Talk:Photon rocket

Missing Primary Reference

Sänger, Eugen (1956). Zur Mechanik der Photonen-Strahlantriebe. München,: R. Oldenbourg aajacksoniv (talk) 16:14, 12 November 2017 (UTC)

I don't understand

What's the meaning of that sentence? "One of the best ways to overcome the inherent inefficiency in producing thrust of the photon thruster is by amplifying the momentum transfer of photons by recycling photons between two high reflectance mirrors"Klinfran (talk) 23:45, 1 December 2019 (UTC)

Expression of concern ((un)resolved)

The speed expression and derivation in this article assume the rocket consumes its entire quantity of fuel in a single burst and thus is more akin to an explosion than to a rocket. Applying the same simplistic momentum conservation argument for a conventional (non-relativistic) rocket would give $\Delta v=v_{\mathrm {ex} }{\frac {m_{\mathrm {initial} }-m_{\mathrm {final} }}{m_{\mathrm {final} }}}$ instead of the Tsiolkovsky rocket equation $\Delta v=v_{\mathrm {ex} }\log \left({\frac {m_{\mathrm {initial} }}{m_{\mathrm {final} }}}\right)$. The qualitative change in behavior from rational function to logarithm (with the associated limitations on performance) comes from the fact that if the fuel is not consumed in a single burst, some of the fuel's energy and momentum are devoted to accelerating fuel that will be burned later. I may be missing something, but I don't see how a photon rocket would be immune to this issue, so the equations given here seem to me to be incorrect. At the very least some additional explanation or qualification of the results given seems warranted. Becarlson (talk) 14:31, 31 March 2021 (UTC)

The Tsiolkovsky rocket equation is a classical equation that stops being valid once you go to relativistic speeds (that's independent of the type of engine, a fusion-powered ion thruster would have the same issue). Unlike conventional rockets, the exhaust of photon rockets is always going at the speed of light for everyone. It doesn't matter when/how you emit the photons. The momentum carried by them will always be E/c, and that's your momentum as well. That is a nice special case that allows the derivation shown in the article. --mfb (talk) 21:44, 31 March 2021 (UTC)

I stand corrected, thanks for the comment. I hadn't worked through the derivation in detail, and thought the "momentum carried by them will always be E/c" reasoning would break down because of the relativistic Doppler effect. Considered in the original rest frame, it does, but in a way that is consistent with relativistic velocity addition. Working through the derivation in more detail and checking its results against relativistic velocity addition and results from the Relativistic rocket equation, things work out fine. I'll leave this exchange here, but feel free to clean up if desired. Cheers! --Becarlson (talk) 15:03, 3 April 2021 (UTC)

you were perfectly right and his argument is completely false. The tsiolkovsky equation does exist in relativity, it is slightly more complicated, but the phenomenon is the same. What happens is, though the momentum is always E/c as stated, E is no longer the same since in the earth frame (say we started on earth), the doppler effect creates a photon which has a lower frequency, and thus a lower energy. That article is way too much a personnal page as far as I readKlinfran (talk) 22:56, 8 April 2021 (UTC)

Please try to understand my comments before you call them false. Or at least point to specific statements you think that are false, then I can explain why they are correct. The Tsiolkovsky rocket equation is nonrelativistic, a generalization is not the Tsiolkovsky rocket equation any more. --mfb (talk) 04:45, 9 April 2021 (UTC)

The speed derivation is correct (although the step "Therefore, taking the Minkowski inner product (see four-vector), we get:" could be phrased better - it involves combining several previous equations). If you calculate the total in-own-frame gradual acceleration required to reach the predicted speed, you will again recover the logarithm of Tsiolkovsky's equation. However, the assumption that all spent fuel remains on board should be restated in the Maximum speed limit section, since that assumes a non-relativistic limit and is thus only valid as long as the total mass is essentially constant. 81.191.115.167 (talk) 22:25, 3 January 2023 (UTC)

Section "Maximum speed limit"

The first paragraph seems to give undue weight to the work of a single person with fringe views. Everything that follows seems to be original research with little relevance - a spacecraft using fusion or fission as power source would use the waste as reaction mass instead of working as far less efficient photon rocket. I propose to remove the section completely. --mfb (talk) 08:11, 5 October 2023 (UTC)