevino , L. "SpaceX Dragon Re-Entry Vehicle: Aerodynamics and Aerothermodynamics with Application to Base Heat-Shield Design," 6th International Planetary Probe Workshop Conference Proceedings, 2008 (Georgia Tech link)
  • Whitnah, A. M. & Howes, D. B. "Summary analysis of the Gemini entry aerodynamics," 1972 (retrieved from NTRS id: 19730014059)
  • Brown, S. W. & Moseley, W. C., Jr. "Summary of wind-tunnel investigations of the static longitudinal stability characteristics of the production Mercury configurations at Mach numbers from 0.05 to 20," 1961 (retrieved from NTRS id: 19710069960)
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    $\begingroup$ Wow. This is not just a hobby, right? $\endgroup$ Nov 7, 2021 at 8:06
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    $\begingroup$ @Peter-ReinstateMonica unfortunately it is just a hobby, for now :( $\endgroup$ Nov 7, 2021 at 13:00
  • $\begingroup$ So much for keeping the real data secret because, you know, that prevents evil actors from designing re-entry. I see. $\endgroup$ Nov 13, 2023 at 16:35
  • $\begingroup$ Interesting that risk assessment didn't find fault with a trajectory squarely aiming at Tallahassee. Apparently, 200 km or so are enough of a safety margin. One argument may be that the debris from a disintegrating capsule would hit the ground earlier, not later. $\endgroup$ Nov 13, 2023 at 16:38

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