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The Wikipedia webpage on Small-lift launch vehicle gives a huge list of dedicated launch vehicles with a maximum payload capacity of 2,000 kg (4,400 lb) for small satellites, which are either retired, operational or under development.

Almost all space agencies, to the best of my knowledge, started launching satellites using these kinds of rockets. For example, SpaceX started their journey with Falcon-1 (A Small-lift launch vehicle) and not with Falcon Heavy (A Heavy-lift launch vehicle). Of course, they learnt from each launch (both failures and success), and using these lessons they developed Medium-lift and Heavy-lift launch vehicles. There are some new countries like Spain, Argentina, etc., developing small-lift launch vehicles.

I don't understand why some countries with very reliable medium-lift and heavy-lift launch vehicles, again start developing small-lift launch vehicles. Examples include the Indian Space Research Organisation (ISRO) which is developing its Small Satellite Launch Vehicle (SSLV) even though it has operational medium-lift launch vehicles. Further, some countries have operational small-lift launch vehicles in addition to very reliable medium and heavy-lift launch vehicles. This list includes the United States, Russia and, China. There are some private companies like Rocket Lab and Firefly which are having either a fully operational or under development small-satellite launch vehicles.

Why hasn't the small-lift launch vehicles completely replaced by the medium and heavy-lift launch vehicles?

The main reason said in many sources is, using small-lift launch vehicles, small satellites (cube-sats, nano-sats, etc.) get a first-class ride rather than piggybacking on a medium or heavy-lift launch vehicles along with heavy satellites. I agree that sometimes these small satellites are secondary to the primary satellites when the success of primary payload is hindered due to a small malfunction in the launch vehicle. In one of the CRS missions, the secondary payloads were left in a lower than intended orbit due to a problem in the launch vehicle. These re-entered the Earth's atmosphere withing days after orbit-injection. Yes, in these cases it seems to be better to have small-lift launch vehicles. But, I am confused by the following question.

Why not utilise a medium-lift launch vehicle to launch only small satellites as primary payload?

Why do we need to develop a new dedicated launch vehicle for smallsats when existing launch vehicles can carry them into orbit? According to this answer for the question Why is SpaceX not creating its own launch vehicle for small satellites?,

They already have one. The Falcon 9. Earlier this month a single Falcon 9 put 64 smallsats on orbit. It was arranged by a rideshare company, Spaceflight, at prices that small launch vehicles would have a hard time competing with, starting at \$300,000.

Even if it were on the basis of cost, existing launch vehicles can tackle it easily based on the above example. If the entire system is fully reusable like the Starship and Super Heavy, then the costs reduce dramatically, and even the turn around time will become much shorter. Or in other words, the higher frequecy of the launch can also be handled easily, with little to no refurbishment.

If different CubeSats require different orbit requirements, why can't we just club the launches of CubeSats having the same requirements? Or in other words instead of say 10 small-lift launches to 400km polar orbit, why not have a single medium-lift launch to the same orbit?

Not only these super heavy-lift launch vehicles can launch small satellites but also very larger ones. Is this not a bonus? On the other hand, small-satellite launch vehicles can only launch small satellites and not big ones.

Why do we need dedicated launch vehicles for small satellites? Are there any other variables to play here? Do these rockets have any advantage over medium and heavy-lifters?

Another analogy to this is, Are we using small aircrafts like Cessna for air travel? No. We use bigger ones like Boeing 747, etc. The first-class ride on Cessna is much costlier than a first-class ride on an Airbus A380.

If it is not clear to you, kindly let me know in the comments. Thank you for reading.

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    $\begingroup$ "Are we using small aircrafts like Cessna for air travel? No." There are forty-odd thousand Cessna 172s in the world. What exactly do you imagine they're all being used for? $\endgroup$
    – anaximander
    Commented Oct 26, 2019 at 15:39
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    $\begingroup$ @anaximander, I meant comparatively. I know they are being used. I actually made it on the basis of cost for running. For transporting say 400 passengers from A to B, we use a 747 and not 100 Cessnas. This was the basis of my logic. Further, the way I meant was understood by those who answered this question, and I realised where my logic went wrong. $\endgroup$
    – Vishnu
    Commented Oct 26, 2019 at 16:03
  • $\begingroup$ @Intellex , if you're thinking of "bulk launches" the simple answer is that is not possible physically. The rocket has to put each individual object in that trajectory. No "bulk launch" is conceptually possible. I guess that's what you're asking? $\endgroup$
    – Fattie
    Commented Oct 26, 2019 at 17:17
  • $\begingroup$ Your question would be just like asking: "100 different people want to go to { insert list of exactly 100 different cities worldwide }. Why not just use one vehicle?" It just doesn't make sense / it's conceptually impossible. Launch rockets can only go to one of the 1000 trillion "places" satellites want to be. $\endgroup$
    – Fattie
    Commented Oct 26, 2019 at 17:18
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    $\begingroup$ Important factor that has been missing yet: Smallsats are put under heavy security constrains when they share the launch vehicle e.g. only allowed cold-gas thrusters instead of hypergolic. @EverydayAstronaut I don't think this is opion based when there are a lot of papers on this topic out there (e.g. this with 16 citations). $\endgroup$
    – Christoph
    Commented Oct 28, 2019 at 10:53

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Why hasn't the small-lift launch vehicles completely replaced by the medium and heavy-lift launch vehicles?

Because small launchers can provide several things:

  1. A small launcher is much cheaper than using a large launcher to launch a single small satellite.

  2. a dedicated launch, instead of having to share a launch which reduces your choice of final orbits

  3. a dedicated launch also provides more scheduling flexibility

Are we using small aircrafts like Cessna for air travel? No.

Yes we are, all the time. We use a Boeing 747 to transport 400 passengers to the same destination. We switch to a Fokker F-27 when we have only 40 passengers for that destination, and a Cessna when we have 4 passengers.

Even if it were on the basis of cost, existing launch vehicles can tackle it easily based on the above example.

Sure, a shared launch can be cheaper than a dedicated launch. But cost is not the only important parameter. Your final orbit is important too.

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    $\begingroup$ I'd also add that some of the smaller launch vehicles are repurposed ICBMs, which drives the cost even lower. $\endgroup$
    – Alice
    Commented Oct 26, 2019 at 6:56
  • $\begingroup$ What is the first point based on? ("much lower cost than a dedicated heavy-lift launcher could") This is a very broad statement, which I guess is wrong and which contradicts the last paragraph. $\endgroup$
    – Everyday Astronaut
    Commented Oct 28, 2019 at 14:01
  • $\begingroup$ "dedicated" ->a heavy-lift launcher used to provide a launch for a single small satellite instead of a rideshare launch $\endgroup$
    – Hobbes
    Commented Oct 28, 2019 at 14:05
  • $\begingroup$ @Hobbes, Small correction - Dedicated for only small satelliteS, not a single small satellite. Small satellites are the only occupants in the rocket. $\endgroup$
    – Vishnu
    Commented Oct 28, 2019 at 14:28
  • $\begingroup$ That's not what I meant. $\endgroup$
    – Hobbes
    Commented Oct 28, 2019 at 14:40
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Apparently lining up a lot of smallsats for a dedicated big rocket launch is like herding cats. Delays on any of the smallsats delay the overall launch. Hence SpaceX's recent announcement that their planned Falcon 9 dedicated smallsat launches will launch on schedule regardless of whether all the satellites are ready.

https://spacenews.com/spacex-says-rideshare-missions-will-launch-on-time-even-if-partly-empty/

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  • $\begingroup$ Will the customers have to pay whether they ride or not? $\endgroup$
    – user20636
    Commented Oct 25, 2019 at 17:17
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    $\begingroup$ @jcrm According to a friend of mine who is in the business (he works at ESA on the Kourou launch-site) all agencies use the same policy: you buy a slot on a shared launcher. If your satellite doesn't show up in time for installation in the launcher you don't get a refund. Sometimes you can swap your slot with another customer that was scheduled for the next launch, but has its own satelite ready ahead of time. $\endgroup$
    – Tonny
    Commented Oct 26, 2019 at 11:48
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    $\begingroup$ That seems to go against the answer, which states a delay on a smallsat delays the overall launch @Tonny $\endgroup$
    – user20636
    Commented Oct 26, 2019 at 13:22
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    $\begingroup$ again @tonny, The answer says "Delays on any of the small sats delay the overall launch" - so your anecdote contradicts this answer. Is your friend perhaps referring to rideshare with a main payload, instead of a payload consisting entirely of rideshare? $\endgroup$
    – user20636
    Commented Oct 26, 2019 at 13:52
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    $\begingroup$ @JCRM I don't really get the issue you seem to have with my comments? My comments don't really contradict what Ags1 said. Yes, delay of one, delays the whole launch, if you allow that to happen. Space-X doesn't (and most agencies have similar policies on ride-share launches these days). On a big primary launch with secondary small ones the main load is normally leading. On a small-sat only launch there is usualy a minimum number of slots that must be filled for the launch to make economic sense. If that number is met the launch is usually a go as planned even if not all sats show up in time... $\endgroup$
    – Tonny
    Commented Oct 26, 2019 at 14:05
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One variable not mentioned here is the increasing ∆V of ion thrusters on satellites. On the starling launches 60 satellites all just take off and find their own orbits. The better electric propulsion gets, the less important it will be to lift into exactly the right orbit.

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Adding to other answers, if you have a small launcher you can put in orbit your very own small satellite without sharing a great deal of information about it with the big rocket operator and/or the public in general.

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    $\begingroup$ To avoid accidental collisons of satellites, there should be no secret orbit unknown to other operators. Radar surveillance of orbits would determine secrets orbits anyway. $\endgroup$
    – Uwe
    Commented Oct 27, 2019 at 18:56
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    $\begingroup$ Orbits are not secret, but the payload may be. And when you use someone else's rocket, you have to disclose a lot. $\endgroup$
    – fraxinus
    Commented Oct 27, 2019 at 18:59

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